Skip to main content

Frequently Asked Questions


  • Can an Active Harmonic Filter be used to protect a specific circuit or machine in a building

      AHF’s are used to mitigate harmonics in a total circuit. Let’s set up an example. A company has 5 work cells and each is producing 20 amps of harmonics. A sixth work cell is introduced, it’s a different machine and it produces 30 amps of harmonics. Some operational issues on the new sixth machine have been determined to be caused by Power Quality issues. Can an AHF be deployed to just protect the new machine?

      • If the new machine is fed from a dedicated transformer, it’s possible an AHF designed to handle at least 30 amps could be installed to mitigate harmonics on the secondary of the transformer feeding this machine.
      • If all six machines are fed by one large distribution transformer with no other isolation transformers between, then an active filter large enough to handle the entire harmonic amperage ((20 x 5) + 30 = 130 amps) must be installed.

  • What is IEEE 519-2014

      IEEE is the Institute of Electrical and Electronics Engineers. IEEE 519-2014 is a document that establishes levels of voltage and current harmonic distortion acceptable to the distribution system based on the input transformer characteristic and the loads on a customer’s facility. Many electrical consultants are including compliance with IEEE 519-2014 in their design specifications to help reduce harmonic problems and avoid penalties that can be imposed by electrical utilities. More information about the levels of harmonics can be found on the IEEE website.

      • The IEEE 519-2014 also outlines the Point of common coupling (PCC) as the point where the utility meets the facility
      • The current and voltage harmonic limits set by IEEE and followed by many specifiers are clearly outlined in the following IEEE tables shown below:

      Voltage Distortion Limits & Maximum Harmonic Current Distortion

  • Does the VFD have to be equipped with a DC link choke to work with Active Harmonic Filter
  • Can the TruWave be used on a 600V system

      Typically the TruWave is rated for 480VAC. However, through the use of a 600V to 480V autotransformer between the TruWave and the load, the TruWave can be used on 600V systems. This same setup can also be used on voltages if needed.

  • What are the benefits of an active filter over a passive filter

      Here are some of the advantages that Active Harmonic Filters can provide over the Passive Filters.

      • Active Harmonic Filters provide far superior flexibility and performance over passive filters.
      • Not all Passive filters can achieve the 8% or 5%THD IEEE-519 specification even at full load. The HPS TruWave AHF will achieve less than 5% THD even until 10% loaded. Passive filters typically provide less overall mitigation as the load decreases.
      • AHF will not cause a leading power factor at no load while passive filters do
      • AHF can be installed anywhere in the lineup, while the passive filters must be installed at each VFD
      • Active filters are cost and space effective with the use of multiple VFD loads compared to passive filters

  • What is an active harmonic filter?

      Due to the increasing usage of non-linear loads such as VFDs, harmonics are being introduced into the power grid which is contributing to poor power quality and leads to overheating of equipment and nuisance faults. Active Harmonic Filters are parallel devices that are used to mitigate harmonics to the levels defined by IEEE-519.

      HPS TruWave AHF utilizes high frequency current sensors to continuously monitor the load and harmonic currents. By utilizing highly sophisticated software and a powerful DSP microcomputer, the system is able to instantaneously inject a corrective current from its IGBT based inverter to dramatically reduce harmonic distortion. The corrective current is equal to but 180 degrees out of phase with the existing harmonic currents to cancel their effect.

      Active filters work on the same principle s as noise cancelling head phones except they cancel harmonic currents and reduce distortion.

  • Do I need to use a line reactor with VFDs to work with an Active Harmonic Filter

      All non-linear loads must have an input line reactor (minimum 3%) or a DC link choke to achieve the desired system performance. While an AHF can correct harmonics without line reactors, issues can occur if there is not sufficient impedance between an AHF and a load.

      Using line reactors is also cost effective since reactors mitigate some of the harmonics and a smaller AHF can be deployed.

  • Can neutral currents such as the 3rd harmonic be reduced by the use of 3rd harmonic blocking filters?

      Some manufacturers are promoting the use of 3rd harmonic (180 Hz) blocking filters for the treatment of high neutral currents caused by non-linear loads such as personal computers. These devices are parallel L-C filters tuned to 180 Hz and are connected in the neutral of 4-wire systems between the transformer secondary and the neutral-to-ground connection.

      Their high impedance to the flow of 3rd harmonic current forces all connected equipment to draw current that does not contain the 3rd harmonic. Although their use will result in a significant reduction in 3rd harmonic current, it is achieved at the risk of rather severe consequences.

      1. The installation raises questions with respect to NEC 2002 compliance. NEC 250.30(A)(2)(a) states that “a grounding electrode conductor for a single separately derived system … shall be used to connect the grounded conductor of the derived system to the grounding electrode…” In addition, “the grounding electrode conductor shall be installed in one continuous length without a splice or joint…” [See NEC 250.64(C)].

      If a simple splice connection is not allowed, then certainly the L-C circuit of the 3rd harmonic blocking filter should not be allowed either. Also, the installation results in an impedance grounded wye system rather than a solidly grounded system. The only reference in NEC that allows for the introduction of impedance between the neutral and the grounding electrode is found in Section 250.36, High-Impedance Grounded Neutral Systems. However, these systems are permitted only at 480V and higher and only if they do not serve line-to-neutral loads. They also require the use of ground fault detectors. None of these requirements is met in the normal application of the 3rd harmonic blocking filter where the loads are primarily 120V, phase-to-neutral connected computer or other power electronic equipment.

      2. Although tuned to 180 Hz, the L-C circuit will introduce some impedance at 60 Hz as well. The consequences are:
      a. Line-neutral short circuit current will be reduced which will limit a circuit breakers ability to clear a line-neutral fault. This can be very dangerous because an uninterrupted fault (commonly referred to as an arcing fault) will often result in an electrical fire.
      b. The neutral point at the transformers wye secondary can shift. This can result in 120V line-neutral voltages that rise and fall unpredictably as the load balance between the phases varies.

      3. High impedance to the flow of 3rd harmonic current will produce voltage distortion in the form of flat-topping – a dramatic reduction in peak to peak voltage. This will:
      a. Significantly reduce the ride-through capability of switch-mode power supplies (SMPS) since the DC smoothing capacitors will not be allowed to fully charge.
      b. Reduce the SMPS DC bus voltage, thereby increasing the current demand the associated I2R losses. Component reliability will be reduced due to higher operating temperatures.
      c. Often cause Single Phase UPS systems to switch to battery back-up.
      d. Force connected equipment to operate without a 3rd harmonic current – an operating mode for which they have not been intended or tested.

      At first, when loading is light, problems may not be extremely obvious. However, as the load increases, voltage distortion and flat-topping will also increase until problems do arise. Although neutral current can be reduced, it is often achieved at the expense of a tremendous increase in voltage distortion. At 30%, the voltage distortion can be up to 4 times the maximum limit of 8% recommended by IEEE std 519. In addition, the measured crest factor can be significantly below the normal sinusoidal crest factor of 1.414.

      4. The 180 Hz L-C blocking filters requires the use of capacitors and it is well known that capacitors are less reliable than inductors and transformers. Failure of the capacitor or its protection could result in a very high impedance ground at the neutral over the full frequency range. This would have a dramatic effect on 60 Hz unbalance and fault currents.

      5. At frequencies above the resonant point (180 Hz), the parallel L-C circuit becomes capacitive which could result in a resonant condition at some higher harmonic frequency.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Can I use PFCC with Active Harmonic Filters

      Power Factor Correction Capacitors can be used on systems with AHF’s. AHF’s harmonic mitigation may even be required to protect PFCC from excessive heating and failure caused by harmonics. PFCC cannot be installed on the load side of AHF current sensors. PFCC should be installed between the AHF and the utility point of common coupling (PCC).

  • Can equipment manufacturers design their products to be free of harmonics

      Yes they can, but lowering the current distortion levels at the input to the SMPS in a computer will add to the cost of the computer. This is not a step that computer manufacturers wish to take because of the continuous and intense cost cutting in the computer industry.

      Actually it is less costly overall to provide a harmonic mitigating transformer to feed several hundred computers than it is to improve the operation of the SMPS in each computer. This is especially true when we consider that the added cost of the improved SMPS will reappear every three years when a new computer system is purchased.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Can and Active Harmonic Filter improve power factor
  • Can I use Active Harmonic Filter for single phase loads

      An active harmonic filter cannot be used to correct harmonics from single-phase harmonic sources. AHF’s correct the harmonics from three-phase sources and therefore are also only designed to run on three-phase systems.

      Isolation transformers and line reactors can mitigate some of these harmonics from single-phase sources. Three-phase system with large loads of single-phase harmonic sources can also use Harmonic Mitigating Transformers (HMT).

  • Why do 3rd harmonic currents overload neutral conductors?

      Sinusoidal currents on the phases of a 3-phase, 4- wire system with linear loads sum to return on the neutral conductor. The 120° phase shift between the sinusoidal load currents causes their vector sum to be quite small. In fact it will be zero if the linear loads are perfectly balanced.

      The instantaneous sum of the currents in the three phases taken at any moment will also be zero if the linear loads are perfectly balanced. If they are not, then there will be a small residual neutral current.

      With linear loads, the neutral conductor can be the same size as the phase conductors because the neutral current will not be larger than the highest phase current. Unfortunately, this is definitely not true for non-linear phase-to-neutral loads.

      120VAC non-linear loads like the SMPS used in computers and in monitors draw current in two distinct pulses per cycle. Because each pulse is narrow (less than 60 degrees), the currents in the second and third phases are zero when the current pulse is occurring in the first phase. Hence no cancellation can occur in the neutral conductor and each pulse of current on a phase becomes a pulse of current on the neutral.

      Even if the phase currents of the SMPS loads are perfectly balanced in RMS amperes, the RMS value of the neutral current can be as much as √3 times the RMS value of the phase current because there are 3 times as many pulses of current in the neutral than in any one phase. If the phase current pulses do overlap because they exceed 60 degrees in width, then there will be some cancellation so that the neutral current will be less than √3 times the phase current. Overlapped or not, because there are 3 times as many pulses in the neutral than in a phase, the predominant component of the neutral current will be the 3rd harmonic (180Hz for a 60Hz system). The linear current completes only 2 cycles in the same time period that the non-linear neutral current completes 6 cycles or 3 times the fundamental.

      Often, in new construction this situation is addressed by simply doubling the neutral conductor ampacity. In existing facilities however, it is most often very difficult and too costly to implement this solution, therefore an alternate method is usually necessary. Question 11 describes how Zero Sequence Harmonic Filters can be used very effectively to reduce 3rd harmonic currents in the neutral conductor.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Are there standards that can help in addressing harmonics?
  • How many CT do I need to use for the Active Filter

      CT’s are used with the HPS TruWave AHF to continuously monitor the load and harmonic currents. Typically, if the system only has three-phase loads downstream to the AHF, two CT’s can be used; the TruWave software will calculate the third phase current. If the system has any single-phase loads, a third CT is required.

      Here are some the installation considerations for the current transformers (CT’s) with the AHF:

      • Must be located upstream of VFD loads requiring correction
      • Two CT’s are required for the correction of three-phase loads
      • A third CT is only required if there are also single-phase (line to neutral) loads
      • The CT’s are sized based on the current rating of the bus

  • What are the main functions performed by an Active Harmonic Filter
  • What communication options come with the HPS TruWave Active Harmonic Filter
  • What does AHF stand for
  • Which applications are best addressed by an Active Harmonic Filter

      AHF’s are used where a significant portion of the load consists of VFD’s or other three-phase non-linear sources such as large three-phase DC power supplies, electric vehicle chargers or UPS’s. VFDs are defined as non-linear loads which generates an enormous amount of harmonics in a system. Harmonics cause a host of electrical problems. AHF’s are great candidates to mitigate harmonics from a system where multiple VFD loads that represent a significant portion of the total load.

      Active filters are designed to reduce harmonics from three-phase sources. For single-phase harmonic sources, solutions such as harmonic mitigating transformers should be considered.

  • What information is needed to size an Active Harmonic Filter

      The following information is all required in order to correctly size an active harmonic filter:

      • One-line diagram of the system. Location and size of VFD’s and Power Factor Correction Capacitors is very useful.
      • Detailed equipment lists can also be used, especially in conjunction with one-line diagrams.
      • VFD information: Horse Power or Current size.
      • Are line reactors being used with each VFD? If so what is the impedance?
      • Will the active filters operate on generator?
      • Are there any large soft start loads located downstream of the AHF?
      • Local Environmental Conditions

  • What information does the TruWave Active Harmonic Filter display give
  • Does HPS build secondary unit substation transformers?

      Secondary Unit Substation Transformers feed low voltage switchgear or switchboards. They typically range from 150 kVA to 2,500 kVA three-phase. The Primary voltage ranges from 2400 VAC to 34,500 VAC and the secondary from 600VAC to 480 VAC. Taps are typically manually changed while the unit is de-energized. The primary connections are typically delta connected while the secondaries are usually wye connected.

      The units are typically installed inside a building and are either dry (VPI) or cast coil because of safety and installation concerns. These units are often connected directly close coupled to the switchgear. HPS can provide both VPI and Cast Coil Secondary Unit Substation Transformers.

  • Does HPS build primary unit substation transformers?

      Primary Unit Substation Transformers are located outside and typically range from 750 kVA to 10,000 kVA three-phase. The Primary voltage ranges from 2400 VAC to over 100,000 VAC and the secondary from 2400 VAC to 34,500 VAC. Taps are typically manually changed while the unit is de-energized. The primary connections are typically delta using bushings, throats or air terminal chambers.

      While often oil, HPS dry-type transformers can offer similar performance with a lighter weight and fewer environmental concerns.

  • What is a Grounded Conductor (Grounding)?
  • Does HPS build substation transformers?

      Substation transformers are located outside and typically range from 750 kVA to 5000 kVA single-phase and 25,000 kVA three-phase. The Primary voltage ranges from 2400 VAC to 46,000 and the secondary from 480 VAC to 15,000 VAC. Taps are typically manually changed while the unit is de-energized.

      While they are typically oil, HPS dry-type transformers can offer similar performance with a lighter weight and fewer environmental concerns.

  • What is a Center Tap?

      It is a center point of a two winding transformer or a true tap in the middle winding of a coil. The voltage from the neutral to the center point will equal the voltage from the center point to end of the coil.

      For three phase units, the center tap is often limited to 5% of the total transformer kVA.

  • What is a Bonded Conductor (Bonding)?

      Bonding all metal parts together and then to the system winding (typically to the X0 terminal of a transformer) is done to provide a low-impedance path to the source (system) to facilitate the opening of the circuit-protection device to remove dangerous voltage on metal parts. In addition, bonding the system to metal parts (typically to the X0 terminal of a transformer) stabilizes the system voltage to the metal parts and it provides a zero system reference (to the metal parts).

  • What does radial-feed mean?
  • What does the term Banked describe?

      Two or three, single-phase transformers can be inter-connected to make a three-phase bank. The primary windings of the single-phase transformers can be connected in Delta or Wye. Likewise the secondary windings can be connected in either a Delta or Wye configuration.

      The equivalent capacity of the bank will be equal to three times the nameplate rating of each single-phase transformer. Usually, this type of installation is more expensive than using a single three-phase transformer. Advantages to banked transformers include:

      • For utility applications, the loss of one transformer in a delta configuration creates an open delta configuration using only two transformers. This will continue to supply power albeit at a reduced total kVA rating.
      • Banked transformers can be used to applications where the size or weight of a three phase transformer is too large. An example would be the ability to move a banked transformer in three lighter and smaller sections in a freight elevator.

  • What are transformer wire leads

      Some transformers, often smaller control or potted units, will use wire leads for their primary and secondary connections instead of copper pads or terminal blocks. Typically these are insulated multistrand wires which are connected to the rest of the circuit using a terminal block, lugs or wire nuts.

  • What does loop-feed mean?
  • What are Primary Voltage Taps?

      In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments.

      Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.

  • Can a transformer convert single-phase power to three-phase power?

      Single-phase power can be derived from a three-phase source. Transformers cannot convert a single-phase source to a three-phase source. The typical method to convert single-phase power to three-phase power is to utilize devices generally termed as rotary or static phase converters.

  • When can you Reverse Connect a transformer

      In general, distribution transformers can be reverse connected without de-rating the nameplates KVA capacity. However, this is rarely considered in modern applications due to NEC code changes. Several precautions need to be taken for reverse connection of some smaller transformers. These would include:
      Dealing with higher current inrush which can cause nuisance tripping.

      HPS transformers under 6kVA three-phase and 3kVA single-phase, there is a “turns ratio compensation” on the low voltage winding. When backfed the turns compensation actually reduces the output voltage. When a three-phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. This modification may violate the warranty and agency listings such as U.L.

      Back-fed transformers increase the installer’s liability since a future user may not realize what is the primary while de-energizing the transformer.

      In general HPS suggest that a proper step up transformer which is designed with the low voltage terminals as the primary terminal be used.

  • Explain Balance Loading on Single and Three Phase Transformers?

      A single-phase transformer with a series/parallel 120/240V secondary winding has two separate 120V secondary windings and is usually connected into a 3-wire system. When the winders are wired in series for 240 VAC, 120 VAC can be obtained at either between the neutral and centerpoint or between the centerpoint and 240VAC. If both 240 VAC and 120 VAC are going to be used, care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total 120VAC load if the 120 VAC load exceeds 5% of the total tranformer rating.

      Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded with single phase loads.

      Failure to balance loads can cause secondary voltage imbalances, additional transformer losses and high neutral currents. Significantly unbalanced loads can reduce the life of a transformer.

  • What is a Delta connection?

      The delta connection is a standard three phase connection with the ends of each phase winding connected in series to form a closed loop with each phase 120 degrees from the other.

  • What does the dot mean on a single phase transformer wiring schematic?

      Typically, a single phase transformer wiring schematic has a dot on both the primary and secondary windings.

      The placement of these dots next to the ends of the primary and secondary windings informs us that the instantaneous voltage polarity seen across the primary winding will be the same across the secondary winding. In other words, the phase shift from primary to secondary will be zero degrees, which is important for some types of circuits. If the wiring to the dots is reversed on one side, the primary and secondary will be 180 degrees out of phase.

  • Describe a Flexible Connection.
  • Can You Use a Control Transformer Connected in Reverse?
  • Can a Control Transformer regulate the output voltage?

      A control transformer will not regulate the voltage. The output (or secondary voltage) of a control transformer is dependent on the magnitude of the primary voltage and the transformer turns ratio. Therefore, fluctuations in the secondary voltage are direct results of any fluctuations in the primary voltage.

  • Can variable frequency drives be powered from an open delta system?

      There are several issues that occur when a Variable Frequency Drive (VFD) is powered from an open delta system:

      • Uneven voltages in an open delta circuit can cause the diode bridge to unevenly draw current which causes additional heating.
      • The current harmonic distortion caused by the diode bridge will also not be balanced line to line which will cause even more additional heating.
      • Some utilities may require a harmonic study to be performed anytime a large VFD load is to be supplied by an open delta system.

      Overall, an open delta system can result in shorted diodes or DC bus capacitor failures on a VFD. Using DC Link Chokes and/or Line Reactors will mitigate some of this additional distortion. The VFD may need to be de-rated for open-delta configurations.

  • What is Common Mode Noise Attenuation (CMNA)?

      It is the attenuation of electrical noise or voltage disturbance, that occurs between all of the line leads and the common ground, or between the ground plane and either line or the neutral. A transformer’s impedance, isolation properties and electrostatic shield may all contribute to the total TMNA level. This is typically expressed in decibels.

  • What is Parallel Operation?

      Single and three phase transformers may be operated in parallel by connecting similarly marked terminals, provided their ratios, voltages, resistances, reactance and ground connections are designed to permit parallel operation. Current and voltage angular displacements are also required to be the same in the case of three phase transformers.

  • What is a Zig-Zag Connection?
  • What is a Wye Connection?

      A standard 3-wire transformer connection with similar ends of the single-phase coils connected. This common point forms the electrical neutral point and may be grounded.

  • What is a Tap?
  • What is a T-Connection?
  • What is a Short Circuit?
  • What is a Scott T Connection?
  • What is a Full Capacity Tap?

      A full capacity tap is one through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise.

      Transformers may be designed with full capacity taps ABOVE or BELOW nominal (FCAN and FCBN, respectively). Full capacity taps allow for the input primary voltage to be increased or decreased from its nominal voltage while retaining its ability to deliver its rated kVA output without exceeding it’s specified temperature rise.

  • Under what circumstance is D.C. Resistance Measurement needed?

      Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of transformer winding heat losses used for winding temperature testing, and as base data for future assessment in the field.

  • What is an Autotransformer?

      It is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits.

      Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autrotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush currents.

  • What is ANSI NETA ATS-2017
  • As an autotransformer, how can a Buck-Boost transformer supply loads significantly higher than its nameplate rating as a low voltage lighting isolation transformer?

      With an autotransformer, only a portion of the current acts as a load on the transformer.  This portion is roughly proportional to the voltage change.  If you increase the voltage from 100VAC to 120VAC, you are roughly adding 20% (20/100).  As a result, only about 20% of the current acts as a load on the transformer.  This would mean that a 500VA low voltage lighting transformer used in an autotransformer (Buck-Boost) application like this could provide a 2500VA (2.5kVA) load even though the nameplate is only rated for 500VA (.5kVA)

      This is a function of changing the voltage by a small amount. For example, if the transformer is connected in such a way that 22 volts is added to a 208 volt primary, a 230-volt output will result.  Only a portion of the current goes through a buck-boost autotransformer roughly equivalent to the voltage change.  As a result, if a buck boost transformer changes the voltage by 10%, only 10% of the current (kVA) go through the unit.  Therefore a transformer rated for 1 kVA when used as an isolation transformer could handle a 10 kVA load if it adjusted voltage by 10% because only 10% of the total load would go through the unit.

      Using this example, the calculation for autotransformer kVA is as follows:

      KVA = (Output Volts x Secondary Amps)/1000

      KVA = (230V x 41.67 Amps)/1000 = 9.58 KVA

  • Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?

      Autotransformers are very common and recognized by all the safety and standard authorities.

      You can refer to N.E.C. Article 450-4, “Autotransformers 600 Volts, Nominal, or Less”, as a reference publication. Item (a) details over-current protection for an autotransformer, and Item (b) covers an isolation transformer being field connected as an autotransformer for a Buck-Boost application.

  • What is the base temperature rise of a transformer

      The base temperature rise of a transformer is the maximum temperature rise at the expected full load capacity. Transformers can be built to run cooler than the base temperature rise, these are typically referred to as lower temperature rise transformers which can either operate in higher ambient temperatures or have additional service factor.

      • 105C Insulation System: 55C Base Temperature Rise
      • 150C Insulation System: 80C Base Temperature Rise
      • 180C Insulation System: 115C Base Temperature Rise
      • 220C Insulation System: 150C Base Temperature Rise

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • How do I use a Grounding Transformer

      Three-phase grounding transformers provide an artificial neutral for grounding. The main requirement is a specific zero-sequence impedance from the Zig-Zag or the Wye/Delta transformer in addition to the fault current withstand rating. For grounding purpose, only the Zig-Zag or a Wye/Delta connected transformer can be used. Autotransformers will have a high zero-sequence impedance and hence, cannot be used for grounding. Air-core reactors can be normally connected between the artificial neutral and ground to provide some additional current-limiting impedance.

      Grounding transformers are often required by the utility to attach a load generator such as solar, wind or a generator to the power grid.

  • Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?

      Buck-Boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For this reason, qualified personnel only should maintain conventional autotransformers.

  • What is preferred for a neutral grounding transformer

      It is up to the user to specify this. Typically, The most common magnetic grounding device is a zig-zag autotransformer. This design offers greater flexibility at a cost and size smaller than a comparable Wye-Delta isolation transformer.

  • What is the difference between a buck-boost transformer and an autotransformer?

      A Buck-Boost transformer is typically a small single-phase low voltage lighting transformer that can be wired as an autotransformer to provide small voltage corrections for single and three phase applications. An autotransformer is a transformer with a direct connection between the primary and secondary and does not act as an isolation transformer. Autotransformers can also include wider classes of products including buck-boost, dedicated three coil distribution style units, motor starting autotransformers and solar grid tie transformers.

  • When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker?

      The NEC Article 450-4 outlines over-current protection for autotransformers. It is reproduced as follows: “NEC 450-4 – Autotransformers 600 Volts, Nominal, or Less

      (a) Over-current Protection. Each autotransformer 600 volts nominal, or less shall be protected by an individual over-current device installed in series with each ungrounded input conductor. Such over-current device shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An over-current device shall not be installed in series with the shunt winding.

      Exception: Where the rated input current of an auto transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes, an over-current device rated or set at not more than 167 percent of the input current shall be permitted.

      (b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as autotransformers shall be identified for use at elevated voltage.”

      Example: A 1kVA transformer, Catalog No. Q1C0ERCB, is rated 120 x 240 to 12 x 24 volts. It is to be connected as an autotransformer to raise 208 to 230 volts single-phase. When connected as an autotransformer in this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining the full load input current and the corresponding size of the over-current protection device, either a fuse or breaker.

      Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts.

      When the full load current is greater than 9 amps, the over-current protection device (usually a fuse or nonadjustable breaker).  Current rating can be up to 125 percent of the full load rating of the autotransformer input current.

      Max. current rating of the over-current device = 46 amps x 125% = 57.5 amps.

      The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the over-current device. This is shown in Article 240-6 of the N.E.C.

      Max. size of the fuse or circuit breaker = 60 amps.

  • Do I need to connect the neutral and ground my HPS three-phase autotransformer?

      If the application needs a neutral (including 3 phase 4 wire systems), the autotransformer must be ordered with the optional neutral terminals (“3L0U” suffix).

      This option will provide the customer with a common (H0/X0) neutral connection point that is connected by the factory to the middle point of the Y winding configuration.

      When selecting this option, both the Line and Load side neutral cables must be connected to the respective neutral terminals in order to ensure the proper operation of the autotransformer.

      HPS does not recommend that the transformer H0/X0 point be grounded locally.

      When an autotransformer without neutral connections is selected, typically the neutral is grounded at the source transformer secondary and is properly referenced throughout the whole installation and carried through to the end load downstream the autotransformer.

      When installing an autotransformer with neutral connections problems can occur when the X0 point of the autotransformer is grounded locally. In such cases a multiple grounding situation may occur which would be against the electrical codes in North America.

      In the above case typically the upstream transformer secondary is grounded at the X0 point of the Y secondary (GND1), in the meantime grounding the X0 point of the autotransformer would create a secondary ground (GND2). Since the two grounds are typically in two different locations, likely far away from each other they will be at different ground potentials.

      This situation can create a number of issues including:

      With the two grounds at different potentials, if the autotransformer center point (X0) is used as a neutral, the line voltages compared to that local neutral would be unbalanced. The extent of the unbalance would depend on the extent of the potential difference between the two grounds (GND1 and GND2). This unbalance could cause issues with the equipment connected to the autotransformer.

      Grounding the X0 of the autotransformer will force the center point of the Y to be always at a certain potential, defined by the local ground. However the voltages of the lines coming into the autotransformer are referenced to the ground point of the upstream transformer. The likely scenario is that the two grounds will be at different potentials which will result in conflicting reference points at the autotransformer. The autotransformer and the electrical system will try to resolve the conflict and equalize the two ground points. The only way that can happen is by having ground current flowing between the two grounds.  Depending on how much of a difference in voltage potential there is between the two grounds and also depending on the ground resistances, there can be a significant current flow through the wye center points.  Adding to this fact that the impedance of an autotransformer is typically low, there could be enough current through the autotransformer to burn out one or more coils of the autotransformer.

      The effects and resulting problems that occur due to improper grounding can be unpredictable and manifest themselves differently in time. Ground potentials can greatly vary depending on environmental conditions.  After installing an autotransformer and grounding the center point of the wye (X0) problems may not surface initially.  However, there is a chance that after a rainstorm or some other event, all of a sudden the user experiences high ground currents just because the grounding conditions have changed.  These problems could be very intermittent in nature and hard to diagnose.

      When an autotransformer with neutral connections is requested, we do not recommend the grounding of the X0 point and recommend that the customer and installing contractor should refer to the local electrical code requirements for grounding and the short circuit protection of a three phase autotransformer.

  • Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?

      Shipped as an isolating transformer, the nameplate is required to show the performance characteristics accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kava’s would be impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.

  • What are some DV/DT applications?

      The dV/dT filter reactors are specifically designed for drive/motor applications with long lead lengths (usually where the motor cable length is 100 feet and greater). They are always installed between the Variable Frequency Drive (VFD) and the motor. Typical installation applications include production process lines, conveyor systems and deep wells.

  • What are Motor Starting Autotransformers?

      Motors have a large inrush current upon energization that can stress the electrical system and cause low voltage conditions. Motor Starting Autotransformers (MSAT’s) are used in reduced voltage starters to temporarily reduce the voltage being applied to the motor. This will extend the time it takes the motor to reach full speed and reducing the overall startup current to the motor.

  • What are DV/DT Filter Reactors?

      The advent of pulse width modulated (PWM) inverters with IGBT high-speed transistors, has resulted in smaller more cost effective drives and increased switching speeds. A waveform with increased harmonics at higher frequencies is the result of these much faster switching devices, usually at frequencies of 10,000 to 20,000 Hertz.

      Drives and motors often need to be separated by significant distances. For deep wells or mines, the motors are usually controlled on the surface. As a result, the distance between the drive and the motor creates long motor lead lengths. In some plant applications, the motors can withstand the harsh environment but the sensitive variable frequency drive cannot. This again results in long lead lengths to the motor.

      Most manufactures of variable frequency drives will publish a recommended maximum distance between their equipment and the motor. Sometimes these recommendations create application difficulties, thus increased motor lead lengths are inevitable.

      DV/DT is explained as the steep-front voltage pulses that travel down these long leads in the circuit to the motor and subsequently reverted back in a “reflective wave”. When the conductors are long enough, usually 20 feet or more, the time for reflection matches the time for transmission resulting in a high amplitude ‘standing wave’ on the circuit. Voltage spikes of up to 2100 volts are frequently experienced for 600-volt systems, and motor winding failures are the result.

      A Filter Reactor, installed in front of the motor, combines the current limiting ability of an AC line reactor plus a resistive capacitance circuit that forms a damped, low pass filter. It provides protection for the motor by slowing the rate of voltage increase and minimizing the peak voltage that occurs at the motor terminals.  The cost of a DV/DT Filter Reactor is little more than the cost of the reactor and can be mounted next to the motor, or inside the PWM enclosure.

  • What Output Problems Can Occur with Variable Frequency Drives (VFD or VSD) and How Can You Mitigate These Issues?

      A voltage-sourced Variable Frequency Drive (VFD) uses Insulated-Gate Bipolar Transistors (IGBTs) to rapidly switch voltage on and off to form a Pulse Width Modulated (PWM) voltage source for the motor. The PWM simulates a sine wave voltage source to the motor and it operates as if it was being powered by a sine wave.  The PWM wave allows the VFD to change the fundamental frequency of the PWM waveform and simulate sine waves.  Since the speed of a motor is directly related to the fundamental frequency of the sine wave, a VFD can control speeds from a fraction of a hertz to hundreds of hertz.

      Output reactors, dV/dT filters or drive isolation transformers can be used to help mitigate some issues caused by the PWM output.  PWM outputs cause rapid switching transitions which can cause over-voltages due to parasitic capacitance and inductance in the motor’s leads. The parasitic currents and voltages can be determined by the equation of V = L × (Δi/Δt).  VFD’s switching frequencies (the amount of pulses used to simulate the sine wave) generally range from 1,000-20,000 pulses per second.  IGBT’s produce an almost perfect square wave which produces a very high Δv/Δt.   High Δv/Δt can cause higher surge currents in the leads. This then causes high voltage pulses across the parasitic inductances.  Therefore the faster the pulses switch, the greater the impact of cable capacitance and inductance. These voltage pulses stress the motor’s windings causing higher audible noise, heat and possibly premature failure of the insulation. There is also capacitance in the motor’s bearings.  The combination of lubrication and air gaps prevent direct and continuous contact of the bearings to the metal traces that contain them.  Parasitic currents [I = C × (Δv/Δt)] causes current to flow through the bearings.  The amount of current will increase as the VFD output switching speed increases. These currents can cause micro pits to form in the bearings and eventually will lead to premature bearing failure.

  • What is the typical dV/dT Filter Reactor performance?

      Category: | Frequently Asked Questions   | DV DT  


      A dV/dT filter reactor combine appropriate values of inductance, capacitance and resistance to form a filter, which reduces dV/dT and peak voltages from the PWM (Pulse Width Modulated) voltage waveform. This will reduce motor heating harmonics and audible noise to significantly increase the life of the motor.

      Long lead lengths between the VSD and motor can experience motor terminal peak voltage spikes two or four times the DC bus voltage. Therefore motor terminal voltage peaks of 1300-2700 volts for 480V drives and 1700-3400 volts for 600V drives are not uncommon.

  • Transformer is overheating

      Transformer insulation is generally rated for 220°C but may be lower for some designs including control or encapsulated. Standards permit the temperature of the transformer enclosure cover to be 65°C over ambient. When temperatures exceed the rating for the insulation system or enclosure, overheating occurs.

      Burned, darkened or damaged insulation may be apparent along with a burnt smell. The hottest part of a transformer is the coil near the top of the core. Energized transformers should not be touched. If the insulation is damaged or smoke is visible, the unit may need to be returned for testing and replaced or repaired.

      Check: Solution:
      Verify total load doesn’t exceed transformer kVA rating. Reduce size or load or replace with larger transformer. In some cases fans can be added to increase cooling and
      maximum load.
      Verify ambient temperature does not exceed transformer ratings. Relocate to area with lower ambient temperature, reduce load, reduce ambient temperature at primary location or
      replace with a low temperature rise transformer. Transformers installed in small rooms will need proper room
      Verify tap connections are set up identically on all coils.
      Verify transformer is correctly rated for harmonic load, check for high neutral currents. Reduce or remove harmonic loads or replace transformer with a larger unit or unit with the proper k-rating.
      Verify that the transformer’s ventilation openings are not blocked. Transformers purchased as core and coil
      units and placed in enclosures not supplied by HPS require that the integrator properly size the enclosure and cooling
      Relocate the transformer to an area of better ventilation. Move the transformer away from walls, equipment or
      overhead projections that may impede airflow. Do not install fans to cool a transformer. Improperly installed fans may
      actually impede airflow and could result in transformer damage.
      Improper Input Voltages Verify taps are correctly set for the input voltage. Depending on the load and transformer type, continuous
      overvoltages or undervoltages as low as 5-10% may cause overheating.
      Check no load current. If no load current is high (varies with transformer efficiency but no load current is typically less than 2-3% of
      total kVA), inspect the core and coils for damage. In most cases you will not be able to inspect the insulation between
      the core and coil without returning to the factory for testing and disassembly. If there is a short between the core
      and coil, the unit will have to be replaced or repaired.
      Excessive and sustained airflow caused by exterior winds or fans generally moving horizontally to the ground can
      disrupt convection cooling and cause overheating at high loads.
      Relocate the transformer to an area with less wind or block the wind.
      Fan cooled transformers have broken or misaligned fans. Fans need to be replaced or realigned.
      Low Power Factor Low power factor can cause excessive current and higher overall loads. Current meters need to be able to register
      total current. Some digital meters may not be accurate.
      Unbalanced loads may cause excessive heating. Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for
      each phase (1/3 of total kVA for three phase units).
      Transformer is installed above a heat source such as another transformer. Move either the transformer or the heat source. Redirect the hot airflow from the lower object away from the
      cooling entrances and surfaces of the higher object. Replace the top unit with a low temperature rise transformer.
      Check if output voltage is distorted. A highly distorted output voltage may be a sign that there is a turns to turns fault and the transformer is in
      danged of immediate failure. The transformer needs to be denergized and meggered. The damaged coil may need to be
      replaced or the transformer scrapped.
      Check the output circuits to make sure each leg of the transformer is functioning and overcurrent protection is
      If a fuse on one or more of the legs has opened, determine and clear the fault and replace the fuse. This is more
      commong on delta transformer outputs, especially if three single phase units are used in a Delta bank.
      If a Drive Isolation Tranformer (DIT) is being used, verify the DIT kVA has been derated per the HP sizing charts in the catalog. If motor HP is unknown, use .746 kW/HP to determine the equivalent HP of the load. DIT’s are not current rated
      devices, the HP selection charts must be used to properly size a DIT. Extrusion applications tend to be the worst.
      Check if two or more transformers are operating in parallel to power one load. Transformers operating in parallel are rare. Large circulating currents and uneven load can result from
      transformers wired in parallel. The transformer s may have to replaced with one unit capable of power the entire
      Cable connections are discolored by heating. Cables should be periodically tightened. The surface should be cleaned of any insulation applied during the vacuum
      pressure impregnation process. Rough edges must be smoothed.
      Sparks or smoke is visible from the base of the transformer but the transformer has not failed and there isn’t any sound of arcing. During the VPI process, icicles of insulation can form under the tarnsformer and occassionally act as a ground. If
      discovered early enough the icicle can be removed and the transformer will not be damaged.
      Excessive dust could block air vents Dust needs to be blown out while transformer is denergized.
      Discolored Insulation The transformer’s insulation may have been damaged and may need to be repaired or replaced.
      Visible Flames or Smoke The transformer’s insulation may have been damaged and may need to be repaired or replaced.

  • Noise & Vibration

      Sound levels for transformers vary from 40 dbA for smaller distribution units to 68 dbA for 3000 kVA power transformers and higher for larger units. All transformers vibrate at 120 Hz because the EMF vibrates the unit due to 60 Hz oscillations. The audible noise is measured at no-load and tested in a low ambient noise environment with walls or reflecting surfaces at least 10’ away from the transformer per NEMA ST-20. When installed in more confining electrical rooms and connected to a load, transformers will exhibit higher sound levels than these standards. Transformers can be ordered that produce less noise that the NEMA ST-20 standard, generally at -3 and -5 dB but lower levels are also available. Rubber pads or springs can also be installed between the enclosure and floor to further reduce vibrations and noise. Harmonics generally don’t increase noise too much.

      Check: Solution:
      High Input Voltage Verify taps are correctly set for the input voltage.
      High Frequency If power is coming from a local generator, adjust the generator. If the frequency can’t be adjusted, the transformer may have to be rebuilt.
      Unbalanced Loads Loads should be balanced to within 20 % of maximum kVA. No individual load should exceed the load specific load for each phase (1/3 of total kVA for three phase units).
      Damaged Core. Check any core welds or clamped brackets when fully denergized to make sure the welds are intact and the brackets haven’t slipped. Loose brackets can be tightened on-site but broken welds or cores that have shifted will need to be sent back to the factory for repair.
      Missing Core to Enclosure anti-vibration pads. All ventilated transformers should have rubber pads between the core and enclosure or mounting points. If these are missing they can either be field replaced or the unit can be sent back to the factory.
      Loose Enclosure Tighten enclosure screws and bolts were necessary.
      Transformer mounted on a suspended floor or wall may create and echo chamber and increase noise. Reinforce the floor/wall or move the transformer to a more solid mounting area.
      Multiple transformers installedin one room. Multiple transformers in one area can sometimes resonate with each other and increase the overall noise. The transformers either have to be moved further apart or isolation pads or springs can be mounted under the transformer enclosure.
      Transformer is running correctly but still producing too much ambient noise. A low audible dB transformer can be used to replace the existing unit and/or vibration isolation pads and springs can be installed between the transformer base and floor.

  • Are there any transformers exempt from this legislation?

      As defined by DOE 10 CFR 431.192 a Distribution transformer means a transformer that—

      • Has an input voltage of 34.5 kV or less;
      • Has an output voltage of 600 V or less;
      • Is rated for operation at a frequency of 60 Hz; and
      • Has a capacity of 10 kVA to 2500 kVA for liquid-immersed units and 15 kVA to 2500 kVA for dry-type units.

      These are the transformers subjected to the DOE 2016 requirements.

      Exceptions are defined by the same DOE 10 CFR 431.192.(5):

      • The term “distribution transformer” does not include a transformer that is an:
      1. Autotransformer;
      2. Drive (isolation) transformer;
      3. Grounding transformer;
      4. Machine-tool (control) transformer;
      5. Non-ventilated transformer;
      6. Rectifier transformer;
      7. Regulating transformer;
      8. Sealed transformer;
      9. Special-impedance transformer;
      10. Testing transformer;
      11. Transformer with tap range of 20 percent or more;
      12. Uninterruptible power supply transformer; or
      13. Welding transformer.

      Drive (isolation) transformer means a transformer that:

      1. Isolates an electric motor from the line;
      2. Accommodates the added loads of drive-created harmonics; and
      3. Is designed to withstand the additional mechanical stresses resulting from an alternating current adjustable frequency motor drive or a direct current motor drive.

  • What are the types of transformers affected by DOE 2016 and NRCan 2019?
  • What are the Energy Efficiency levels mandated by DOE as of January 1st 2016?

      The US Department of Energy (DOE) has regulated the energy efficiency level of low-voltage (LV) dry-type distribution transformers in US since 2007, and liquid-immersed and medium-voltage (MV) dry-type distribution transformers since 2010.

      DOE’s CFR (Code of Federal Regulation) title 10, part 431 defines the current energy efficiency standards for distribution transformers sold in US also known as TP1 energy efficiency levels as adopted by NEMA. Effective Jan. 1st 2016 DOE’s CFR 10 p.431 will require new higher levels of Energy Efficiency for transformers installed in any US territory as published in the Federal Register Vol. 78, No. 75 on April 18, 2013.

      Any Distribution transformer manufactured on or after Jan. 1st 2016 and sold in any US state will have to comply with the new energy efficiency levels defined by this document.

      Q2: What are the types of transformers affected?

      The three types of distribution transformers covered by the standard are: low-voltage dry-type, liquid-immersed, and medium-voltage dry-type distribution transformers.

      Q3: What are the Energy Efficiency levels mandated by DOE as of January 1st 2016?

      The new Energy Efficiency levels mandated as of Jan.1st 2016 are as follows:

      Amended Energy Conservation Standards for Low-Voltage Dry-Type Distribution Transformers

      Single phase   Three phase  
      kVA Efficiency (%) kVA Efficiency (%)
      15 97.7 15 97.89
      25 98 30 98.23
      37.5 98.2 45 98.4
      50 98.3 75 98.6
      75 98.5 112.5 98.74
      100 98.6 150 98.83
      167 98.7 225 98.94
      250 98.8 300 99.02
      333 98.9 500 99.14
          750 99.23
          1000 99.28
      Note: All efficiency values are at 35 percent of nameplate-rated load, determined according to the DOE Test Method for Measuring the Energy Consumption of Distribution Transformers under Appendix A to Subpart K of 10 CFR part 431.

      Bottom of Form



      Amended Energy Conservation Standards for Medium-Voltage Dry-Type Distribution Transformers

      Single phase       Three phase      
      BIL* kVA 20–45 kV efficiency (%) 46–95 kV efficiency (%) 96 kV efficiency (%) BIL kVA 20–45 kV efficiency (%) 46–95 kV efficiency (%) 96 kV efficiency (%
      15 98.1 97.86 15 97.5 97.18
      25 98.33 98.12 30 97.9 97.63
      37.5 98.49 98.3 45 98.1 97.86
      50 98.6 98.42 75 98.33 98.13
      75 98.73 98.57 98.53 112.5 98.52 98.36
      100 98.82 98.67 98.63 150 98.65 98.51
      167 98.96 98.83 98.8 225 98.82 98.69 98.57
      250 99.07 98.95 98.91 300 98.93 98.81 98.69
      333 99.14 99.03 98.99 500 99.09 98.99 98.89
      500 99.22 99.12 99.09 750 99.21 99.12 99.02
      667 99.27 99.18 99.15 1000 99.28 99.2 99.11
      833 99.31 99.23 99.2 1500 99.37 99.3 99.21
              2000 99.43 99.36 99.28

      Bottom of Form

              2500 99.47 99.41 99.33
      BIL means basic impulse insulation level              
      Note: All efficiency values are at 50 percent of nameplate rated load, determined according to the DOE Test Method for Measuring the Energy Consumption of Distribution Transformers under Appendix A to Subpart K of 10 CFR part 431.

      Bottom of Form


  • Can I still sell my C802.2 efficiency transformers that I have in stock after NRCan 2019
  • What are the environmental benefits of this change?

      According to DOE, the new amendments to the existing efficiency standards would further decrease electrical losses by about 8 percent for liquid-immersed transformers, 13 percent for medium-voltage dry-type transformers, and 18 percent for low-voltage dry-type transformers. In addition, about 264.7 million metric tons of carbon dioxide emissions will be avoided, equivalent to the annual greenhouse gas emissions of about 51.75 million automobiles.

      Beginning in 2016, newly amended energy efficiency standards for distribution transformers will save up to $12.9 billion in total costs to consumers — ultimately saving families and businesses money while also reducing energy consumption. The new distribution transformer standards will also save 3.63 quadrillion British thermal units of energy for equipment sold over the 30-year period of 2016 to 2045.

  • What is the energy efficiency regulation compliance in the U.S. and Canada?

      In the past several years, there has been an accelerated rate of change in updating energy efficiency standards for transformers in North America.

      Governments in US and Canada are encouraging users to use higher energy efficiency dry-type transformers, to help reduce carbon dioxide emissions. There is also a long term cost savings in operating higher efficiency transformers translated in lower energy usage, lower cooling cost, etc.

      In U.S.A. the Department of Energy (DOE) has mandated new higher efficiency levels effective Jan. 1st 2016.

      In Canada Natural Resources Canada (NRCan) published SOR/2016-311 which amends the Energy Efficiency Act to align the via amendment 14 the minimum energy efficiency levels for dry type transformers to the ones implemented by DOE in Jan 2016.

      The new NRCan 2019 regulation is going to be enforced across Canada on May 1st, 2019. The Ontario government already adopted these new efficiency levels by publishing the ON Reg.404-12 which in schedule 6 defines the new energy efficiency levels that dry type transformers sold in ON must comply with starting Jan.1st 2018 (Ontario Energy Efficiency Compliance).

      The rest of Canada (including Quebec) is still following the current energy efficiency levels prescribed by CSA C802.2, until the new NRCan regulations come in effect on May 1st 2019.

      To help our valued customers in estimating the cost savings resulting from upgrading their old dry type transformer to the new DOE2016/NRCan2019 efficiency levels, HPS has developed an Energy Savings Calculator available on its website. To find out how HPS can help reduce your energy consumption, click here.

      To visit the Canadian Gazette for more information about the Canadian energy efficiency standards, click here.

      For the Ontario Energy efficiency regulation please click here.

      To view an electronic copy of the U.S. DOE energy efficient standards, click here.

  • Will NEMA Premium transformers continue to be offered?
  • What are the new Energy Efficiency levels in place for the Canada in 2019?

      Transformers have been and remain an essential part of our electrical infrastructure. Everywhere we look there is a transformer supplying power to industrial, commercial or residential applications.

      In the past decades the greenhouse gas emissions and the effects on our planet have become the focus of many governments, agencies and individuals. Energy generation is a major contributor to the greenhouse gas emissions. In addition to widespread efforts to make energy generation more environmentally friendly, there is also a goal to lower energy consumption within most industrial, commercial and residential areas. Achieving increased energy efficiency levels for equipment and consumer products has become a priority for many manufacturers.

      Improving the energy efficiency of new transformers is a primary goal of the Department of Natural Resources (French: Ministère des Ressources naturelles), operating under the FIP applied title Natural Resources Canada (NRCan). It has the legal authority to define efficiency levels and enforce compliance. Environmentally conscious consumers also recognize that buying a higher energy efficiency transformer will have a societal payback over many years.

      NRCan has established new and more stringent Energy Efficiency levels for Transformers in Canada effective May 1st, 2019 that is generically referred to as NRCan 2019. The new efficiency levels for Medium Voltage Liquid-Filled, Medium Voltage and Low Voltage Dry-Type Distribution Transformers are defined byNRCan and largely follow the U.S.A.’s efficiency leves in the DOE’s 2016 CFR (Code of Federal Regulations) title 10 part 431. The new efficiency levels are expected to reduce energy losses by an average of 18% in low-voltage dry-type distribution transformers and 13% for medium-voltage dry-type transformers, over the current C802.2 efficiency levels.

      To put the benefits of this change in perspective, the U.S.A’s DOE projects savings up to $12.9 billion (U.S.) in total costs to consumers and 3.63 quadrillion Btu of energy over a 30 year period. In addition, about 265 million metric tons of carbon dioxide emissions will be avoided, equivalent to the annual greenhouse gas emissions of about 52 million automobiles. Canada can expect similar benefits but scaled to Canada’s overall economy.

      The subject of energy efficiency for transformers raises two main considerations:

      (1) Under normal operation a transformer is always on (typically at 35% average loading), making any energy efficiency improvements more significant over an extended period of time. This means that customers will be rewarded in two manners: they are reducing greenhouse gas emissions and there is an economic payback through reduced energy costs.? Considering the life expectancy of a transformer and the fact that the transformer will be on 24 hours a day, 7 days a week for the next 25-30 years, even small energy efficiency improvements will pay dividends for decades.? A secondary benefit is that more efficient transformers generate less heat, and in many cases this translates into lower costs to cool the environment in which they are utilized.

      (2) The currently mandated energy efficiency levels are already hovering around the 98-99% mark, depending on the type of transformer and ratings. This means that any further efficiency improvements become more challenging to achieve, typically requiring more and/or better core and conductor materials.? This will directly impact the cost of the transformer in most cases. However, as noted in point 1 above, there is an economic benefit to offset the higher initial transformer costs. The new NRCan 2019 compliant transformers that will come on the market will also be somewhat heavier than the current C802.2 efficiency level transformers.

      Hammond Power Solutions (HPS) has an online Energy Savings Calculator to help to our customers determine the savings they can achieve by installing a higher efficiency transformer.? It includes a comparison of transformers with older efficiencies to those of higher efficiency (NRCan 2019 and DOE 2016) as well as specifics of the application and the customer’s cost of energy.

      The Electricity savings resulting from upgrading one three phase 75 kVA transformer can be translated into one of the following:

      • 1.19 Metric Tons of CO2
      • 121 Gallons of Gasoline
      • About 1/6th of the energy used by an average household annually
      • Planting 28 Trees
      • 0.9 Acres of Forest
      • Recycling 0.34 Metric Tons of Waste
      • Savings of $166 per year at $0.12 per kW-Hr

      Dense Forest

      At some kVA ratings NEMA Premium energy efficiency levels meet or slightly exceed the DOE 2016 levels, some are slightly below the new requirements.? However, the NEMA Premium products are optional within the market today, and many consumers do not take advantage of the benefits they afford.? Hence, the DOE will require that all transformers manufactured after January 1st, 2016 will meet the new efficiency levels.

      The environmental impact and savings for our customers resulting from the DOE changes are positive and significant.? HPS fully embraces and supports this change, and the environmental benefits our society will receive as a result.? We proudly offer high quality transformers meeting the most stringent Energy efficiency requirements today and will be in a position to support the migration to the new DOE 2016 higher-efficiency designs for our valued partners and customers, beginning in the latter half of 2015.

      View HPS Transformer Savings Analyzer.

  • Did the DOE 2016 and NRCan 2019 efficiency regulations result in product brand changes for HPS?

      The obsolete HPS TP1/C802.2 rated Sentinel, Synergy, Centurion and Express lines were still available in the Canadian until the NRCan 2019 efficiency levels (same as DOE 2016) were mandated. In addition the older medium voltage Millennium line was still available in Canada until NRCan 2019 May 1st, 2019.

      The older lines were replaced with DOE 2016 and NRCan 2019 compliant low voltage Sentinal G (general purpose), Sentinel K (K rated), Sentinel H (harmonic mitigation) Express G and Tribune E (DIT, Canada only) product lines. In addition the medium voltage Millennium G, Millennium E and EnduraCoil lines also meet the updated DOE 2016 and NRCan 2019 efficiency levels. While exempt from regulations, the Titan N encapsulated transformers often meet or exceed the DOE 2016 and NRCan 2019 regulations.

  • How will the new DOE 2016 and NRCan 2019 compliant Sentinel G, K and H be different from the older TP1/C802.2 lines?
  • What should I do if I need to quote a project that will ship after NRCan 2019 is implemented?
  • Can I still sell my TP1 efficiency transformers that I have in stock?
  • Are there any restrictions on the type of load that can be operated from a Buck-Boost transformer?

      Buck-Boost transformers can be used for any application that allows for autotransformers.  Some applications require an isolation transformer only.

  • How does a buck-boost transformer differ from an isolating transformer?

      A Buck-Boost transformer is manufactured as an isolating transformer, with separate primary and secondary windings and is shipped from the factory in that configuration. When field connected for a buck-boost application, the primary and secondary windings are wired together which changes the transformer’s electrical characteristics to those of an autotransformer. The primary and secondary windings are no longer isolated as they are connected together.

  • Can Buck-Boost transformers be used to power low voltage circuits?

      Installed as two-winding, isolation transformers, these units can be used to power low voltage circuits including control, lighting circuits, or other low voltage applications that require 12, 16, 24, 32 or 48 volts output, consistent with the secondary of these designs. The unit is connected as an isolating transformer and the nameplate kVA rating is the transformer’s capacity.

  • Can Buck-Boost transformers be used on 3 phase systems?
  • Can 60Hz Buck-Boost transformers be operated on 50Hz?

      HPS Universal Buck-Boost transformers are 50/60Hz rated and are okay to operate at both frequencies. Units rated for 50Hz can also be operated at 60Hz. However, units which are only rated for 60Hz cannot safely be operated at 50Hz.

  • How does the sound level differ between Buck-Boost and isolation transformers?

      Buck-Boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer that is rated for the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10kVA is 35dB and a 75kVA is 50dB.

  • Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?

      No – a three-phase “Wye” buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.

  • How do the costs compare between a Buck-Boost transformer and an Isolation transformer handling the same load?
  • What are Buck-Boost Transformers and where are they used?

      Buck-Boost transformers are potted transformers with low voltage secondary windings. By field connecting the primary and secondary windings in an autotransformer configuration (not isolated), they offer an economical solution to the adjustment of line voltages that are slightly above or below normal. These transformers should be used to adjust stable voltages only.

  • What is the function of a Buck-Boost Transformer?

      A Buck-Boost transformer is a simple and effective way of correcting off-standard voltages. Electrical and electronic equipment is designed to operate within a standard tolerance of nominal supply voltages. When the supply voltage is consistently too high or low – typically more than 10%, the equipment will operate below peak efficiency.

  • What is the life expectancy of a Buck-Boost transformer?
  • Why do Buck-Boost transformers have 4 windings?

      A four winding buck-boost transformer with 2 primary and 2 secondary windings can be connected eight different ways to provide a multitude of voltages and kVA’s. This provides the flexibility necessary for the broad variety of applications. A two-winding transformer can only be connected in two different ways.

  • What is a Buck Boost transformer?
  • Why isn’t a ‘closed Delta’ Buck-Boost connection recommended?

      This connection requires more kVA power than a “Wye” or open delta connection, and phase shifting occurs on the output. The closed delta connection is more expensive and electrically inferior to other three-phase connections.

  • Why are Buck-Boost transformers shipped from the factory connected as isolating transformers, and not pre-connected autotransformers?

      The same 4-winding Buck-Boost transformer can be connected eight different ways to provide a multitude of voltage combinations. The user when assessing the supply voltage at site can best determine the correct connection.

  • Will a Buck-Boost transformer stabilize voltage?

      Autotransformers will not stabilize supply line voltage. The output voltage of an autotransformer is a function of the input voltage. If the input voltage varies, then the output voltage will also vary by the same percentage.

  • Can wire or cable be located above a transformers core and coil

      HPS recommends side or bottom entry for conduit into a type 3R, 12, 4 and 4X enclosure. Neither U.L. or NEC addresses this specifically with transformers. C.S.A. C22.2 No. 47 does address this and disallowed cable entry from the top of the enclosure. The issue with insulated cables is a transformer cools by convection with air moving from the bottom to the top of the coils. The air exiting from the top of the coils can be heated significantly higher than ambient. Cables which enter through the top of the enclosure will experience this heated air which causes three issues:

      • Cable insulation damage within the enclosure.
      • Hot air entering the conduit can cause insulation damage with the conduit.
      • Hubs must be used to prevent water intrusion

      Because of these issues, HPS does not recommend that cable enter through the top of a transformer’s enclosure in any application. Note that some of these issues can be mitigated if using non-jacketed hard bussing instead of cable.

  • common transformer installation issues

      Improper Secondary Ground
      If the secondary of the transformer is not grounded properly, the output voltage will look ok between the phases but it will float and not be referenced to earth ground.

      Back-Feeding Delta Primary/Wye Secondary Transformers
      While a base wye secondary transformer can be field modified to backfed, the field modifications may violate U.L., NEC or local code and the transformer’s warranty. Don’t back-feed delta/wye transformers.

      Back-Feeding Transformers above 1 kVA
      Back feeding larger transformers can result in high inrush currents upon transformer energization and nuisance tripping of circuit breakers and fuses. Due to a number of factors which affect inrush, this issue is difficult to predict and costly to fix. The best way to handle this is to purchase transformers wound as step-up. If this isn’t feasible, transformers should be sized to the maximum amperage protection allowed by code, the larger the transformer, the more potential for this to occur.

      Power Wires Routed over the core and coils
      The are being ventilated through the core and coils can be very hot, in excess of 100oC. This can cause wire insulation failure.

      Power Wires terminated in the bottom of the transformer compartment
      Conduit should not be terminated in the bottom of the transformer with a grated floor. The grated floor is needed to provide airflow to cool the transformer but the grates provide a poor surface to mount a coupling and may also violate NEC code.

      Missing Vibration Pads or Vibration Isolators
      All transformers vibrate at 120 hz because of the electromagnetic field in the core. These vibrations and audible noise can transfer through the floor, vibration pads and isolators help to minimize this issue in commercial applications.

      Missing Drip Shields
      While all outdoor applications need a minimum of a NEMA 3R enclosure, even indoor applications near sprinklers would require a minimum NEMA 2S enclosure and therefore drip shields.

      Transformer Harmonic Heating
      Due to the prevalence of non-linear loads and the harmonics they produce, transformers can overheat if not specified properly. As a rule of thumb, if a load contains 25-50% non-linear sources, use K=4, if a load exceeds 50% non-linear sources use K=13.

      Transformer Ambient Heating
      Transformers need to be placed in locations that allow proper ventilation to remove the heat they produce during normal operation.

      More troubleshooting

  • Can conduit be installed through the top of an enclosure

      HPS recommends side or bottom entry for conduit into a type 3R, 12, 4 and 4X enclosure. Neither U.L. or NEC addresses this specifically with transformers. C.S.A. C22.2 No. 47 does address this and disallowed cable entry from the top of the enclosure. The issue with insulated cables is a transformer cools by convection with air moving from the bottom to the top of the coils. The air exiting from the top of the coils can be heated significantly higher than ambient. Cables which enter through the top of the enclosure will experience this heated air which causes three issues:

      • Cable insulation damage within the enclosure.
      • Hot air entering the conduit can causing insulation damage with the conduit.
      • Hubs must be used to prevent water intrusion

      Because of these issues, HPS does not recommend that cable enter through the top of a transformer’s enclosure in any application. Note that come of these issues can be mitigated if using non-jacketed hard bussing instead of cable.

  • What is the difference in enclosures for indoor and outdoor non-hazardous applications?
  • Why is the insulation rating for some distribution transformer set at 220°C and for others the rating is 200°C?

      Most standard ventilated distribution transformers use a 220°C insulation system. This insulation system provides a 150°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      However, if a transformer is wound using copper wires, a few of the smaller frame sizes (15kVA and 30kVA three phase, 15kVA and 25kVA single phase) utilize a 200°C insulation with a 130°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      The differences in these smaller kVA sizes using copper wire are the result of U.L. ratings of the wire’s insulation temperature rating. Copper wire has a significantly smaller diameter than an equivalent ampacity aluminum wire.

      Small copper wire with 220°C insulation is not always available, so insulation systems are limited to 200°C (Note: U.L. does not cover 200°C insulation systems for units greater than 1.2kV, they will use 180°C insulation systems).

      Small control and potted transformers have insulation systems well below 220°C because of the resin used. As a result, transformer manufactures generally rate transformers using copper conductors 30kVA and below as having a 200°C insulation system.

      Manufacturers compensate for this by building the transformers to run cooler at full load and as a result have a lower 130°C temperature rise and can operate in a 40°C environment.

      Because both units can be operated in a 40°C ambient, we say copper transformers 30kVA and below with a 130°C temperature rise are equivalent to larger units with a 150°C temperature rise.

  • What is the difference between impregnation and encapsulation

      Impregnation is the complete penetration of the process materials (generically called varnish or epoxy) into the windings of the transformers coils. This is often accomplished by using a Vacuum Pressure Impregnation (VPI) cycle.

      Encapsulation is the complete encasement by the process materials (generically called varnish or epoxy) onto the windings of the transformers coils. In simple terms, the process materials have a thicker, more complete coating than an impregnated transformer. Encapsulation can be done by performing two or more VPI cycles or using a higher viscosity process material. Sometimes “potted” will be used for “encapsulated”.  A Cast Coil transformer would be considered to be both impregnated and encapsulated.

      There is not an industry standard for encapsulation. A winding can be encapsulated, it can be impregnated or it can be encapsulated and impregnated.

  • What is a Dielectric System in a transformer
  • What is Dielectric Material in a transformer?
  • What do I need to specify a neutral grounding transformer

      Neutral grounding transformers are not sized by kVA. To properly specify a grounding transformer, the following parameters must be known:

      1. System Voltage & System BIL
      2. Continuous Neutral Current
      3. Fault Current & Duration
      4. Impedance
      5. Connection Type: Zig-Zag Autotransformer or Wye-Delta
      6. Enclosure Type
      7. Ambient Temperature
      8. Other Environmental Concerns

  • What are the Advantages and Disadvantages to Using a Fan-Cooled Transformer?


      • Smaller size; fans may add some height but may reduce width and depth
      • Lower costs for larger units (generally above 1000kVA) to add fans instead of conductor and core
      • Potentially better low-load efficiencies



      • Increased complexity and maintenance
      • Increased cost as fan packages may cost more than just adding material in smaller units
      • Additional energy losses and noise when fan motors are operated in higher loads

  • What are Important Specifications for a Marine Duty Transformer?

      Marine Duty is often left to the manufacturer to define.  HPS provides American Bureau of Shipping (ABS) approved transformers which include:

      • Copper or aluminum windings
      • Dry-type convection-cooled
      • Standard NEMA taps
      • 150ºC, 115ºC or 80ºC temperature rise available
      • 220ºC, 200ºC or 180ºC insulation system available
      • VPI Impregnation for salt environments
      • Fungus resistant
      • Braced for marine applications
      • Enclosures:  NEMA 2 or NEMA 3R (per ABS requirements)
      • Stainless steel hardware (on units over 1000kVA)
      • UV protection for outdoor enclosures
      • UL50 frames, channels, etc.
      • CSA, UL and ABS approval (Lloyd’s Register and DNV approval available upon request)

  • What are the types of shielded isolation transformers?

      Shielded Isolation Transformers with Single Electrostatic Shield

      This is the simplest type of shielded transformer with one grounded shield extending from top to bottom between the primary and secondary windings. This will typically supply 60dB of common mode noise attenuation from 100Hz through 1MHz. Up to 100dB of TMNA and 40dB at 1000kHz of CMNA can be obtained with effective close coupling and low capacitance.


      Shielded Isolation Transformer with Double Electrostatic Shields

      This has two grounded shields extending top to bottom between the primary and secondary windings and between the secondary windings and the core. This will typically supply 60-80dB of common mode noise attenuation from 100Hz through 1MHz.


      Shielded Isolation Transformer with Triple Electrostatic Shields

      This has three grounded shields extending top to bottom between the primary and secondary windings and between the secondary windings and core and covering the outer winding. Little benefit is gained by having the third shield. This will typically supply 65-80dB of common mode noise attenuation from 100Hz through 1MHz.


      Note that recent testing may indicate that electrostatic shields have little to no benefit in typical applications where the secondary is grounded.

  • What are some of the solutions to Dirty Power?

      The solutions are as wide ranging as the problems. So are the prices. This Table summarizes some solutions and their price ranges.

      Table for dirty power

      Unfiltered Surge Fuses are very inexpensive, and may provide damage protection from lightning strikes or other surges, but they do not filter out adverse noise.

      Filtered Surge Suppressors are inexpensive solutions to noise suppression and surge protection. The better units inhibit surges above 5000 volts, 200 amps. They should also provide noise filtration of 10dB or more to cover average power disturbances.

      Computer Regulators or Line Voltage Conditioners protect equipment from both noise and voltage fluctuations. They are an inexpensive solution, available in both portable and hardwired models. They provide ideal protection in high noise areas where voltage fluctuations exceed the regulating range of the computers power supply.

      Super Isolation Transformers provide inexpensive protection against frequency variation or noise related disturbances. This is adequate where voltage fluctuations are not a serious problem. Most high-end computers have built-in voltage regulation, but still require protection from line noise.

      U.P.S. Systems are in effect self-contained power centers. They provide backup power for a period of time when utility power is interrupted. Most U.P.S. systems also provide noise filtration and surge suppression.

  • What is the maximum surface temperature of a transformers enclosure

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 65C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 50C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum enclosure temperatures.

  • How are HPS transformers designed to shield against voltage transients?

      Electrostatically shielded transformers may help minimize or limit the effects of voltage transients. Common Mode noise is measured from line to ground and is usually the most troublesome. Transverse Mode noise is measured from line to line. Attenuation is the difference of an incoming transient on the primary of the transformer to the secondary side.

      An electrostatic or Faraday shield is simply a thin piece of grounded non ferrous metal (generally copper foil) placed between the primary and secondary windings of a transformer. The shield extends from the top to the bottom of the windings. Some manufactures use shields that don’t extend the full length of the coil face. While less expensive, they will not offer as much protection as a full shield.

      There is no national standard that gives test methods for measuring CMNA and TMNA. Hence, in the industry, various companies have different claims that they have succeeded in getting into customer or consultant specifications. A lot of the confusion for shielded transformers results from differing claims made by various manufacturers and experts. Some recent reports indicate that and electrostatic shield may have little to no benefit where the secondary is grounded which is in most applications. The difference in the claims results from many variables:

      1. Standard single shielded distribution and drive isolation transformers may theoretically provide typical values of CMNA =60 dB and TMNA = 10 dB.
      2. A single shielded transformer with a low capacitive coupling of less than 30 pF may theoretically provide typical values of CMNA = 100 dB and TMNA = 40 dB.

      A manufacturer should be willing to share their testing procedures and test circuit to verify their

      Note attenuation ratings vary by frequency. As the frequency increases, dB ratings go down. The ratings given above may be best case for a wide range of frequencies from 100Hz to 1MHz. Actual attenuation might be significantly higher at the lower frequencies. Some manufactures may claim higher dB’s attenuation by using much lower frequency ranges.

      There may be a large difference between calculated dB and actual dB due to real-life inconsistencies in material and manufacturing. Manufacturers should have actual test data to back up their attenuation claims.

  • What salt spray rating test have Hammond’s painted enclosures passed?
  • What is a Low Voltage General Purpose Transformer?

      HPS’s low voltage general-purpose transformers provide a safe, long lasting, highly reliable power source. They are designed for general lighting and other low voltage applications. They are UL listed and CSA certified.

  • Can transformers be operated above a 1000m/3300′ altitude?

      There are two main considerations for operating transformers at altitudes above 1000m/3300′. Current standards state designs must be valid to these heights. Above this height, the density of air no longer works as effectively to remove heat. As a result the functional kVA of the transformer must be reduced at higher altitudes, typically about .3% for every 100m/330′. The second issue is the dielectric constant of air is reduced at higher altitudes. Dry type transformers use air gaps as an important component of the electrical insulation properties. At higher altitudes, this lower insulation values, typically in medium voltage BIL levels. Ideally, if transformers will be installed above 1000m, inform the manufacturer and the design can be adjusted to meet all requirements at the higher altitudes.

  • Can I increase the kVA rating of an existing transformer?

      The most common way to increase the available kVA rating of an existing transformer is to add additional fan cooling. This typically requires modifications including raising the transformers core, adding fans and fan brackets, a motor power supply and controls to start fan cooling when the transformer’s components reach a preset temperature. Fans should never be added without contacting and following specific instructions from the manufacturer. Low temperature rise transformers (115C and 80C rise with 220C insulation) can maintain higher loads in lower ambient transformers. Always follow proper ventilation and clearance instructions.

  • Do dry-type, potted or cast resin transformers contain PCB’s?
  • How do you choose the correct, most cost-effective Clean Power Solution?

      Not everyone has the same power problem. Finding the most cost-effective solution requires some analysis of your equipment, the power system and the available solutions in the market. The table below lists causes and effects of many common power problems. You or your electrician can determine the most likely cause of power problems based on knowledge of your location, the kinds of equipment you operate in that location, and the kind of power distribution system in your building.

      The following table lists the types of Clean Power products available from HPS to solve your power problems.

      Table for clean power


      HPS offers the following products for clean power solutions:


  • How do you calculate the Total Losses in a transformer?

      It is the transformer electrical losses, which include no-load losses (core losses) and load losses (winding losses). There is not a specific formula to calculate total losses since they vary by size and manufacturer. Manufacturers will often provide charts that show losses at one or more load points.

      HPS does offer an efficiency calculator for distribution transformers.

      HPS Efficiency Calculator

  • What is the ambient temperature specs for a standard transformer?

      A standard transformer with 220°C insulation and a 150°C temperature rise, will be rated to run full load in an average 30°C ambient environment over 24 hours with a maximum 40°C ambient temperature.

      Other magnetics may have lower or higher ambient ratings depending on the design and application.

  • What Seismic ratings Does HPS Use?

      HPS units meet Risk Category IV (Ip=1.5) for Sds=2.0 per ASCE 7-16 (and equivalent factors in NBCC 2015) for ground-level installations only (z/h=0) for all locations in North America.

      HPS units can be designed to meet California OSHPD (Office of Statewide health Planning and Development) requirements. Please see the California OSHPD site for a listing of HPS transformers which have been tested and certified.

  • What is the maximum surface temperature of a transformers terminals wire leads or connections points

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 50C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 35C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum connection point temperatures.

  • benefit of vacuum pressure impregnation

      All HPS transformers and reactors are vacuum impregnated with a “Polyester Resin” and oven cured which seals the surface. Impregnating the entire unit provides a strong mechanical bond and offers protection from environmental conditions.

      Using a vacuum during this process eliminate moisture and provides deeper penetration of insulation into transformer cavities.

  • Can You Use a Control Transformer Connected in Reverse

      Theoretically, yes. However, the output voltage will be less than its rating, due to the voltage compensation factor of the windings. Care must be taken in sizing fuses due to higher inrush when back-feeding. Back-feeding may also violate the 2014 NEC Code unless it is specifically allowed on on the nameplate.

  • What is a Control Transformer?

      A control transformer is an isolation transformer that provides good voltage regulation, and is also designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as “inrush current”). Control transformers are also known as Machine Tool Transformers, Industrial Control Transformers or Control Power Transformers.

  • What Type of Fuses are Recommended for HPS Control Transformers?
  • What is a “Dry Type” transformer?
  • What are Drive Isolation Transformers and where are they used?

      Drive Isolation Transformers (DIT) are designed to supply power to AC and DC variable speed drives. The harmonics created by SCR type drives requires careful designing to match the rated hp of each drive system. The duty cycle included is approximately one start every 2 hours. The windings are designed for an over-current of 150% for 60 seconds, or 200% for 30 seconds.

      DIT’s are covered by NRCan efficiency regulations in Canada but are exempt from efficiency regulations in the U.S.A.

  • Does HPS have NRTL certification?
  • Charging reactor – definition
  • What is a Cast Coil transformer?
  • Capacitor switching reactor – definition
  • What does the term BIL mean?
  • What is an Applied Voltage test?
  • What does ANSI stand for?
  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What is an Ampere?
  • What is Ambient Noise Level?
  • What is an air terminal chamber (ATC) or line terminal compartment?

      This is an air filled terminal compartment, typically 12″-24″ wide that is bolted to one or both sides of a substation transformer. This typically contains either the primary or secondary connections with a steel barrier separating it from the larger chamber containing the actual transformer core and coil. The ATC may also contain additional connections for loop feeds and/or lightning arresters.

  • Air core motor starting reactor – definition
  • Will current TP1 Distribution efficiency units be available into Q4/2015 and Q1/2016?

      The supply will be hard to predict.  Many manufacturers will stop production of these units well before January 1st 2016.

      • Supply channel may be hesitant to stock the current TP1 units if specifications are largely updated to support the new regulations.
      • Manufacturers will establish cut-offs for stock replenishment and custom orders 2-5 months before January 1st, 2016.

  • What is a “Dual Winding”?
  • What are dust filters?

      Dust filters are placed over ventilation openings to mitigate dust accumulation within the transformer’s enclosure. Filters are typically made to be removable and washable. Care must be taken for regular maintenance since accumulating dust will limit airflow and reduce air cooling. For this reason, dust filters are typically avoided through using non-ventilated designs or moving the transformer’s location. Because of reduced airflow, transformers cannot be retrofitted with dust filters without derating or using fan forced venting.

  • What defines the audible noise levels in a dry type transformer?

      NEMA ST-20 (2014) defines the noise level in transformers up to 1.2 kV and up to 1000 kVA.

      NEMA TR-1 defines the sound level for medium voltage transformers above 1.2 kV class up to 7500 kVA.

  • What makes up a Coil?
  • What does Eddy Current mean?

      Eddy currents (also called Foucault’s currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday’s law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field.

      Click here for more information

  • What does Dry-Type Self-Cooled Transformer Class AA mean?
  • What does Dry-Type Self-Cooled Future-Forced-Air-Cooled Transformer Class AA FFA mean?

      Per of NEMA ST-20, a dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and which contains the provision for the addition of forced-air-cooling equipment at a later date.

  • What does Dry-Type Self-Cooled Force Air-Cooled Transformer Class AA FA mean?

      A dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and a forced-air cooled rating with cooling obtained by the forced circulation of air. This is sometimes referred to as fan-cooled. Fan cooling can increase a transformer’s kVA rating by 25% to 50% depending on the type and size of the transformer.

  • What does Dry-Type Forced-Air-Cooled Transformer Class AFA mean?
  • What does DIT stand for
  • What does Combustible Materials mean?
  • What does Ampacity mean?

      Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

  • What does a One-Line Diagram or Single-Line Diagram mean

      It is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols.

      Click here for more information

  • What are Distributed Static VAR and Synchronous Compensators (D-SVCs/D-STATCOMs)?
  • What does the term “Encapsulated” describe?

      A transformer with its coils either encased or cast in an epoxy resin or other encapsulating materials. This includes cast coil and potted transformers. Manufacturers may also consider transformers which go through multiple dip and bake cycles creating a thicker coating as encapsulated.

  • What is Temperature Rise?
  • What are solar transformers?

      Solar transformers covers a broad selection of transformers which are designed for the unique requirements of a solar power system. These transformers can include solar inverter transformers, grid tie transformers and zig-zag autotransformers or isolation transformers specially designed to be used in grounding banks for utility hook-ups. Transformers used to directly deliver power to utilities must often be capable of bidirectional current flow.

  • What are Low Temperature Rise Transformers?

      All transformers have operating losses, and heat is the product of these losses. Hammond low temperature rise transformers are designed with reduced 115°C or 80°C full load operating temperature rises. These units decrease total operating losses by 20% and 35% respectively, compared with the standard 150°C rise operating system. Hammond low temperature rise transformers provide greater efficiency under normal operating conditions, and overload capability without harm to their service life or reliability.

  • What is a Type 2-S enclosure?
  • What is a Type 12 enclosure?

      This is a non-ventilated indoor enclosure designed primarily for providing a degree of protection against circulating dust, falling dirt, and dripping non-corrosive liquids. This enclosure is both oil and rust resistant suitable for applications such as oil refineries where oil or other chemical liquids may be prevalent. (Note: not watertight)

  • What is a Type 1-N enclosure?

      This is a general-purpose non-ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.

  • Line reactor – definition
  • Explain the K-Factor rating?

      K factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. Transformers with a rated K factor of 4, 9, 13, 20 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 9 should be specified when 100% of the load is non-linear. For critical applications, K=13 can be considered.

      As the K-factor of the transformer increases, generally the size and cost increase, low load efficiency (<25% load) decreases and impedance decreases. That is the trade-off to be able to handle higher harmonic factors.

      Learn more about HPS Sentinel K

      Learn more about HPS Sentinel H

  • Define K-Factor?

      K-Factor is defined as a ratio between the additional losses created by the harmonics and the eddy losses at the rated 60 Hz. This factor is used to specify the size of the transformer to meet the magnitude of the harmonic load in the circuit. A standard general-purpose transformer does not have the shielding, conductor sizes, core cross-section, or the capacity in the neutral to provide the same service.

  • Iron core filter reactor – definition
  • Iron core motor starting reactor – definition
  • Interphase reactor – definition
  • What is an Induced Voltage test?

      The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation

  • What is an incoming line interrupter switch (electrical disconnect)?

      This is typically a two position, three phase switch designed to disconnect a transformer on the line side. The switch may or may not also have fuses. The switch assembly is typically attached directly to the transformer enclosure and electrically connected through close coupled bussing.

  • What are Impedance Voltage and Load Loss tests?

      The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature.

      Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.

  • What are Harmonics?

      Harmonics, in an electrical system, are currents created by non-linear loads that generate non-sinusoidal (non-linear) current waveforms. These current and voltage wave forms operate on frequencies that are in multiples of the fundamental 60hz frequency. That is, the fundamental frequency is at 60 hertz, the 2nd harmonic is at 120hz frequency (60 x 2), the 3rd at 180 hertz, and so forth. Harmonics are principally the by-product of switch-mode power supply technology where AC is rectified to DC, and back again. In the process, a capacitor is charged in the first half-cycle, and then discharged in the next half-cycle, in supplying current to the load. This cycle is repeated. This action of recharging causes AC current to flow only during a portion of the AC voltage wave, in abrupt pulses. These abrupt pulses distort the fundamental wave shape causing distortion to the various harmonic frequencies.

  • What is Flux Density?
  • What does the abbreviation FCBN stand for?

      “Full Capacity Below Normal.” This designates that a transformer will deliver its rated kVA when connected to a voltage source that is lower than the rated voltage.

  • What does the abbreviation FCAN stand for?

      “Full Capacity Above Normal.” This designates that a transformer will deliver its rated kVA when connected to a voltage source that is higher than the rated voltage.

  • What is a Fan Cooled transformer?
  • Explain Frequency?

      On AC circuits, designates the number of times the polarity alternates from positive to negative and back again . . . such as 60 cycles per second. Measured in Hertz.

  • What is an exciting or excitation current?

      A transformer exciting current is the current or amperes required to energize the core. Even with zero load, a transformer will draw a small amount of current due to internal loss. The excitation current is made up of two components. The real component in the form of losses that are commonly referred to as no-load losses. The second form is reactive power measured in KVAR.

  • What is “Exciting Current (No-Load Current)”?

      Current which flows in any winding used to excite the transformer when all other windings are open-circuited. It is usually expressed in percent of the rated current of a winding in which it is measured.

      A transformer will always consume a small amount of current to energize the windings even if there is no load. This energizing current can create a large inrush several times the transformer’s rated current for a few cycles during initial energization before it reduces to a much lower steady draw.

  • What are Energy Efficient (TP1) Transformers?

      TP1 was often used to generically refer to the minimim efficiency levels that were originally require in Canada in 2005 and in the U.S. in 2006 for low voltage ventilated transformers. Specifically, the TP1 specification covers energy efficiency in transformers based on the NEMA Standards Publication, TP-1-1996, “Guide for Determining Energy Efficiency for Distribution Transformers”. The TP1’s recomendations did consider the total owning cost of ownership unique for industrial or commercial installations. TP1 is measured per TP2 and displayed on the nameplate per TP3.

      The TP1 specifcations have now been replaced by higher efficiency specifications:

      • U.S.A.: DOE 2016 efficiency levels (January 1st, 2016)
      • Canada: NRCan 2019 efficiency levels (May 1st, 2019)

  • What are Encapsulated Transformers and where are they used?

      Encapsulated units are covered in a thicker coating of insulation than typical. Often the coils are completely encased in epoxy or an epoxy and aggregate mixture. Sometimes they are referred to as “potted” or “cast coil”.

      The encapsulated design is especially suited for installations in harsh environments where dust, lint, moisture and corrosive contaminants are present. Typical applications include: pulp and paper plants; steel mills; food processing plants; breweries; mines; marine and shipboard installations.

  • What is an “Electrostatic Shield”?

      Electrostatically shielded (Faraday Shield) transformers provides a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts some noise and transients to the ground path rather than passing them through to the secondary. Transformers having a K-Rating are required to have an electrostatic shield.

      Electrostatically shielded transformers often preferred for electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to ‘noise’. Recent testing of electrostatically shielded transformers has questioned their perceived effectiveness where the transformer’s secondary is grounded which would cover most applications.

  • What is electrical noise?

      Noise is a very broad term that can be applied to a number of AC power line disturbances. Lightening surges or any other sudden changes in load, such as switching motor loads or power factor correcting capacitors can produce voltage spikes and ringing. Phase controlled rectifier loads and arcing devices produce continuous noise unless adequately filtered. Noise sources are either common mode, which appears between both sides of a power line and ground or of transverse mode, which appears from line to line. HPS Clean Power products, such as our Computer Regulators remove these noise sources.

  • What is the “Efficiency” of a transformer?
  • Shunt reactor – definition
  • What is a Type 3R-E enclosure?

      Although similar to the Type 3R enclosure, a Type 3RE also provides added protection against snow and particulate materials. It is more suitable for outdoor installations where snow or other particulate materials are present. In certain conditions, where sustained high wind conditions exist (typically > 75-80 km/hour), external measures to reduce the flow of snow and/or particulate materials may be required. Please consult HPS for assistance with applications where sustained high wind conditions exist.

  • What is a Type 1 enclosure?

      This is a general-purpose ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.

  • What is a Ventilated enclosure?
  • What is a solar grounding bank?
  • Air core filter reactor – definition
  • What is an Air Cooled transformer?

      It is a transformer that uses “air” as the cooling medium. This term is abbreviated with the ANSI designation AA, indicating open, natural draft ventilated construction.

  • What is a Polarity and Phase-Relation test for?

      Polarity and phase-relation tests are made to determine angular displacement and relative phase sequence to facilitate connections in a transformer. Determining polarity is also essential when paralleling or banking two or more transformers.

  • What is a pad mounted transformer?

      A pad mounted transformer typically refers to a specific style of enclosure for larger transformers that is capable of being installed in areas accessible to the general public. The transformer will typically have features including tamper resistant construction, tamper proof bolts and screws, lockable compartments with hinged doors, bottom entry of primary and secondary cabling and baffled ventilation openings if applicable. These should not be confused with the general statement that electrical transformers are often installed on a concrete pad.

  • What is an Open Delta transformer?

      An open delta transformer is a three phase transformer that only has two primary and secondary windings, with one side of the delta phase diagram “open”. Open delta transformers are rare and are typically only used for small loads where cost is important. More common is critical loads being wired with three single phase transformers in a banked configuration. Should one of these transformers fail, the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. This allows a circuit to remain powered during a failure of a transformer, albeit at a lower overall load factor.

      Open Delta

  • What are Non-Linear Loads?

      Today, non-linear loads make up a large percentage of all electrical demand. Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. Harmonic currents and voltages produced by single phase, non-linear loads which are connected phase-to-neutral in a three phase four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples – 3rd, 9th, 15th, 21st, etc., phasors displaced by zero degrees). These third order, zero sequence harmonic currents, do not cancel but add up arithmetically on the neutral bus, creating a primary source of excessive neutral current.

  • What is Nomex®?
  • Neutral grounding reactors – definition
  • What is NEMA TP2?

      NEMA TP2 defines how energy efficiency is measured. Typically, it uses a sinusoidal wave with no harmonics at unity (1.0) power factor at 35% load for 600 volt class units and 50% load for medium voltage units.

      This regulation has been replaced by similar test standards described in DOE 2016 and NRCan 2019 regulations.

  • What is NEMA TP3?
  • What is NEMA ST 20?
  • What are Medium Voltage Transformers and where are they used?

      Medium voltage transformers that have one or more windings above 1.2 k-volts. Ventilated medium voltage transformers up to 2500 kVA are covered by DOE 2016 efficiency regulation in the U.S.A. and up to 5000 kVA by NRCan 2019 efficiency regulations in Canada.

      They are primarily for use in stepping down medium voltage power to a lower operating voltage for commercial, institutional or industrial applications. However, medium voltage transformers may also be used to step-up voltage.

  • What are Voltage Fluctuations?

      Under voltages and over voltages are caused by faults on power lines, and the subsequent actions of fault clearing devices. High current loads which temporarily exceed the FLA of a transformer, such as across the line motor starts or contactor soil energiztion, can cause voltage drops of 20% or more. This can result in expensive and time consuming errors, loss of information, downtime, recovery and rerun costs and possible equipment damage. Voltage fluctuation on the input of a transformer or autotransformer will be passed through to the secondary.

      These high inrush currents need to either be mitigated (e.g. reduced voltage starters) or have the transformer sized large enough to handle. Control transformers are recommended for loads with magnetic coils such as contactors.

      Snubber circuits should be considered in medium voltage applications which also use vacuum breakers.

  • What does is a reconnectable transformer?

      A reconnectable transformer typically refers to a transformer with two primary connections. These may be used for mobile equipment, test transformers or to accommodate future voltage changes and upgrades in a facility without having to change the transformer. Depending on the difference in voltages and application reconnectable transformers may be exempt from current efficiency regulations.

  • Are obsolete TP1/C802.2 transformers still available in North America?

      TP1 transformers built and in the U.S.A. before January 1st, 2016 can still be sold and installed in the U.S.A. At the time of this writing, stocks of new TP1 transformers are no longer readily available but may still be found in the used market.

      In Canada, NRCan 2019 will ban manufacturers from selling transformers after May 1st, 2019 which don’t meet the new 2019 efficiency regulations. Older C802.2 units can still be sold by distributors and installed after the regulations.

  • dV/dt reactor – definition
  • What is Dirty Power?

      Dirty power is caused by a number of things. Simply put, dirty power is what causes your radio or telephone to ‘crackle’ during an electrical storm; or what causes ‘snow’ on your TV when someone is using a power tool, sewing machine or other appliances in your house. This dirty power, or electrical noise, is a nuisance when it appears on your radio, TV or telephone. When it gets into your computer, it can cause serious errors; improper readouts, printing problems, or even damage your computers circuit.

  • What are Dielectric tests?

      The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the voltages associated with specified insulation levels.

  • DC smoothing reactor – definition
  • What is a Current Transformer?
  • Current limiting reactor – definition
  • What is the Core of a transformer?
  • What is Core Loss?

      Losses in watts caused by magnetization of the core and its resistance to magnetic flux when excited or energized at rated voltage and frequency. Also referred to as excitation loss or no-load loss.

  • What is the term Continuous Rating?
  • What is a Compensated Transformer?

      It is a transformer with a turn’s ratio is adjusted to compensate for the natural voltage drop caused by the coils resistance from the wire. Typically large compensation factors at only found in smaller transformers below 5 kVA.

      Winding compensation can cause lower output voltages if a transformer is backfed.

  • What is Coil Hot-Spot Temperature?

      It is the absolute maximum temperature present in the transformer. This number is equal to the sum of the ambient temperature, temperature rise and a variable.

      T Hot Spot = T ambient + T rise + (10-20) °C.

  • What is the International Building Code

      The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted for use as a base code standard by most jurisdictions in the United States. The IBC addresses both health and safety concerns for buildings based upon prescriptive and performance related requirements. The IBC is fully compatible with all other published ICC codes. The code provisions are intended to protect public health and safety while avoiding both unnecessary costs and preferential treatment of specific materials or methods of construction.

  • Clean Power Products
  • What is NFPA (National Fire Protection Association)?

      The National Fire Protection Association (NFPA) is a United States trade association, albeit with some international members, that creates and maintains private, copyrighted standards and codes for usage and adoption by local governments.

      NFPA 70E covers the Standard for Electrical Safety in the Workplace.

  • What is Subtractive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

  • What is meant by “Class” in insulation?

      The insulation rating is the maximum allowable winding (hot spot) temperature of a transformer operating at an ambient temperature of 40°C. Insulation systems are classified by the temperature rating. The following table summarizes the different insulation systems available.

      Insulation Rating

      Insulation Class

      Average Winding Temperature Rise

      Hot Spot Temperature Rise

      Maximum Winding Temperature

      Class 105 A 55 degree C 65 degree C 105 degree C
      Class 150 or 130 B 80 degree C 110 degree C 150 degree C
      Class 180 F 115 degree C 145 degree C 180 degree C
      Class 200 N 130 degree C 160 degree C 200 degree C
      Class 220 H 150 degree C 180 degree C 220 degree C
      Note: the maximum acceptable temperature rise based on an average ambient of 30 degree C during any 24 hour period and a maximum ambient of 40 degree C at any time.

  • What is IEEE 1584-2018?

      IEEE 1584-2018 provides mathematical models for designers and facility operators to apply in determining the arc-flash hazard distance and the incident energy to which workers could be exposed during their work on or near electrical equipment.

      It generally indicates that systems with an available short circuit current of 2000 Amps or higher should be assessed for arc-flash potential. A rule of thumb would indicate that most systems fed by a 45 kVA or larger transformer will need to be assessed if impedance (%Z) of 45 kVA is less than 6%, 30 kVA if %Z is less than 4% or 15 kVA if %Z is less than 2%.

  • What is IR drop

      IR drops relates to Ohm’s Law: Voltage = Current x Resistance. Transformers utilize conductors which have resistance and can cause a slight voltage drop. Typically manufacturers compensate the winding ratio to mitigate any effects of IR voltage drop in the secondary voltage.

  • What is Additive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

  • What is an audio transformer

      Audio transformers are typically electronic transformers used in microphone and amplifier applications. Audio transformers are typically rated by power (VA), frequency response and distortion. They are often essential in impedance (power) matching functions.

      HPS does not manufacturer audio transformers and is often confused with Hammond Manufacturing at

  • What is a Non-ventilated enclosure

      A non-ventilated enclosure is constructed to restrict unintentional circulation of external air through the enclosure. Common industry descriptions would include Type 12, 4 and 4X in North America.

  • What is a vault room

      A vault room is a reinforced concrete structure used for the purpose of housing liquid cooled transformers, switchgear and other electrical distribution equipment. Requirements for vault rooms for liquid filled transformers are defined by many specifications including NEC 450.26. Vault requirements are outlined in NEC 450, Part III, beginning with 450.4. Typical requirements might include:

      • Ventilation with outside air via a dedicated ductwork
      • Three hour fire-resistant construction including a minimum 4″ concrete floor
      • Oil containment capable of containing the entire liquid contents of the largest transformer
      • Some exceptions will allow lower construction standards such as the use of sprinklers, carbon dioxide or halon systems and/or the use of “less flammable” liquids.

  • What is a Cast Resin transformer
  • What is a Epoxy Cast transformer?
  • What does Two-Phase Circuit mean?

      Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. This has been entirely replaced by three-phase power in a modern power grid.

  • What does Readily Accessible mean?

      Per 1.2.2 of NEMA ST-20, Capable of being reached quickly for operation, renewal, or inspections, without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc.

  • What does MSR stand for?
  • What does MSAT stand for?
  • What does LV stand for
  • What is a Type 2 enclosure?
  • What does 50/60 Hertz mean?

      Transformers that are designed to specifically run at 60 Hz can’t be run at 50 Hz or in some cases only with significant derating. Magnetic flux is proportional to frequency so a 50 Hz transformer has a core 20% larger to handle 20% more magnetic flux than a 60 Hz unit. A 50 Hz transformer will simply run cooler at 60 Hz given the proper voltage is applied. Transformers cannot change frequency, the primary frequency equals the secondary frequency.

  • VAR compensator reactor – definition