Air Cooled: A transformer which uses air as the cooling medium. This may be natural draft cooling by convection or forced air with the use of fans.

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

ANSI: American National Standards Institute is the recognized body which specifies the standards on transformers.

ANSI C 89.1 and C 57.12 are the standards most often used for dry type transformers.

Auto-Transformer: A transformer which has only one winding per phase, part of which is common to both primary and secondary circuits.


Banked: This term is used when two or more single-phase transformers are connected or banked together to supply power to a three-phase load. The three single-phase transformers banked together will produce a KVA capacity of three times the nameplate rating of each of the single-phase transformers. Example, three 5 KVA single-phase transformers connected together for a three-phase load will have a KVA capacity of 15 KVA.

BIL: Basic Insulation Level is 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.


Center Tap: A reduced capacity tap at the mid-point in a winding.

Certified Test: Actual production or laboratory tests which certify values as having been taken on a specific item, on a specific order.

Compensated Transformer: A transformer with a turns ratio which provides a higher than rated voltage at no load rated voltage at rated load.

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

Conductor Losses: Losses caused by the resistance of the transformer winding and usually measured at 25, 50, 75, and 100 percent of load.

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

Current Transformer: A transformer designed to have its primary winding connected in series with the circuit and used for transforming current to a value suitable for measurement or control.


Decibel (dB): The standard unit used in the measurement of sound intensity.

Delta (): A standard three-phase connection in which each phase winding is connected in series to form a closed loop.

Delta Wye (-Y): The term used when indicating a method of connection for both primary and secondary windings of a three-phase transformer bank.

Dielectric Tests: A series of tests conducted at much higher than rated nameplate voltage to determine the effectiveness of insulating materials and electrical clearances.

Distribution Transformer: Any transformer rated between 3 and 500 KVA, inclusive with a primary voltage of 601 volts or greater.

Dry Type Transformer: A dry type transformer is one which does not employ a liquid as a cooling or insulating medium.

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.


Electrostatic Shield: A grounded conductor sheet placed between the primary and secondary winding to greatly reduce or eliminate line to line or line to ground noise. It is often referred to as a Faraday shield.

Encapsulated: A style of dry type transformer utilizing a solid resin to encase and seal the core and coil assembly.

Exciting Current: The current which maintains the excitation of a transformer (the current that flows in the primary) when the load is disconnected. Usually expressed in percent of the rated current of the winding.

Extended Winding: A winding with additional turns to provide application of a higher voltage. This effectively changes the turns ratio of the transformer when the extended winding is employed in the circuit. Example: FCAN taps.


FCAN: Full capacity above nominal taps. The designation used to indicate 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. Same as FCAN except that the taps are below 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.

Filter Press: A device for filtering and absorbing moisture from oil. Liquid is force-circulated through absorbent filter pads.

Flexible Connector: A pliable conductor designed to compensate for thermal expansion and contraction, or reduce the transmission of objectionable noise.

Frequency: The number of times an AC voltage will alternate from positive to negative and back again within a specified period of time, usually expressed in cycles per second and identified as Hz as in 60 Hz.

Full Capacity Tap: Tap that is designed to deliver the rated capacity of the transformer.


Harmonic Mitigating Transformer: A transformer whose primary function is the elimination of harmonics produced by certain types of non-linear loads, such as computer power supplies and a number of special drives.
Hazardous Duty Transformer: A transformer designed for use in a typically corrosive environment as described by Article 500 of the NEC, Class 1, Division 2, Group C and D.

Hertz (Hz): A term for AC frequency in cycles per second. Transformers rated for 60-Hertz service should not be applied to 50 Hertz, as overheating will occur. Certain transformers are rated 50/60 Hertz and therefore, suitable for either frequency.
High Resistance Transformer: A design which has high leakage to reactance to limit the output current to a designated value in the event of a fault. Also called Energy Limiting transformer.

Hi Pot: A standard dielectric test to check insulating materials and clearances between windings and between windings and ground


Impulse Tests: The test employed to determine Basic Insulation Level (see also BIL).

Impedance: The vector sum of resistance and reactance, which limits the current flow in an AC circuit. Impedance is identified in percentage and is used to determine the interrupting capacity of circuit breakers, which protect the primary circuit.

Induced Potential Test: A standard dielectric test which verifies the integrity of insulating materials and electrical clearances between turns and layers of a transformer winding.

Inrush Current: An abnormally high current which is occasionally drawn when a transformer is first energized, due to residual flux in the core.

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.

Insulating Material: Organic or inorganic substances which prevent or greatly reduce the current flow between live parts or between live parts and ground.

Insulating Transformer: One in which the primary winding is insulated from the secondary winding.

Interleaving: A method of winding where the primary and secondary are wound in sections with a section of the primary between two sections of the secondary and vice versa. This technique is used to reduce the leakage reactance of the transformer when desirable to do so. Industrial control transformers frequently have interleaved windings.

Isolating Transformer: A transformer which insulates the primary circuit from secondary circuit as with an insulating transformer, but frequently with no voltage change.


Jack Pads: Structural member at bottom of transformer to accommodate hydraulic jacks which lift onto rollers for moving into position.


K Factor: A relative number (4, 9, 13, 20, 30, etc.) assigned to electrical apparatus such as a transformer to express the devices compensating ability in dealing with additional, non-fundamental loading. This loading is typically caused by harmonics.

KVA: Kilovolt Ampere rating designates the output which a transformer can deliver at rated voltage and frequency without exceeding a specified temperature rise.


Load: The KVA or VA requirement which the transformer must supply.

Load Center Unit Substation Transformer: A transformer that is mechanically and electrically connected to and coordinated in design with switch-gear or motor control assemblies.

Load Losses: The losses which are the result of a current flowing to the load. Load losses would include all losses incurred above and beyond the no-load losses.


Multiple Winding: A winding which consists of two or more sections that can be paralleled for specific mode of operation.


NEMA Standard: Any standard recommended by the National Electrical Manufacturers Association.

Network Transformer: A transformer which is electrically and mechanically connected to and coordinated in design with switch-gear or motor control assemblies for use on a utility network power system.

Noise Attenuating Transformer: A transformer designed to minimize electrical noise associated with certain loads.

No-Load Losses: The losses incurred when a transformer is excited, but without a load connected to the secondary. These include core loss, dielectric loss, and exciting current I2R loss.


Overload: When a transformer is overloaded, excessive heat develops, and the insulation system begins to break down out of proportion to the amount of overload. Life expectancy of the transformer is greatly decreased due to heat exceeding the rating of the insulation system.


Parallel Operation: Transformers or transformer windings may be connected in parallel provided that the electrical characteristics are suitable for such operation.

Percent IR (%IR) Percent Resistance: The voltage drop due to conductor resistance at rated current expressed in a percent of the rated voltage.

Percent IX (%IX) Percent Reactance: The voltage drop due to reactance, at rated current, expressed in a percent of the rated voltage.

Percent IZ (%IZ) Percent Impedance: The voltage drop due to impedance, at rated current, expressed in a percent of the rated voltage.

Phase: Classification of an AC circuit. Usually, circuits are rated single-phase two wire or three wire or three-phase three wire or four wire. 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. The secondary will be single-phase.

Polarity: A designation of the relative instantaneous direction of the current in a secondary lead as compared with a primary lead. The two leads are said to be of the same polarity when, at any instant, the currents in the two leads are flowing in the same direction as if the leads were a single piece of wire. The polarity of a single-phase transformer is classed as either additive or subtractive.

Polyphase: More than one phase.

Potential Transformer: A transformer that is designed to have its primary winding connected parallel with a circuit and used for transforming voltage to a value suitable for measurement or control.

Power Factor: The ration of watts to volt amperes in a circuit. Expressed as % watts / VA.

Pressure Relief Device: Used to relieve excessive pressure inside tank. Generally operates at 71/2 psi. Self-resealing with target indicator to show operation. Alarm contacts are optional.

Pressure Vacuum Bleeder Valve: Automatic valve which opens and closes to maintain internal pressure within predetermined limits.


Radiators: Tank extensions used to assist cooling the oil and transformer. Panel type is generally used for better efficiency and appearance.

Rating: This refers to the characteristics such as volt ampere capacity, voltages, frequency and temperature rise for which the transformer is designed.

Ratio: A reference to either the turns ration or to the voltage ration of the transformer. It’s the ration between the primary and the secondary. For example, a transformer with a 480-volt primary and a 120-volt secondary has a ratio of 4 to 1.

Reactance: A component of impedance produced by either inductance or capacitance in an AC circuit.

Reactor: A device, generally a single winding on an air core or an iron core, which produces a specific amount of inductive reactance for which it is designed.

Rectifier Transformer: A transformer designed to supply the AC input to a rectifier at the value necessary to obtain the desired DC output. Also to have the ability to withstand the usual heating effects caused by rectifier commutation or ripple.

Reduced Capacity Taps: Tap that is not designed to deliver the rated capacity of the transformer.

Regulation: The percent change in output voltage from full load to no load.


Scott Connection: A transformer connection usually used to get a two-phase output from the secondary of a transformer with a three-phase input to the primary, or vice versa. It can also be used to provide three-phase to three-phase transformation.

Secondary Winding: The transformer winding connected to the load or output side.

Sealed Dry Transformer: dry type core and coils in a tank containing an inert gas such as nitrogen or Freon under slight pressure.

Series/Multiple: A winding consisting of several sections, which can be connected for series or parallel operation.

Star Connection: See “Wye Connection”.

Step Down Transformer: One in which the high voltage winding is connected to the input or power source and the low voltage winding to the output or load.

Step Up Transformer: A transformer in which the low voltage winding is connected to the power source or input and the high voltage winding is connected to the output load.

Sudden Pressure Relay: Pressure switch device, which operates instantaneously to disconnect transformer from the line. Sudden pressure is usually caused by an internal transformer fault.


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

Tap: A connection provided in a transformer winding which has the effect of changing the nominal voltage ratio of the transformer. The taps are usually placed on the high voltage winding to correct for high or low voltage conditions found on the low voltage output side. Taps are expressed as either full capacity above nominal (FCAN) or full capacity below nominal (FCBN).

Temperature Indicator: Measures winding temperature. Has provision for resetting maximum temperature indication.

Temperature Rise: The temperature increase over ambient due to load. This is measured as either average rise by resistance or as hot-spot.

Thermal Overload Relay: Allows loading transformer to full thermal capacity by analyzing loads, temperatures and transformer time constant. Operates contacts for alarm and/or forced air cooling fans.

Transformer: A transformer is a static electrical device which by electro-magnetic induction, transfers electrical energy from one circuit to another circuit usually with changed values of voltage and current.

Turns Ratio: See “Ratio”.


Volt Amperes: The current flowing in a circuit multiplied by the voltage of that circuit. An expression of the output rating of a transformer.

Voltage Ratio: See “Ratio”.

Voltage Regulation: See “Regulation”.


Wye Connection: A three-phase connection in which similar ends of each phase winding are connected together at a common point which forms the electrical neutral and often grounded.


Zig-Zag Transformer: A transformer where the windings are physically manipulated to achieve certain voltage and current phase relationships. A zig-zag winding arrangement is commonly found in harmonic mitigating or grounding styles of transformers