Transformer Technical Terms Glossary

Transformer Technical Terms Glossary


Copper vs. Aluminum

This is a common question when you begin comparing AC transformers. The beginnings of Copper vs. Aluminum can be traced to a time when copper was hard to come by because of World War II weapons efforts. Seeking a cost effective alternative to copper, many industries began looking to alternatives, like aluminum. Aluminum offered several benefits in that it was readily available and the cost was less expensive than copper and price. Today most standard AC transformer lines now use aluminum windings.

Both copper and aluminum provide inherent benefits which are listed below:

Aluminum Copper
Aluminum, after developing an oxide barrier, inhibits chemical reactions and its oxide is an excellent insulator. Inherently develops an oxide barrier which resists corrosion and maintains strength and conductivity that makes it better suited to corrosive environments than Aluminum.
Superior thermal storage capacity with the ability to withstand overload currents and more surge than comparable metals. Highly efficient. Can produce a more compact package than Aluminum.
Excellent conductivity with a long life span. Excellent conductivity with more current capacity then comparable metals.
Generally more expensive for the same performance than Aluminum.

Connection Types

Transformers can be wired in several different ways.


Shown as Δ, 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.


A standard three phase transformer connection with similar ends of the single phase coils connected. This common point forms the electrical neutral and may be grounded depending on the application. 208/120V is a common example of this as is 380/220V in Europe.

Scott T

The Scott-T transformer connection may be used in a back to back T to T arrangement for a three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the X0 Neutral tap is part way up on the secondary teaser transformer (see below). The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned.

Lighting Tap

Also known as a center tap, the lighting tap is a connection brought out of the secondary windings to provide for a lower voltage supply for lighting accessories. This tap is typically located in the center of the winding providing for a voltage that is ½ of that produced by the coil. Example: A 240 VCT transformer will measure 240 VAC across the outer two taps (winding as a whole), and 120VAC from each outer tap to the center-tap (half winding). These two 120 VAC supplies are 180 degrees out of phase with each other.

The reasons for choosing a Y or Δ configuration for transformer winding connections are the same as for any other three-phase application: Y connections provide the opportunity for multiple voltages, while Δ connections enjoy a higher level of reliability; if one winding fails open, the other two can still maintain full line voltages to the load. Delta primary Delta secondary is acceptable as is Delta-Wye but Wye primary with Wye secondary is unstable with very poor voltage regulation and is seldom used except for special applications where this can be accounted for.

Enclosure Ratings

Transformer enclosure ratings are defined by NEMA standards.

NEMA Rating Indoor/Outdoor Use Protection Against
NEMA 1 Indoor Limited amounts of falling dirt.
NEMA 3R Outdoor Rain, sleet, and damage from external ice formation.
NEMA 3S Outdoor Rain, sleet, windblown dust and to provide for operation of external mechanisms when ice laden.
NEMA 4X Indoor or outdoor Corrosion, windblown dust and rain, splashing water, hose-directed water, and damage from external ice formation.
NEMA 6 Indoor or outdoor Hose-directed water, and the entry of water during occasional temporary submersion at a limited depth and damage from external ice formation.
NEMA 6P Indoor or outdoor Hose-directed water, the entry of water during prolonged submersion at a limited depth and damage from external ice formation.
NEMA 7 Indoor in locations classified as Class I, Division 1, Groups A, B, C or D hazardous locations as defined in the National Electric Code (NFPA 70) (Commonly referred to as explosion-proof).
NEMA 8 Indoor or outdoor For use in locations classified as Class I, Division 2, Groups A, B, C or D hazardous locations as defined in the National Electric Code (NFPA 70) (commonly referred to as oil immersed).
NEMA 9 Indoor For use in locations classified as Class II, Division 1, Groups E, F and G hazardous locations as defined in the National Electric Code (NFPA 70) (commonly referred to as dust-ignition proof).
NEMA 10 Indoor or outdoor Intended to meet the applicable requirements of the Mine Safety and Health Administration (MSHA).
NEMA 12/K Indoor Circulating dust, falling dirt, and dripping noncorrosive liquids.


Frequency, expressed in Hertz (Hz), is the number of times the polarity alternates from positive to negative and back in the AC electric circuit. Standard US frequency is 60Hz.


Electrical impedance describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.


K-factor is a classification of harmonic load currents according to their effects on transformer heating, as derived from ANSI/IEEE C57.110. A K-factor of 1.0 indicates a linear load (no harmonics). The higher the K-factor, the greater the harmonic heating effects. When a non-linear load is supplied from a transformer, it is sometimes necessary to de-rate the transformer capacity to avoid overheating and subsequent insulation failure. The reason for this is that the increased eddy currents caused by the harmonics increase transformer losses and thus generate additional heat. Also, the RMS load current could be much higher than the kVA rating of the load would indicate. A Transformer rated for the expected load will have insufficient capacity. The K-Factor is used by transformer manufacturers and their customers to adjust the load rating as a function of the harmonic currents caused by the load(s).


kVA (Kilo-volt-ampere) is the amount of apparent power in an electrical circuit, equal to the product of voltage and current (1000 VA = 1 kVA). It is equal to the electrical power measured in watts for Direct Current (DC) circuits but with no consideration of powerfactor. The apparent power may differ from the real power for Alternating Current (AC) circuits, where voltage and current may be out of phase. The real power is equal to the apparent power multiplied by the power factor.

You should ALWAYS choose a transformer with a kVA that is LARGER than that of the load. This is necessary for safety purposes and in some cases allows for expansion in case a larger load is connected in the future. NEVER undersize a transformer, this will cause unnecessary heating of the transformer and can result in the failure of the unit and possible damage to the loads attached to it.

Determining kVA

Single Phase Three Phase
Volts & Amps Known kVA & Volts Known Volts & Amps Known kVA & Volts Known
kVA = (Volts x Amps) / 1000 Amps = (kVA x 1000) / Volts Three Phase kVA = (Volts x Amps x 1.73) / 1000 Amps = (3 Phase kVA x 1000) / Volts x 1.73


Single Phase & Three Phase Electric Power

Single phase refers to the distribution of alternating current electric power with only two active conductors. Single phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single phase motors need additional circuits for starting, and such motors are uncommon above 10 Hp/7Kw.

In a three phase system, three active conductors carry three alternating currents, of the same frequency, which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between phases has the effect of giving constant power transfer over each cycle of the current, and also makes it possible to produce a rotating magnetic field in an electric motor.

Primary Voltage

A piece of equipment's rated input voltage.

Secondary Amps

A piece of equipment's rated output amperage.

Secondary Voltage

A piece of equipment's rated output voltage.


Taps are physical connections, usually on the primary winding, that allow for changes in the ratio between the primary and the secondary. Changing the winding ratio results in changes in voltage produced on the secondary. Standard taps are at 2.5% and 5% above nameplate primary and 2.5% and 5% below the nameplate primary.

Temperature Rise

Temperature rise in a transformer is the temperature of the windings and insulation above the existing ambient or surrounding temperature.

Transformer Types

Isolation Transformers

This is the most common type of transformer for most applications. An Isolation Transformer is commonly used to isolate two electrical circuits. An isolation transformer allows AC power to be taken from one circuit and fed into another without electrically connecting the two circuits. They also block interference caused by ground loops. Isolation transformers with electrostatic shields are used in power supplies for sensitive equipment such as computers or laboratory instruments.

Auto Transformer

An Autotransformer is an electrical transformer with only one winding. The auto- prefix refers to the single coil rather than any automatic mechanism. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding. The winding has at least three taps where electrical connections are made. The voltage source and the electrical load are both connected to two taps with one of the taps at the end of the winding being a common connection to both circuits. Each tap along the winding corresponds to a different source or load voltage.

Drive Isolation Transformer

A Drive Isolation Transformer is an Isolation Transformer with grounded electrostatic (Faraday) shielding separating the primary and secondary windings that provides a decrease in the capacitive coupling involved in transferring common-mode voltage disturbance. Without such shielding, that capacitance allows passage through the transformer of high-frequency "noise" and transient voltage spikes. Common-mode transients are those appearing between ground and neutral of the a-c system. (Although those two parts of the circuit are normally bonded together at one point, they cannot be presumed to be at the same potential throughout an entire power system.) Such disturbances arise from switch-mode power supplies, motor drive operation, arc welders, lightning, or even from normal operation of such equipment as stepper motors. Some isolation transformers can also block "normal-mode" transients, appearing between line and neutral.

Motor Starting Auto Transformer

During start up, large squirrel cage induction motors have the undesirable characteristic of drawing to 6 or 8 times or more their normal running current from the power source. This start up current is at its peak the moment power is applied and gradually diminishes to running-load current as the motor reaches normal speed. Substantial voltage drop on the feeder lines often is the result and it may be severe enough to cause light flickering and malfunction of electrical or electronic equipment connected to the feeder lines. Motor Starting Auto Transformers have voltage taps commonly at 25%, 50%, and 75% that are switched in successive order to gradually increase voltage supply to the motor. By reducing the supply voltage during start up the current draw is greatly reduced.

Harmonic Mitigating Transformer

Harmonic Mitigating Transformers are used for reducing harmonic currents from switch-mode power supplies, motor drive operation, arc welders and other such devices. Harmonic Mitigating Transformers use a combination of phase shifted electromagnetic flux and impedance to cancel the distortive affects of non linear loads.

K rated Transformer

K-Rated transformers are designed to prevent their overheating when subjected to heavy non-linear loading such as switch-mode power supplies, motor drive operation, arc welders and lightning.