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Air cooled Transformer

A transformer which uses air as the cooling medium. This may be a forced air with the use of fans.

Alternating Current (AC)

An electrical current flow of continuously changing polarity, which rises to a maximum voltage in one direction, decreases to zero and then sinks to the maximum voltage in the other direction before changing polarity once again. This pattern is referred to as a sinusoidal wave and the number of cycles per second is equal to the frequency, which is measured in Hertz.

Ambient Noise level

The level of acoustic noise existing at a given location like room or compartment etc. Ambient noise level is measured based on sound level meter or in decibels (db).

Ambient Temperature

The normal surrounding temperature of the environment in which a transformer will operate.


ANSI was formed in 1918. American National Standards Institute is a recognized body which approves standards for transformers. ANSI 89.1 is mostly used for dry type transformers.


Breakdown voltage

The voltage at which an electrical breakdown occurs. It is also known as breakdown potential, sparking potential or sparking voltage.

BIL Basic Insulation level

It is an insulation system that can withstand very high voltage surges.



The ferrous center part of a transformer or inductor used to increase the strength of the magnetic field. It carries the flux and forms the magnetic coupling between primary and secondary.

Core Loss

Core loss is also known as iron loss. Core loss is a form of energy loss that occurs in electrical transformers and other inductors. Core losses do not include the losses due to resistance in the conductors of the windings, which is often termed copper loss. It does not vary with load and hence also called constant losses. It mainly consists of eddy current and hysteresis losses.

Core Saturation

Condition that occurs when an inductor or transformer core has reached maximum magnetic strength.

Compensated Transformer

A transformer with a turn’s ratio which provides a higher than rated voltage at no load, and, rated voltage at rated load.

Current Transformer (CT)

A transformer used in instrumentation to assist in measuring current. It utilizes the strength of the magnetic field around the conductor to form an induced current that can then be applied across a resistance to form a proportional voltage.


Duty Cycle

The percentage of time a transformer will be supplying the Full Rated Power to the load. Percentage of time a unit is expected to perform at Full Rated power versus time spent in idle can significantly affect the physical size of a transformer.


Delta is a three phase connection where the ends of each phase winding connection in series to form a closed loop with each phase 120 electrical degrees from the other.


The connection between a delta source and a wye load.


The connection between a delta source and a delta load.

Drive Isolation Transformer

A transformer built with a K factor of K4 or greater and an electrostatic shield. Drive isolation transformers are designed this way to mitigate the negative affects of harmonics (caused by non-linear loads) on the transformer itself.


Eddy Currents

It is induced into a metal when magnetic lines of force move across it.

Effective Voltage or current

The amount of power being delivered to a DC circuit load can be calculated easily by dividing the load resistance into the applied DC voltage squared.


Ratio of its power output to its total power input.

Electrostatic Shield

A grounded conductor sheet which provides a ground shield between primary and secondary windings to decrease or eliminate line to line or line to ground noise. It is also known as Faraday Shield.

Electrostatic Shielding

Placed between windings (usually the primary and secondary) to provide maximum isolation. Additional Electrostatic Shields can be placed between secondary windings as required. Shielding is normally connected to the transformer’s ground.


Faraday Shield

A grounded metallic barrier that can be used for improved isolation between the windings of a transformer. In this application, the shield basically reduces the leakage capacitance between the primary and secondary.

Faraday’s Law

A law that states an electro motive force is induced in any system in which a magnetic field is changing with time and is directly proportional to the rate of change of flux.


Full Capacity above nominal. This is used to specify that a transformer will deliver rated KVA when connected to a voltage source which is higher than rated voltage.

FCBN (Full Capacity below Nominal) Taps

It is the same as FCAN except that the taps are below rated voltage


Resonance resulting when the iron core of an inductive component of an LC circuit is saturated, increasing the inductive reactance with respect to the capacitance reactance.



A device that converts mechanical energy into electrical energy by magnetic induction.


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

Ground Fault

Any undesirable current flow from a current carrying conductor to ground.

Ground Fault Interrupter (GFI)

A device whose function is to interrupt the electric circuit to the load when a fault current to ground exceeds a predetermined value that is less than that required to operate the over-current protective device of the supply circuit.


Connected to the earth or some other conductor.


Hertz (Hz)

Cycles per second

Horsepower (HP)

One horsepower is 33,000 lb.-ft /minute or 746 watts.


Electricity produced by turbines that are turned by water flow.



Forces, including resistance and capacitive or inductive reactance, which resist current flow in AC circuits.


The ability of a coil to store energy and oppose changes in current flowing through it. A function of the cross sectional area, number of turns of coil, length of coil and core material.


A coiled conductor that opposes change in current.

Inrush Current

A brief and momentary surge of current through the transformer, due to residual flux, experienced at the instant the transformer is energized.

Instrument Transformer

A transformer designed to transform the conditions of current or voltage and phase position in the primary with a specified accuracy of the secondary circuit.


Jack Pads

Structural member at bottom of transformer which provides lifting points which are used to lift the device onto rollers for repositioning.



This is a rating used to denote a rated transformer which is specifically designed to handle non-linear loads. Numerical values indicate both the magnitude and frequency of any component of a current waveform which have been considered in the transformer design. Industry standard ratings are K4, K9, K13, K30, and K50.

Kilowatt (KW)

1,000 Watts.


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


Kilowatt hour, one kilowatt for one hour – a unit of energy.



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

Line voltage

Voltage of a power line.

Linear Load

A load in which the relationship between current and voltage is directly proportional. For example: water heater, resistance heating etc.


Magnetic Shielding

Conductive material placed around a transformer’s coils to attenuate stray magnetic fields.

Multiple Winding

A winding which consists of two or more sections that can be paralleled to specific mode of operation.


NEMA Enclosure

(National Electrical Manufacturers Association) an enclosure conforming to NEMA specifications, usually constructed of metal that provides some measure of protection against weather and other elements. Different NEMA ratings determine the degree of protection, ranging from “some measure of protection” to “fully weatherproof.” Suitable for outdoor use or where indoor location may constitute a shock hazard if connections are left exposed.

No-load current

The current drawn by a transformer at nominal input voltage in its unloaded (open-circuit) condition. Known also as exciting current or magnetizing current.

Natural Convection

Method of temperature regulation in which the normal convection of ambient air surrounding the transformer will provide its only cooling.

NEMA Standard

Any standard published or sponsored by the National Electrical manufacturers Association (NEMA).

Nominal Voltage

The normal or designed voltage level. For three phase wye systems, nominal voltages are 480/277 (600/346 Canada) and 208/120 where the first number expresses phase to phase (or line to line) voltages and the second number is the phase to neutral voltage. The nominal voltage for most single-phase systems is 240/120.


Ohm’s Law

The relationship between voltage (pressure), current (electron flow), and resistance. The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. E=IR, or I=E/R, or R=E/I. Where E=voltage, I=current, and R=resistance.

Open Frame

A method of transformer construction utilized when the transformer will be combined with other components inside an enclosure.


The variation, usually with time, of magnitude with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.


When a transformer’s connected load exceeds the rated output. When overloaded, excessive heat is developed, and insulation system begins to break down prematurely. Life expectancy decreases due to the over heating.


Peak voltage

Highest voltage measured during an event. Or the maximum voltage obtained from an oscillating voltage wave. With an AC source, this occurs twice and lasts for only a fraction of the cycle. Direct current voltage is considered peak voltage at all times.


Electrical circuits can be single-phase, three-phase or poly-phase depending on the number of source conductors usually lighting and residential circuits are rated single-phase. Single-phase transformers can be used on a three-phase source when two wires of the three-phase system are connected to the primary of the single-phase transformer. Conversely, three single phase sources can be combined to provide three phase power. Typical power distribution networks combine single phase generators to provide three phase distribution voltages.

Power Factor

Watts divided by volt amps, kW divided by kVA. Power factor: leading and lagging of voltage versus current caused by inductive or capacitive loads, and harmonic power factor: from nonlinear current.

Primary winding

The coil winding that is directly connected to the input supply.


Rated Power

The total output power available from all secondary windings, expressed in Volt-amperes (VA) or Kilovolt amperes (kVA).


Opposition to changes in flow of alternating current. Capacitive reactance is opposition in change from a capacitor, and inductive reactance is the opposition in change from a coil or other inductor.


An electrical device used to change AC power into DC power.


The percentage difference between a secondary winding’s output voltage when operating under no-load and open-circuit and full load conditions.

Resin Filled Construction

The core and coil assembly is completely encapsulated with a resin-sand compound and contained in a metal enclosure.



(Semiconductor, or silicon controlled rectifier) An electronic DC switch which can be triggered into conduction by a pulse to a gate electrode, but can only be cut off by reducing the main current below a predetermined level (usually zero).

Secondary Winding(s)

The coil winding(s) supplying the output voltage to the load(s).


Imposing a metallic barrier to reduce the coupling of undesirable electromagnetic signals.

Short circuit

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

Single Phase

(With a three phase source) one or two phase conductors. (Single phase source) A single output which may be center tapped for dual voltage levels.



A Scott connected three-phase transformer utilizing two primary and two secondary coils called the main and teaser coils.

Taps or Voltage Taps

Additional connections to winding allowing different voltages to be obtained from the same winding. Often used on the primary winding to allow the transformer to be used in different countries having different line voltages available.

Temperature Rise

The additional heat, above ambient temperature, that the transformer itself will generate in the normal course of operation.

Test Potential

A voltage applied to a winding to ensure adequate insulation performance.

Three Phase Power

Three separate outputs from a single source with a phase differential of 120 electrical degrees between any two adjacent voltages or currents.


Weather shield

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


Zigzag Transformer

A transformer where the windings are physically inter-connected to achieve specified voltage and current phase relationships. A zigzag winding arrangement is commonly found in harmonic mitigating, phase shifting, or grounding transformers.

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