Click on a letter to see the list of defined words.

American National Standards Institute. A recognized body which approves standards for transformers. ANSI C 57.12 series contains the standards most often used for dry type transformers.

A transformer cooled by the natural circulation of air over and/or through the core and coils. Alternating Current (or voltage) Current that alternates regularly in direction, is periodic and has an average value (over a period of time) of zero.

Ambient Noise Level
The existing or inherent sound level of the area surrounding a transformer installation. Measured in decibels.

Ambient Temperature
The temperature of the air surrounding the transformer.

The current-carrying capacity of an electrical conductor or device.

The practical unit of electric current.

Decrease in signal voltage or power.

A transformer in which part of one winding is common to both the primary and the secondary circuits associated with that winding.

Basic Insulation Level. A measure of the ability of the insulation system to withstand very high voltage surges. For example, a 600 volt class transformer has a 10 KV BIL rating.

Two or more single-phase transformers connected together, or banked, to supply power. Three single-phase transformers banked together will produce a kVA capacity of three times the nameplate rating of the individual single-phase transformers. For example, three 5 kVA single-phase transformers connected together for a three-phase load will have a 15 kVA capacity.

Mark to indicate third party approved or self-certification to European Community requirements.

Canadian Standards Association. The Canadian equivalent of Underwriter’s Laboratories (UL).

Mark to indicate UL certification to CSA standards.

Same as Centigrade. To convert Centigrade to Fahrenheit, use the following formula: °F = 1.8 x °C + 32.

A number of turns of conductor wound as a coil.

Compensated Transformer
A transformer with a turns ratio which provides a higher rated voltage at no-load and rated voltage at rated load. Normally used on units rated 2 kVA or smaller.

Continuous Duty
The service requirement that demands operation at a constant load for an indefinite period of time.

Continuous Rating
The load that a transformer can handle indefinitely without exceeding the specified temperature rise.

Control Transformer
Usually referred to as an Industrial Control Transformer. Designed for good voltage regulation characteristics when low power factor and/or large inrush currents are drawn (5 to 15 times normal).

Conductor Losses
Losses in the transformer winding that are incidental to the carrying of the load. These losses include those due to resistance as well as to stray and eddy currents.

The steel that carries the magnetic flux in a transformer.

Core Loss
Losses caused by a magnetization of the core and its resistance to magnetic flux.

One complete sequence of values of an alternating quantity, including a rise to maximum in one direction, a return to zero, a rise to a maximum in the opposite direction, and a return to zero.

Decibel (db)
A unit used to express the magnitude of a change in signal or sound level, either an increase or a decrease.

Delta Connection
A method used for connecting the three windings of a three-phase transformer (or three single-phase transformers). The windings are connected in series, the three-phase supply being taken from or supplied to the junctions.

Delta Wye
The method of connection for both primary and secondary windings of a three-phase transformer bank.

Dielectric Tests
A series of tests conducted at a much higher than rated nameplate voltage to assure the integrity of insulating materials and electrical clearances.

Distribution Transformer
Any transformer rated between 3 and 500 kVA and a primary voltage of 601 volts or less.

Double Wound
Transformer See Isolating Transformer.

Drive Isolation Transformer
A transformer designed to withstand the additional heat and mechanical stress caused by DC drives.

Dry Type Transformer
A transformer cooled by a medium other than a liquid, usually through the circulation of air.

Dual Winding
A winding that consists of two separate windings which can be connected in series to handle a specific voltage and kVA or in parallel to handle the same kVA at one-half the series connected voltage.

Currents Additional currents caused by a time varying magnetic field.

Voltage or Current 0.707 times the peak value of AC voltage or current. Effective value is also designated RMS value (Root Mean Square). When AC voltage is referred to, the effective value is understood unless otherwise noted. Symbols “E” and “I” without subscripts indicate effective values.

The efficiency of a transformer is the ratio of its power output to its total power input.

Electrostatic Shield
A grounded conductor placed between the primary and secondary winding to greatly reduce or eliminate line-to-line or line-to-ground noise. Often referred to as a “Faraday shield.”

Excitation Current
The steady rate current that keeps the transformer energized after the inrush has dissipated, with all other windings open-circuited. Also called “magnetizing” or “no-load current.”

Exciting Wattage
The no-load loss of a transformer.

FCAN and FCBN Taps
Full Capacity Above Nominal and Full Capacity Below Nominal. The FCAN designation is used to indicate that a transformer will deliver rated kVA when connected to a voltage source which is higher than rated voltage. The FCBN designation indicates that a transformer will deliver rated kVA when connected to a voltage source which is lower than rated voltage.

Fan Cooled
A means of accelerating heat dissipation to lower the temperature rise of the transformer. This has the effect of increasing the transformer rating.


The number of complete cycles per unit for a periodic quantity such as alternating current, sound waves or vibrating objects.

An overcurrent protective device with a circuit-opening fusible member which is directly heated and severed by the passage of overcurrent through it, or by a fault.

A conducting path, whether intentional or accidental, between an electric circuit or equipment and the earth, or some other conductor.

Connected to the earth or some other conductor.

Horsepower. Energy required to raise 33,000 pounds one foot in one minute. Equals 746 watts, or .746 KW.

A sinusoidal waveform with a frequency that is an integral multiple of the fundamental 60 Hz frequency.

1202nd Harmonic
1803rd Harmonic
2404th Harmonic
Current waveforms from non-linear loads appear distorted because the non-linear waveform is the result of adding harmonic components to the fundamental current.

Harmonic Distortion
Non-linear distortion of a system characterized by the appearance in the output of harmonic currents when the input is sinusoidal.

Harmonic Distortion, Total
The square root of the sum of the squares of all harmonic currents present in the load, excluding the 60 Hz fundamental. Usually expressed as a percent of the fundamental.Hertz (Hz)Cycles per second. High Voltage Windings in a transformer with two windings, designates the winding with the greater voltage. Usually marked with an “H” designation. Hysteresis Tendency of a magnetic substance to persist in any state of magnetization.

Total opposition of a component or circuit to the flow of an alternating or varying current (symbol Z).

That property of a circuit or circuit element opposing a change in current flow (symbol L). Measured in Henrys.

The power or signal fed into an electrical device, or to the terminals involved.

Inrush Current
The initial high peak of current during the first few cycles of energization which can be 30 to 40 times the rated current.

Isolating Transformer
Transformer in which input winding(s) connected to the line are completely isolated from those connected to the load.

Material with high electrical resistance.

Device used for supporting or separating conductors of electricity.

Insulating Transformer
Another term for isolation transformer.

A numerical value taking into account both the magnitude and frequency of the component of a current waveform. Used to indicate a full-rated transformer specifically designed to handle non-linear loads.

Kilowatt (KW)
1,000 Watts.

Kilowatt hour, one kilowatt for one hour.

Kilovolt-ampere, or thousand volt-ampere. When multiplied by the power factor, will give kilowatts, or KW.

Linear Loads
Loads where the current waveform conforms to that of the applied voltage, or loads where a change in current is directly proportional to a change in applied voltage. For example: resistance heating, incandescent lighting, water heater.

Thin sheets of steel making up the core of the transformer.

Line Voltage
The voltage of the power line.

Non-Linear Loads
Loads where the current waveform does not conform to that of the applied voltage, or where a change in current is not proportional to change in applied voltage. For example: computer power supplies, motor drives, fluorescent lighting.

Non-Ventilated Construction
The core and coil assembly is mounted inside an enclosure which has no ventilation openings.

The core and coil assembly is completely encapsulated (contained within protecting material) with a resin-sand compound and contained in a metal enclosure.

Power Factor (PF)
A capacitive or inductive circuit condition that results in the applied current leading or lagging the applied voltage.

Peak Voltage
The voltage or current of an AC sinusoidal wave when it reaches its peak or maximum level. This occurs twice and lasts for only a fraction of the cycle. Direct current voltage is peak voltage at all times.

Short Circuit
A low resistance connection, usually accidental, across part of a circuit, resulting in excessive current flow.

Having the form of a sine (or cosine) wave.

Step-Up/Step-Down Transformers
A transformer can either step up or step down voltage. A step-up transformer is one in which the output voltage is greater than the input voltage. With a step-down transformer, the input voltage is greater than the output voltage.

Incoming plant voltage varies according to the distance from the substation and other factors. Taps allow a distribution transformer to provide secondary voltage as close as possible to the desired operating voltage. Taps are usually supplied on the primary winding to allow matching of the supply voltage to the voltage rating of the transformer connection. A tap position above the nominal connection will lower the secondary output and vice-versa.

Transformer Regulation
The percentage difference between voltage at the secondary terminals under noload condition versus voltage under full-load. This value depends on the load power factor and is usually reported at 1.0 PF and 0.8 PF.

Turn Ratio
The relationship between the number of turns on the transformer’s two windings. Voltage is always transformed in exact accordance to this ratio. The amperes, or amount of current, changes in an inverse ratio to the turns ratio. When voltage increases, current decreases in the same proportion, and vice-versa.

Underwriter’s Laboratories. A non-profit safety testing organization.

Providing circulation of external air.

Ventilated Enclosure
Enclosure with openings which allow air to flow directly over the core and coil assembly for cooling.

Transformers are rated in volt-amperes (the product of volts and amperes in the input winding). The capacities of very large transformers are rated in thousands of volt-amperes (kilovolt-amperes or kVA) and in millions of volt-amperes (megavoltamperes or mVA). For all practical purposes, input kVA is equal to output kVA.

Unit of electrical power when the current in the circuit is one ampere and the voltage is one volt.

When added to ventilated enclosures, allow indoor-rated units to be situated outdoors, changing the enclosure rating to NEMA 3R.

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