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

Auto Transformer: A transformer used to step voltage up or down. The primary and secondary windings share common turns and thus provide no electrical isolation.

ANSI: 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.

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

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).

Arc voltage: The amount of voltage present between electrodes of different potential or between an electrode and ground. The magnitude is determined by the distance between electrodes and the dielectric constant of the medium surrounding them.


BIL Basic Insulation level: It is an insulation system that can withstand very high voltage surges.

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


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

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.

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.

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.


Double conversion: A UPS design in which the primary power path consists of a rectifier and inverter.

Dropout voltage: The voltage at which a device fails to operate properly or safely. Computer systems will reboot, reset, or lose data.

Delta: 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.

Delta-Delta: The connection between a delta source and a delta load.

Delta-Wye: The connection between a delta source and a wye load.

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.


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.

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.

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

Efficiency: Ratio of its power output to its total power input

Excitation Current: Current required magnetizing a core..

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.

Encapsulation: A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally performed when a unit might encounter harsh environmental conditions such as moisture, salt spray, full-water submersion or corrosive elements.

Exciting Current: The current drawn by a transformer at nominal input voltage in its unloaded (open-circuit) condition.


FCAN Taps: 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.

Filter Press: A device for filtering and absorbing moisture from oil.

Frequency: It means the number of times an AC voltage will change from positive to negative and vice versa within a precise time, usually expressed in cycles per second and identified as Hz as in 60 Hz.

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.

Filtered: Removing the ripple effect caused by a rectifier. Can also refer to reduced non-sinusoidal or unwanted harmonic frequencies in a power sources.

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.

Ferroresonance: 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.

Ferroresonant Transformer: A voltage-regulating transformer that depends on core saturation and output capacitance.

Filter: A selective network of resistor, inductors, or capacitors which offers comparatively little opposition to certain frequencies or direct current, while blocking or attenuating other frequencies.

Flux: The lines of forces of a magnetic field.

Forced Air: A method of temperature regulation that involves air from an external environment being forcibly exchanged with a transformer’s enclosed environment.


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

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

Grounded: Connected to 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.


Horsepower (HP): One horsepower is 33,000 lb.-ft /minute or 746 watts

H type core: It is one type of transformer core that surrounds the windings on four sides. This helps lessen flux leakage.

Hydroelectric: Electricity produced by turbines that are turned by water flow.

Hertz (Hz): Cycles per second


Isolating Transformer: Transformer in which input windings are connected to the line and are completely isolated from those connected to the load.

Insulation: Material with high electrical resistance.

Insulator: Device used for supporting or separating electrical conductors

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.

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

Inductance: 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.

Inductor: 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.

Inverter: A device used to change DC into AC power.


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


Kilowatt (KW): 1,000 Watts

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

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

K-Factor: 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.


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

Line voltage: Voltage of a power line

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


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.


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

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.

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.

Nonlinear load: A load in which the relationship between current and voltage is not directly proportional.

Network Transformer: 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.

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

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

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.


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.

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

Overload: 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.


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.

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.

Phase: 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.


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

Reactance: 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.

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

Regulation: 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.


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

SCR: (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).

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

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.

Sinusoidal Waveform: A waveform that can be expressed mathematically by using the sine function.

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

Step up/step-down transformers: A step-up transformer is one in which the output voltage is greater than input voltage. In a step-down transformer, the input voltage is greater than the output voltage.


Transformer Bank / Bank of transformers: Two or more single-phase transformers connected together to supply a three-phase load.

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.

Transformer: An electrical device, which, by electromagnetic induction, regenerates AC power from one circuit into another. Transformers are also used to change voltage from one level to another. This is accomplished by the ratio of turns on the primary to turns on the secondary (turns ratio).

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

Transformer Regulation: The percentage difference between voltages at the secondary terminals under no-load 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.

Transient: A high amplitude, short duration pulse superimposed on the normal voltage wave form or ground line.


Voltage Regulation: Maintaining stability of output voltage under conditions of fluctuating input voltage.

Ventilated: Providing circulation of external air.

Ventilated Enclosure: Ventilated Enclosure allows air to flow directly over the core and coil assembly for cooling.

Volt-Amperes: Transformer capacity is rated in volt-amperes (the product of volts and amperes in the input winding). Capacities of very large transformers are rated in thousands of volt-amperes (kVA) and in millions of volt-amperes (MVA). Input VA is equal to output VA.


Working Voltage: The voltage that a winding will operate at, but not necessarily the output voltage of the winding.

Wye: A wye connection refers to a three-phase electrical supply where the source transformer has the conductors connected to the terminals in a physical arrangement resembling a Y. Each point of the Y represents the connection of a hot conductor. The angular displacement between each point of the Y is 120 degrees. The center point is the common return point for the neutral conductor.

Watt: Unit of electrical power when the current in the circuit is one ampere and the voltage is one volt (for DC) and for AC, even the p.f. should be unity.

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.