Power Transformer Home

Power Transformer Co.   
The Power Transformer Resource     

Power Transformer

Understanding Power Transformer "K Factor Rating"

A K-Factor rating is an index of the transformers ability to supply harmonic content in its load current while operating within it temperature limits. For Dry Type Transformers a K-Factor calculation is made to determine the amount of the Harmonic Content present in a power system.

To Order Power Transformers Call TEMCo Transformer
Or 1-510-490-2187

K-Rated transformers are sized to handle 100% of the fundamental 60 Hz load, plus the non-linear load specified. The neutral of the K-Rated transformer is sized at 300% of the current rating of the phase connections. Industry literature and commentary refers to a limited number of K-factor ratings: K-1, K-4,K9, K-13, K-20, K-30, K-40. Aa transformer could be designed for other K-factor rating in between those values, as well as for higher values. The commonly referenced ratings calculated according to ANSI/IEEE C57.11-1986 are as following

K-Factor 1: A transformer with this rating has been designed to handle only the heating effects of eddy currents and other losses resulting from 60 Hz, sine-wave current loading on the transformer. Such a transformer may or may not be designed to handle the increased heating of harmonics in its load current. Applications are motors, incandescent lighting, resistance heating, motor generators (without solid state drives).

K-Factor 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 Hz, 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 trough the 25th harmonic. The "4" indicates its ability to accommodate four times the eddy current losses of a K-1 transformer. Uses are HID lighting, induction heaters, Welders, UPS with optional input filtering, PLC and solid state controls.

K-Factor 9: A K-9 transformer can accommodate 163% of the harmonic loading of a K-4 transformer.

K-Factor 13: A K-13 transformer can accommodate 200% of the harmonic loading of a K-4 rated transformer.  These transformers are used for multiple receptacle circuits in health care facilities, UPS without optional input filtering, Production or assemble line equipment, Schools and classroom facilities.

K-Factor 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 without overheating. Some of these transformers are used in SCR variable speed drives, Circuits with exclusive data processing equipment, Critical care facilities and Hospital operating room.

Today's modern electronic, electric components and circuitry such as computers, copiers, printers, fax machines and display terminals utilize switching mode power supplies for their operation. These switching mode power supplies are non-linear in nature and can cause significant power system problems:

  • Circuit breakers and fuses blowing far below current rating.
  • Neutrals in transformers and panel boards are much hotter than their ratings.
  • Distributions Transformers are overheating even when operating well within their specified nameplate rating.

Solid state switching elements such as SCR, Transistors and capacitors which  are found in computers, fax machines, solid state drives, energy efficient drive, etc.  The Continuously switching on/off producing non-linear or non-sinusoidal wave shapes in the current supplied from the power source. These problems are the results of Harmonics.

A linear load uses current from the power source continuously over the sinusoidal cycle. A non-linear load uses current in large pulses from the power source thereby creating harmonic distortion.

Harmonic components are represented by periodic waves that have a frequency that is a multiple of the fundamental frequency.


  • Fundamental Frequency=60Hz
  • 3rd Harmonic Frequency =3 x 60Hz=180Hz
  • 5th Harmonic Frequency=5 x 60Hz=300 Hz

Harmonic Frequencies superimpose themselves upon the fundamental waveform distorting and changing its magnitude.

Effects of Harmonic Components on the Sinusoidal Cycle:

The major components in the harmonic currents of switching mode power supplies are the 3rd and the 5th Harmonics. Harmonics which are even multiples of 3, such as 3rd, 6th, 9th, etc. are called Triplen Harmonics. When Triplen Harmonics are present in a 3-Phase system, they add together in the neutral conductor. 3rd Harmonics result in high 180 Hz current flowing through the transformer's neutral terminal. The core steel of the transformer has stray losses that will increase dramatically with Harmonic Distortion. These effects cause the transformers to operate at significantly higher temperatures.  

Great Power Transformer Articles

1. Top 20 Things to Know before Selecting a Power Transformer  - This is a great article to help educate and inform you on how to select the right transformer before you buy. It will help you make an informed decision and covers questions including, what voltage power output do you need?, will it be single phase or three phase?, etc. We answer the 20 most asked questions so you can be well informed and choose the right transformer for the job. Choosing the right transformer can be a daunting task for the inexperienced. This section takes the first step toward becoming a confident, knowledgeable consumer. This article addresses the process of choosing these transformers at its most fundamental level.

2. How Does Transformer Manufacturer Company Brand Affect Power Transformer Quality? - For manufacturers of large power transformers, product design and features seem fairly standard. But different manufacturers offer unique features. there are several standards such as ASTM D 3487 and IEEE Standard C57.12.90. Quality transformers can have a significant impact on cost. Did you know that some transformers brands improved materials, design and quality can save you 30%, or more, in energy cost? Understanding the differences can play a key role in making an informed selection.

3. How Does Winding Metal Type Change a Transformers Properties? - gain knowledge on how the winding and types of metal used can change transformer properties. The conducting material used for the winding depends upon the application. Small power and signal transformers are often wound with solid copper wire. Larger power transformers may be wound with copper wire, or aluminum and may include rectangular conducts. When copper wiring is used it will increase the efficiency of the transformer and will generally generate a lot less heat. Read more to learn about the many other efficiencies gained by choosing a transformer with a metal composition that is fit for your needs

4. Understanding Power Transformer "K-Factor Rating" - A great information article on what "K-Factor Rating" is and the effects they have on transformer choice. The K-Factor rating assigned to a transformer and marked on the transformer case in accordance with the listing of Underwriter Laboratories. It is an index of the transformer's ability to supply harmonic content in its load current while remaining within its operating temperature limits.

Read Addition Power Transformer Articles - Additional informal helpful articles about power transformers.

 Power Transformer Information:

Power Transformer HomeContact Power Transformer Co.

Power Transformer Types

Step Up and Step Down Transformers Step Up and Step Down Transformers to Power transformers to step-up ( raise) or step-down (lower) the electrical voltage.
Isolation Transformers Isolation Transformers allows signal or power to be taken from one device and fed into another without electrically connecting the two.
Toroidal Transformers Toroidal Transformers are devices that transfer electrical energy from one electric circuit to another, without changing the frequency, by electromagnetic induction.
Custom Transformers
Custom Transformers are designed to meet certain performance specifications and size requirement that you require. There is a wide range of custom transformer types.
Buck Boost Transformers
Buck Boost Transformers is a ideal solution for changing line voltage by small amounts. Often used to buck (lower), or boost (raise) the voltage from 208v to 240v for lighting applications.
Pole Mounted Transformers
Pole Mounted Transformers are mounted to poles for overhead electrical lines. Used in various applications. Are available in single phase or three phase transformers.
Medium Voltage Transformers
Medium Voltage Transformers are used with a medium range of voltages. They come in a full range from liquid-filled, convention dry type as well as cast coil.
Pad Mounted Transformers Pad Mounted Transformers are a excellent choice for commercial and industrial such as manufacturing facilities, refineries, office buildings, schools, hospitals, restaurants, and retail stores. They come in various sizes and can be used underground as well.
High Voltage Transformers High Voltage Transformers typically these voltage transformers are used in power transmission applications. High voltage transformers are also used in microwave.

  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 TransformersFederal 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 TransformersHammond 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.

Additional Useful Resources:
Transformer Selection Guide Custom Transformers Dry-Type Transformers Auto Transformers Control Transformers Step-Up Transformers Step-Down Transformers Harmonic Cancellation Isolation Transformers K-Factor Rated Transformers 3-Phase Transformers European Voltage Transformers Drive Isolation Transformers High Voltage Transformers Epoxy Encapsulated NEMA 4 Transformers Weatherproof NEMA 3R Transformers Transformers AC Transformer Voltage Transformer Buy Transformer New Transformer Output Transformer Transformer KVA Converter Transformer Line Transformer Toroidal Transformer Oil Filled Transformers Voltage Regulator Automatic Voltage Regulator DC Power Supply PDU Rack PDU Power Conditioner Power Line Conditioner Load Center Switchgear Voltage Converter Transformer Wiring Transformer Circuit Variable Transformer Pole Transformer Transformer Pad VA Transformer WYE Transformer Potential Transformer Transformer Protection Variac Transformer Rating Sunbelt Transformer Pacific Transformer Jefferson Transformer Electric Transformer Power Transformer Transformer Sizing Transformer Rectifier Center Tap Transformer Power Distribution Industrial Transformer Replacement Transformer Insulation Transformer Micron Transformers Westinghouse Transformer Power Supply Transformer Instrument Transformer Pulse Transformer Substation Transformer Furnace Transformer Pad Mounted Transformers Transformer Manufacturer Distribution Transformers GE Transformers Step Up Transformers Step Down Transformers Buck Boost Transformers High Voltage Transformers Isolation Transformer Single Phase Transformer Hammond Transformers Buck Boost Transformers Diesel Generators Diesel Power Generators Diesel Generator Sets Power Generators Onan Diesel Generators Kipor Diesel Generator Kubota Diesel Generator Cummins Diesel Generator Yanmar Diesel Generator Variable Frequency Drive Soft Start Drives Motor Control Baldor Motors Weg Motors