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Can a non-inverter duty rated motor be used if load reactor or dV/dT filter is installed?

Due to wide variations in non-inverter duty rated motors, it can’t be stated that a line reactor or dV/dT filter will fully protect the motor. It’s suggested that as a minimum, a dV/dT filter should be used.

A full sine save filter may be needed to fully protect non-inverter duty rated motors.

  • 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 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.

  • 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.

  • Are there voltage drop concerns when using a load reactor or dV/dT filter for long lead lengths?
  • Can a non-inverter duty rated motor be used if load reactor or dV/dT filter is installed?
  • If an output load reactor or dV/dT filter is installed on a VFD, should VFD Cable be installed from the drive to the motor?
  • What does DIT stand for?
  • 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 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 EMF (Electric and Magnetic Fields)?

      Electric and Magnetic Fields are produced by the distribution of electricity through current-carrying devices.  The extremely low frequency EMF or power frequencies fields 50 to 60 Hz are produced by the generations, transmissions and use electrical energy.

  • What Effects Does EMF (Electric and Magnetic Fields) Have on Equipment?

      Computers, servers, monitors, medical equipment and other complex electronic equipment can all be affected by the presence of EMF fields. These fields have been known to distort monitors, cause faulty readings, computer errors and even equipment lock-ups.

  • What is the difference between a Variable Frequency Drive’s (VFD) fundamental frequency and the VFD’s carrier (switching) frequency?

      A VFD’s fundamental frequency is the frequency of the output current, typically >0 to 400HZ for most VFD’s. This frequency determines the speed of the motor and is simulated by the drive’s higher carrier frequency output.

      The VFD’s Carrier (Switching) frequency is the frequency of the PWM (Pulse Width Modulation) pulses, i.e. the frequency at which the VFD’s output transistors; usually IGBTs (Insulated Gate Bipolar Transistors), are switching. This typically ranges from 2 to 20 kHz. This high frequency PWM current output simulates a lower frequency sine wave and provides the drive’s variable fundamental frequency output.

  • 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.

  • 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 Dielectric Material in a transformer?

      The turn-to-turn and layer-to-layer insulating materials of the transformer winding can be referred to as “Dielectric Material”. This typically consists of fiberglass, mica, aramid fiber paper, NOEMX, Mylar, etc.

  • 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 salt spray rating test have Hammond’s painted enclosures passed?
  • 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.

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  • 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.

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  • 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?
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  • 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.

  • What is U.L. 1562?

      U.L. 1562 covers medium voltage dry-type transformers:

      1.1 These requirements cover single-phase or three-phase, dry-type, distribution transformers, including solid cast and resin encapsulated transformers. The transformers are provided with either ventilated or non-ventilated enclosures and are rated for a primary or secondary voltage from 601 to 35000 V.

      1.2 These transformers are intended for installation in accordance with the National Electrical Code, ANSI/NFPA 70.

      1.3 These requirements do not cover the following transformers:

      1. Instrument transformers
      2. Step-voltage and induction voltage regulators
      3. Current regulators
      4. Arc furnace transformers
      5. Rectifier transformers
      6. Specialty transformers (such as rectifier, ignition, gas tube sign transformers, and the like)
      7. Mining transformers
      8. Motor-starting reactors and transformers

      1.4 These requirements do not cover transformers under the exclusive control of electrical utilities utilized for communication, metering, generation, control, transformation, transmission, and distribution of electric energy regardless of whether such transformers are located indoors, in buildings and rooms used exclusively by utilities for such purposes; or outdoors on property owned, leased, established rights on private property or on public rights of way (highways, streets, roads, and the like).

  • What is U.L. 1561?

      UL1561 covers 600 Volt Class Transformers:

      1.1 These requirements cover:

      1. General purpose and power transformers of the air-cooled, dry, ventilated, and non-ventilated types to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Construction types include step up, step down, insulating, and autotransformer type transformers as well as air-cooled and dry-type reactors


      1. General purpose and power transformers of the exposed core, air-cooled, dry, and compound-filled types rated more than 10 kVA to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Constructions include step up, step down, insulating, and autotransformer type transformers as well as air-cooled, dry, and compound-filled type reactors.

      1.2 These requirements do not cover ballasts for high intensity discharge (HID) lamps (metal halide, mercury vapor, and sodium types) or fluorescent lamps, exposed core transformers, compound-filled transformers, liquid-filled transformers, voltage regulators, general use or special types of transformers covered in requirements for other electrical equipment, autotransformers forming part of industrial control equipment, motor-starting autotransformers, variable voltage autotransformers, transformers having a nominal primary or secondary rating of more than 600 volts, or overvoltage taps rated greater than 660 volts.

      1.3 These requirements do not cover transformers provided with waveshaping or rectifying circuitry. Waveshaping or rectifying circuits may include components such as diodes and transistors. Components such as capacitors, transient voltage surge suppressors, and surge arresters are not considered to be waveshaping or rectifying devices.

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  • What are NEMA Premium Efficiency Transformers?

      NEMA Premium provides 30% fewer losses than the 2006 DOE 10 CFR Part 431 (commonly called TP1) or Canadian C802.2. This covers low voltage distribution transformers and is measured at 35% load.

      NEMA Premium is largely obsolete as a term and has been replaced by similar efficiencies in the U.S.A’s DOE 2016 and Canada’s NRCan 2019 energy efficieny regulations. NEMA no longer promotes the NEMA Premium specification or specifications higher than the current DOE 2016 and NRCan 2019 levels.

  • What problems can occur if I undersize the short circuit protection on a transformer?

      Nuisance tripping is a concern when short circuit protection is undersized from the National Electric Code recommendations in NEC 450.3.  All transformers experience an inrush current during any energization. The inrush current results from the transformer establishing the initial electromagnetic field and is not linked to load. If short circuit protection is undersized, there is a chance for nuisance tripping of fuses and circuit breakers anytime the transformer is energized.

  • Do any performance issues arise during high ambient temperatures?

      Temperatures which exceed the rated ambient temperatures for which the insulation system is designed can cause insulation damage and premature failure. This can often occur in hotter
      environments or in rooms which have inadequate ventilation. Care should be taken in installing stacked transformers because the top transformer may use air that has been heated by the lower unit. Damage from high ambient temperatures often does not cause an immediate failure but can cause damage that results in a failure weeks, months or years later.

      High ambient temperatures can be mitigated several ways:

      • Order a transformer designed with a lower temperature rise.
      • Use fan cooling, this is typically an economical solution when a unit exceeds 500-1500kVA.
      • Place the transformer in a temperature controlled location.
      • Properly ventilate the location that the transformer is located in.

      Never try to use cooling fans directly on a transformer or blow across a transformer’s windings.

      Manufacturers use special fans, specific locations, and cooling patterns to cool transformers. Improper placement of airflow could cause disruption of the convection airflow and cause the transformer to overheat.

  • Do any performance issues arise during low ambient temperatures?

      Generally low ambient temperatures do not affect an energized transformer. No-load losses on an energized transformer typically generate enough heat to operate effectively in temperatures to -20°C or lower.

      The main issue with lower temperatures is when the unit is not energized. Extremely low temperatures or if the transformer heats up too quickly may cause welds and insulation to become brittle and crack, especially if the transformer experiences any mechanical stresses.

      More importantly, low temperatures can cause moisture (dew, frost) to form on the unit. This can be absorbed into the insulation system and not be apparent.

      If ambient goes below -30°C, special designs and cold start procedures may be necessary. Care should be taken to store transformers in dry areas with temperature control. Installation manuals typically suggest that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present.

      Damage and injury can result from energizing a transformer which has had its insulation system compromised by moisture.

  • How does a UPS transformer’s efficiency affect the overall system efficiency?

      ASHRAE 90.4 Section notes that UPS transformer’s efficiencies at given loads must be included in the total losses for evaluation. Active single feed systems should be evaluated at 100% and 50% ITE load while active dual feed systems should be evaluated at 50% and 25% loads.

  • Can a damaged coil be repaired, refurbished or replaced?

      Low and medium voltage coils which have been damaged can be repaired, refurbished or replaced. Repairing involves fixing the issue without removing the coil from the core. This may be an option for transformers which have sustained much physical damage or if an issue such as lowering insulation levels is diagnosed before actual failure. This may also be possible for transformers which are dirty or have been submerged. In some cases the the coil can be removed and refurbished. This would typically be done for larger, often medium voltage transformers which have sustained damage to the outer, primary winding. The last option is to replace the coil entirely which is often done if the coil is severely damage, the inner winding is damaged or the transformer can’t be sufficiently cleaned.

      While coils can be repaired, refurbished and replaced, this must be compared to the cost of providing a new unit. Repairs must factor in additional transportation and testing costs, higher disassembly and assembly costs, core damage and replacement and the benefits of using a new transformer, often with modern higher efficiencies. Repair is often only practical for large custom transformers.

  • Can I increase the efficiency of an existing transformer?

      There are two main loss area of a transformer, load losses in the coils and no-load losses in the core. Coil losses can only be lowered by completely replacing the coils and using a larger conductor which typically would not be compatible with existing core. In order to reduce core losses, better grades of steel and/or more advanced core design would require the complete replacement of the core while still reusing the coils. While possible, it’d typically be uneconomical to do except for large, custom transformers. The last option for increasing transformer efficiency is to provide line power meeting IEEE 519 THDi and/or removing harmonics on the load side before they reach the transformer.

      In most cases the most economical method to increase a transformer’s efficiency is to replace older transformers with modern high efficiency models meeting current North American efficiency requirements.

  • How do you properly size a distribution transformer?

      Distribution transformers need to take several items into consideration when sizing including:

      • Maximum Load
      • Potential future load growth (typical is 25%)
      • Load Inrush and voltage regulation
      • Harmonics and Power Factor
      • Ambient Temperature
      • Additional Service Factor

      For reference, NEC Article 210, Branch Circuits, and NEC Article 230, Services is used to select panelboards and the size of branch circuits. Typically a transformer must be sized to support the load requirements of the switchgear, panelboards and branch circuits. For drive isolation transformers, it is suggested to take sizing charts provided by manufacturers into consideration due to derating for harmonics. In addition to sizing a transformer, the general types including general purpose, K-Rated, Harmonic Mitigating and Drive Isolation also need to be chosen.

      Distribution transformers are often sized from loads based on NEC Article 220. NEC Article 220.87 does allow transformers to be sized based on peak-load data over a 1-year period. NEC also allows loads to be sized using metered data over 30 days if the additional maximum anticipated heating and/or cooling load is also factored in. This often allows a transformer to be sized lower than the base calculations from NEC Article 220. Peak efficiency for 600V class distribution transformers is typically at 35%. Peak efficiency for medium voltage transformers is typically at 50% load.

      Additional capacity for future loads can be obtained by A) specifying a lower temperature rise (15%-30% for dry type) or B) utilizing fans (25%-50% for dry type).

  • Is ice snow or frost conductive?

      Yes, ice, snow and frost are conductive and should be considered in the same terms as the liquid form of water. If ice, snow or frost come into contact with energized parts of a transformer, a fault and damage can occur. If a fault doesn’t occur initially, the heat from an energized transformer will cause ice, snow and frost to melt which can further increase the chances of a damaging fault occurring.