Federal Pacific K-Factor Transformers

Application
With today’s modern electronic, electrical components and circuitry constantly changing, the demand is forced upon the electrical power industry to produce and supply a clean source of electrical energy.

The Problem
The extensive utilization of solid state power conversion technologies has created new problems for the power industry and power engineer designer. This technology, called Switch Mode Power Systems (SMPS), consists of various types of solid state switching elements. These switching elements are solid state devices such as: SCR’s, DIAC’s, transistors and capacitors. These switching devices are in computers, copy machines, fax machines, tele-communications equipment, solid-state drives, and controls, energy-efficient lighting ballasts, and numerous types of DC-Power Loads. These solid state elements continuously switch on and off producing non-linear or non-sinusoidal wave shapes in the current supplied from the energy source.

While a linear load uses current from the AC source continuously over the sinusoidal cycle, a non-linear load (such as the SMPS) uses current in large pulses from the AC source, which creates harmonic distortion. These non-linear current pulses may exceed the nameplate ampere rating of the power source and may cause transformers to run hotter than expected, even when these transformers are supplying less than 50% of their rated nameplate capacity.

With non-linear loads, overloaded neutrals are also showing up in three-phase panel boards serving single-phase loads. In some cases the neutral conductor carries 180 Hertz currents, rather than 60 Hertz currents. This phenomenon is called triplen harmonics. Triplens are multiples of three, which do not cancel but are additive in the neutral conductor.

What Are Harmonics
As defined by ANSI/IEEE Std. 519-1981

Harmonic components are represented by a periodic wave having a frequency that is an integral multiple of the fundamental frequency. In other words, harmonics are voltages of currents at frequencies that are integer multiples of the fundamental (60Hz) frequency , e.g. 120 Hz, 180 Hz, 240 Hz, 300 Hz, etc. Harmonics are designed by their harmonic number, or multiple of the fundamental frequency. Thus, a harmonic with a frequency of 180 Hz (three times the 60 HX fundamental Frequency) is called the 3rd harmonic.

Harmonics superimpose themselves on the fundamental waveform, distorting it and changing its magnitude. For instance, when a sine wave voltage source is applied to a non-linear load connected from phase-to-neutral on a 3-phase, 4-wire wye circuit, the load itself will draw a current wave made up of the 60-Hz fundamental frequency of the voltage source plus 3rd and higher order odd harmonic (multiples of the 60 Hz fundamental frequency), which are all generated by the non-linear load. It is not uncommon for portions of an industrial power system to have 15 to 25% of Total Harmonic Distortion (THD). THD is calculated as the square root of the sum of the squares of all harmonics, divided by the normal 60 hertz value. This yields a root-mean-square (RMS) value of distortion as a percentage of the fundamental 60 hertz waveform. Therefore, THD is the percent of odd harmonics (3rd, 5th, 7th, …, 25th,…) present in the load which can affect the transformer. This condition is called a “Non-Linear Load” or “Non-Sinusoidal Load”.

For dry type transformers, to determine what amount of harmonic content is present, a “K” factor calculation is made instead of using the THD formula. The total amount of harmonics will determine the percentage of non-linear load, which can be specified with the following examples:
(A) 50% Non-Linear Load (K-4 Rating)
16.7% of the rated current at the 3rd Harmonic
10% of the rated current at the 5th Harmonic
7.1% of the rated current at the 7th Harmonic
5.6% of the rated current at the 9th Harmonic

Beyond the 9th Harmonic the percentages of the fundamental current through the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 0.5.

The FPType FHK-4 series transformer is designed for 100% linear load plus 50% non-linear load which can operated at a total 1 (pu) h K-factor load value of 4.0.

(B) 100% Non-Linear Load (K-13 Rating)
33.3% of the rated current at the 3rd harmonic
20.0% of the rated current at the 5th harmonic
14.3% of the rated current at the 7th harmonic
11.1% of the rated current at the 9th harmonic

Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.0.

The FPType FHK 13 series transformer is designed for 100% linear load plus 100% non-linear load which can operate at a total (1 (pu) h) K-factor load value of 13.0

(C) 125% Non-Linear Load (K-20 Rating)
41.7% of the rated current at the 3rd harmonic
25.0% of the rated current at the 5th harmonic.
17.9% of the rated current at the 7th harmonic
13.9% of the rated current at the 9th harmonic

Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.25.

The FPType FHK20 series transformer is designed for 100% linear load plus 125% non-linear load which can operate at a total (1 (pu) h) K-factor load value of 20.

(D) 150% Non-Linear Load (K-30 Rating)
50.0% of the rated current at the 3rd harmonic
30.0% of the rated current at the 5th harmonic
21.4% of the rated current at the 7th harmonic
16.7% of the rated current at the 9th harmonic

Beyond the 9th Harmonic the percentages of the fundamental current through the 25th Harmonic shall be equal to the reciprocal of the odd harmonic number involved times 1.50.

The FPType FHK30 series transformer is designed for 100% linear load plus 150% non-linear load which can operate at a total (1 (pu) h) K-factor load value of 30.

Note: In these examples the amount of non-linear load specified, the percentage of fundamental current, and the percentages of harmonic factor are arbitrary values; actual values may vary. (Consult manufacturer for your specific application or current values for each harmonic.)

Transformers shall be sized to account for harmonic non-linear loads of 50% minimum (K-4), 100% (K-13), 125% (K-20), 150% (K-30).

The neutral connection shall be sized at 200% of the current rating of the phase connections.

The conductors of the transformer winding shall be sized to handle circulation of 3rd harmonic current and not exceed the rated temperature rise.

Transformers shall be capable of operating within the specified temperature rise while supplying 100% of the 60 Hertz fundamental rated current values plus the following harmonics as calculated by ANSI/IEEE 57.110-1986.

K-Factor Transformer Ratings
The K-Factor rating assigned to a transformer and marked on the transformer case in accordance with the listing of Underwriters Laboratories, is an index of the transformer’s ability to supply harmonic content in its load current while remaining within its operating temperature limits. A specific K-factor rating indicates a transformer can supply its rated KVA load output to a load of specified amount of harmonic content. At present, industry literature and commentary refers to a limited number of K-factor ratings: K-1, K-4, K-9, K-12, K-20, K-30, and K-40. In theory, a transformer could be designed for other K-factor ratings in-between those values, as well as for higher values. The commonly referenced ratings calculated according to ANSI/IEEE C57.110-1986 are as follows:

K-1: This is the rating of any conventional transformer that has been designed to handle only the heating effects of eddy currents and other losses resulting from 60 Hertz, sine-waves current loading on the transformer. Such a unit may or may not be designed to handle the increased heating of harmonics in its load current.

K-4: A transformer with this rating has been designed to supply rated KVA, without overheating, to a load made-up of 100% of the normal 60 Hertz, sine-wave, fundamental current plus: 16% of the fundamental as 3rd harmonic current; 10% of the fundamental as 5th; 7% of the fundamental as 7th; 5.5% of the fundamental as 9th; and smaller percentages through the 25th harmonic. The “4” indicates its ability to accommodate four times the eddy current losses of a K-1 transformer.

K-9: A K-9 transformer can accommodate 165% of the harmonic loading of a K-4 rated transformer.

K-13: AK-13 transformer can accommodate 200% of the harmonic loading of a K-4 rated transformer.

K-20, K-30, K-40: The higher number of each of these K-factor ratings indicates ability to handle successively larger amounts of harmonic load content with overheating.


Triplen Harmonics
Triplen harmonic currents are phase currents which flow from each of the phases into the fourth wire neutral and have frequencies in integer multiples of three times the 60 hertz base frequency (180 Hz, 360Hz, 540Hz, etc). At each of these third multiple triplen frequencies, these triplen phase currents are in phase with each other and when flowing in the neutral as zero sequence currents, are equal to three times their RMS phase values.

In a 3-phase, 4-wire system, single-phase line-to-neutral currents, flow in each phase conductor and return in the common neutral. Since the three 60 hertz currents are separated by 120 degrees, when balanced they cancel each other. The measured resultant current is equal to zero. Theory also states that for even harmonics, starting with the second order, when balanced the even harmonic will cancel in the common neutral. Other odd harmonics add in the common neutral, but their magnitude is considerable less than triplens. The RMS value of the total current is the square root of the RMS value of the individual currents squared.


The UL Approach for Transformers Supplying Non-Sinusoidal Loads
A. A transformer intended for use with loads drawing non-sinusoidal currents shall be marked “Suitable for non-sinusoidal current load with K-factor not to exceed x. (x= 4, 9, 13, 20, 30, 40 or 50)

B. Formulas to determine eddy losses and total losses where the transformer load losses (PLL) are to be determined as follows:

PLL = PDC (1+K(PEC))
Where:
PDC = the total 1 R losses

K= the K-factor rating at the transformer (4,9,13,20,30,40 or 50)

PEC = assumed eddy current losses calculated as follows:

PAC-PDC     for transformers rated
  PDC          300 KVA or less, and

C (PAC-PDC)  for transformers rated
     PDC          4 more than 300 KVA
In which:
PAC = the impedance loss

C=0.7 for transformers having a turn ratio greater than 4:1 and having one or more windings with a current rating greater than 1000 amperes, or C=0.6 for all other transformers.

PDC4 = the 1 R losses for the inner winding

The impedance losses and the FR losses shall be determined in accordance with the Test Code for Dry Type Distribution and Power Transformers, ANSI/IEEE C57.12.91-1979.

DC Components of Load Current
As stated in ANSI/IEEE C57.110-1986

Harmonic load currents may be accompanied by DC components in the load current, which are frequently caused by the loss of a diode in a rectifier circuit. A DC component of load current will increase the transformer core less slightly, and may increase the magnetizing current and audible sound level.

Relatively small DC components (up to the RMS magnitude of the transformer excitation current at rated voltage) are expected to have no significant effect on the load carrying capability of a transformer determined by this recommended practice. Higher DC load current components may adversely affect transformer capability and must be corrected by the user.

Harmonic currents flowing through transformer leakage impedance and through system impedance may also produce some small harmonic distortion in the voltage waveform at the transformer terminals. Such voltage harmonics may cause extra harmonic losses in the transformer core. However, operating experience has indicated that core temperature rise usually will not be the limiting parameter for determination of safe magnitudes of non-sinusoidal load currents.

Noise Isolation Transformer
The Noise Isolation Transformer suppresses common mode noise by introducing a grounded shield between its primary and secondary windings. The grounded shield provides a low impedance path to ground by capacitive coupling, which prevents unwanted high frequency signals contained in the source voltage from reaching the transformer secondary.

The grounded shield between the primary and secondary windings is called an electrostatic shield. This shield does not perform any function with regard to harmonic current or voltage distortion wave forms. However, the shield is extremely valuable in protecting sensitive equipment from common-mode electrical noise and transients generated on the line side of the transformers.

The ration of the common mode noise attenuation (CMA) on the input to that of the output of the transformer is expressed in decibels as shown in Equation 3. An isolation transformer with an electrostatic shield can have a ratio of input noise voltage (VIN) to output noise voltage (VOUT) within the range of 10:1 to 1000:1 or even higher.


Federal Pacific Type DIT Drive Isolation Transformers are designed to meet the requirements of SCR controlled, variable speed motor drives. They are specifically constructed to withstand the mechanical forces associated with SCR drive duty cycles and to isolate the source voltage circuit from low frequency noise generated from SCR voltage spikes and transient feedback. Whereas the electrostatic shielded transformer attenuates higher frequency noise in the 10kHz – 100 kHz range.

Multiplying Effect of Cascading Shielded Transformers

Having the presence of an upline transformer with an electrostatic shield may mean that other shielded transformers would not be required in the system. However, if a shielded transformer feeds another shielded transformer, there is an effect of the attenuation ratio multiplying. If the attenuation ratio is 100:1 in each of the transformers, the total attenuation will be 100x100=10,000:1. Obviously, cascading inherently multiplies the attenuation effectiveness of shielded transformers. The term cascading means that two or more transformers are connected in series on the same system.

Estimating K-Factor Loads

For the most part, each designer or installer must make his/her own decision regarding what K-factor to assign to any load or load category. The following is intended to assist in that determination be presenting what we believe are realistic, yet conservative, K-factors for a number of loads and load categories based on their relative harmonic producing capabilities.

Calculating K-Factor Loads

1. List the KVA value for each load category to be supplied. Next, assign an ILK value that corresponds to the relative level of harmonics drawn by each type of load.

2. Multiply the KVA of each load times the ILK rating that corresponds to the assigned K-factor rating. This result is an indexed KVA-1LK value:

KVA x ILK = KVA – ILK

3. Tabulate the total connected load KVA for all load categories to be supplied.

4. Next, add-up the KVA ILK values for all loads or load categories to be supplied by the transformer.

5. Divide the grand total KVA-ILK value by the total KVA load to be supplied. This will give an average ILK for that combination of loads. (Total KVA-ILK) divided by (Total KVA) = average ILK

SELECTION CHARTS
TYPE FHK-K-FACTOR DRY-TYPE TRANSFORMERS

K-FACTOR RATED - 80 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ

K-FACTOR RATED - 80 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ ELECTROSTATICALLY SHIELD

K-FACTOR RATED - 80 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ ELECTROSTATICALLY SHIELD COPPER

K-FACTOR RATED - 80 DEGREES C - THREE PHASE -K4
480-240/120, 60HZ    LT (LIGHTING TAP)

K-FACTOR RATED - 80 DEGREES C - THREE PHASE -K4
480-240/120, 60HZ    LT (LIGHTING TAP) ELECTROSTATICALLY SHIELD

K-FACTOR RATED - 115 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ 

K-FACTOR RATED - 115 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ  ELECTROSTATICALLY SHIELD

K-FACTOR RATED - 115 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ  ELECTROSTATICALLY SHIELD COPPER

K-FACTOR RATED - 115 DEGREES C - THREE PHASE -K4
480-240/120, 60HZ    LT (LIGHTING TAP)

K-FACTOR RATED - 115 DEGREES C - THREE PHASE -K4
480-240/120, 60HZ    LT (LIGHTING TAP) ELECTROSTATICALLY SHIELD

K-FACTOR RATED - 150 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ 

K-FACTOR RATED - 150 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ  ELECTROSTATICALLY SHIELD

K-FACTOR RATED - 150 DEGREES C - THREE PHASE -K4
480-208Y/120, 60HZ  ELECTROSTATICALLY SHIELD, COPPER

K-FACTOR RATED - 150 DEGREES C - THREE PHASE -K4
480-240/120,  LT (LIGHTING TAP)

K-FACTOR RATED - 150 DEGREES C - THREE PHASE -K4
480-240/120,  LT (LIGHTING TAP) ELECTROSTATICALLY SHIELD

FEDERAL PACIFIC ALSO HAS K-13 AND K-20 CONSULT MANUFACTURER 

• Federal Pacific Industrial Control Transformers
• Federal Pacific Type FB Encapsulated Transformers
• Federal Pacific Type FH Ventilated Transformers
• Federal Pacific High Voltage General Purpose Transformers
• Federal Pacific Motor Drive Isolation Transformers
• Federal Pacific Unit Substation Transformers
• Federal Pacific VPI  Epoxy Shielded Transformers
• Federal Pacific K-Factor Transformers

 Power Transformer Information:

 Power Transformer Manufacturer

• ACME Transformers - With Acme Electric being in business over 80 years, they have always believed in offering there customers superior service, quality and technical expertise in the transformer market.
• AMVECO Transformers - AMVECO designs and manufactures toroids transformers, current transformers, and auto transformers. Most AMVECO products are custom designed utilizing their state-of-art proprietary CAD programs.  The AMVECO engineers can quickly generate designs in a matter of hours, if needed.
• Federal Pacific Transformers -  Federal Pacific is a division of Electro- Mechanical Corporation, a privately held, American owned company founded in 1958. Federal pacific offers dry-type transformers from .050 KVA through 10,000 KVA single and three phase, up to 34.5 KV, 150 KV BIL with UL approval through 15 KV.
• Marcus Transformer - Ever since they opened their doors for business a half a century ago, they have been a leader in innovative transformer design. As a family-owned company they are proud of the reputation they have earned for making quality-built transformers that deliver exceptional performance and savings.
• Hammond Transformers -  Hammond Manufacturing was founded in 1917 in Guelph, Ontario, Canada. In the last 3 decades it has expanded to the US and the international markets offering many types of power transformers. 
• TEMCo Transformers - TEMCo Transformer, a family-owned business which has been manufacturing and distributing electrical products since 1968. They focus on transformers that significantly reduce power consumption over 30 percent compared to competitive makes.
• GE Transformers - GE has been a key player in the energy industry for more than a century.  Since the installation of their first steam turbine in 1901. They have become number one provider of high-technology power generation and distribution equipment.
• Jefferson Electric Transformers - Jefferson Electric has been a pioneer and innovator of magnetic products since 1915. Jefferson broad line of dry-type transformers are backed by quality assurance systems so stringent that each and every unit gets thoroughly tested before it goes out there door.
• More power transformer brands - Check out more companies by clinking this link.

 Power Transformer Types

• Distribution Transformers - Distribution transformers are generally used in electrical power distribution and transmission power. This class of transformer has the highest power, or volt-ampere ratings. and the highest continuous  voltage rating.
• Substation Transformers - Substation Transformers are large devices which usually weigh tens of thousands of pounds.   They are filled with tens of thousands of gallons of heat transfer fluid.  Although they are typically 99.8% efficient in the transforming of electricity from one voltage to another, processing hundreds of Mega Volts-Amps of electricity force the liberation of hundreds of BTUs per second.
• Medical Grade Isolation
   Transformer - Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers.
• Drive Isolation Transformer - They are used to isolate a drive from a main power line to prevent the transmission of harmonics that the drives produce back into the power line.  They stop drive harmonics from disrupting computers and other sensitive equipment.
• Toroidal Transformers - Toroidal Transformers are more efficient than the cheaper laminated EI types of similar power level. Some of the advantages are smaller size, lower weight, less mechanical hum, (making them superior in audio amplifier), low-off-load loss.

 Power Transformer Types

• Step-Up Transformers - A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "steps up" the voltage applied to it. -
• Step-Down Transformers - A Step-Down Transformer is  designed to reduce voltage from primary to secondary.  They can range from sizes from .05 KVA to 500 KVA
• Isolation Transformers - An Isolation Transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics.
• Buck Boost Transformers - Buck Boost Transformers make small adjustments to the incoming voltage. They are often used to change voltage from 208v to 240v for lighting applications.  Major advantages of Buck boost transformers include; low cost, compact size and light weight. 
• High Voltage Transformer - There are many different types of voltage transformers. A High Voltage Transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
• Medium Voltage Transformers - A Medium Voltage Transformer can be connected directly to a primary distribution circuit and generally has the most load diversity. These voltage transformers have installation practices that are generally in accordance with application recommendations from the Institute of Electrical and Electronic Engineers (IEEE).
• Low Voltage Transformers - A Low Voltage Transformer is an electrical device that transforms 120 volts (line voltage) into 12 volts or 24 volts (low voltage). Some uses for low voltage transformer are in landscaping lighting.
• Single Phase Transformers - In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
• Three Phase Transformers - Three Phase Transformers must have 3 coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage needed while maintaining the proper phasing or polarity.
• Custom Transformers - Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification. 
• Industrial Control Transformers - Industrial Control Transformers are used to convert the available supply voltage to the required voltage to supply industrial control circuits and motor control loads.
• Pad Mounted Transformers - Pad Mounted Transformers are usually single phase, or three phase, and used where safety is a main concern. Typical applications; restaurant, commercial building, shopping mall, institutional. 
• Pole Mounted Transformers - Pole Mounted Transformers are used for distribution in areas with overhead primary lines. Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
• Oil Filled Transformers - Oil Filled Transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.
• Dry Type Transformers - Dry-Type Transformers are available for voltages up through 34.5 kV (although the most common upper limit is 15) and KVA ratings up through 10,000 (with 5000 as the usual limit). Dry-type use air as a coolant, lowering health and environmentally concerns.
• Auto Transformers - An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on the local 600 V supply.
• More power transformer types - Read further about additional transformer types and their uses.

 Power Transformer Term Definitions

• Electrical Transformers - Electrical Transformers are devices used to raise or lower the voltage of alternating current. For instance, power is transported over long distance in high voltage power lines and then transformers lower the voltage so that the power can be used by a business or household.
• Isolating Transformers - An Isolating Transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits.  An Isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. 
• Transmission Power Lines - A Transmission Line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic or acoustic waves as well as electric power transmission. Components of transmission lines include wires, coaxial cables,  dielectric slabs, option fibers, electric power lines, and waveguides.
• Transformer Voltage - The measure of the amount of force on a unit charge because of the surrounding charge.
• Transformer Phase - Most transformer are either single phase or three phase.
• Transformer Frequency - The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.
• Transformer K Factor - Some transformers are now being offered with a k-factor rating. This measure the transformer's ability to withstand the heating effects of non-sinusoidal harmonic currents produced by much of today's electronic equipment and certain electrical equipment.
• Primary Voltage - The coil winding that is directly connected to the input power.
• Secondary Voltage - The coil winding  supplying the output voltage.
• Harmonic Cancellation - Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st.
• Weatherproof - Enclosed transformers come with a weatherproof standard set by NEMA.
• Epoxy Encapsulated - A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally preferred when a unit might encounter harsh environmental conditions.
• More power transformer terms - Such as inductor, ground fault, core saturation, current transformer, faraday shield, etc.

Related Transformer Products

• Voltage Regulators - A Voltage Regulator is an electrical regulator designed to automatically maintain a constant voltage level.  It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
• AC Line Reactor - AC Line Reactors is a three phase transformer used in conjunction with AC variable frequency and DC motor drive. They are a bi-directional protective filtering device.
• Line Power Conditioners - Power or Line Conditioners regulate, filter, and suppress noise in AC power for sensitive computer and other solid state equipment.
• DC Power Supplies - Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company to a well-regulated lower voltage DC for electronic devices.
• Rotary Phase Converters - Rotary Phase Converters are commonly used in home or small commercial or industrial settings. Rotary phase converters convert single-phase power into three-phase power. This is a very cost-effective way to power three-phase electric motors and other three phase equipment.
• Frequency Converters - A Frequency Changer or Frequency Converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency.
• Voltage Converters - A Voltage Converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply.
• Magnetic Motor Starters - Magnetic Motor Starters are essentially heavy duty relays mounted in boxes, often equipped with heater/thermal overloads matched to the motor they start.
• Motor Starting Auto Transformers - An Auto Transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter.

For an additional resource the Best of Industry Web Directory : Electrical Power Transformer Directory section is quite useful.