• Transformer Terms and Definitions

    Improve your transformer vocabulary with our cheat sheet of 125+ terms and acronyms.

    AA — An Ansi (American National Standard Institute) cooling class designation indicating open, natural-draft ventilated transformer construction, usually for dry-type transformers.

    Air cooled —  A transformer that is cooled by the natural circulation of air around, or through, the core and coils.

    Ambient noise level — The existing or inherent sound level of the area surrounding the transformer, prior to energizing the transformer. Measured in decibels.

    Ambient temperature — The temperature of the air surrounding the transformer into which the heat of the transformer is dissipated.

    Ampacity — The current-carrying capacity of an electrical conductor under stated thermal conditions. Expressed in amperes.

    Ampere — The practical unit of electric current.

    ANSI — (American National Standards Institute) An organization that provides written standards on transformer.

    Attenuation — A decrease in signal power or voltage. Unit of measure is dB.

    Autotransformer — A transformer in which part of the winding is common to both the primary and the secondary circuits.

    Banked — Two or more single-phase transformers wired together to supply a three-phase load. Three single-phase transformers can be 'banked' together to support a three-phase load. For example, three 10 kVA single-phase transformers 'banked' together will have a 30 kVA three-phase capacity.

    BIL — Basic impulse level. The ability of a transformer's insulation system to withstand high voltage surges.

    BTU — British thermal unit. In North America, the term 'BTU' is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills and air conditioners. When used as a unit of power, BTU 'per hour' (BTU/h) is understood, though this is often abbreviated to just 'BTU.'

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    Buck-boost — The name of a standard, single-phase, two-winding transformer application with the low voltage secondary windings connected as an autotransformer for boosting (increasing) or bucking (decreasing) voltages in small amounts. Applications can either be single-phase or three-phase.

    Bushing — An electrical insulator (porcelain, epoxy, etc.) that is used to control the high voltage stresses that occur when an energized cable must pass through a grounded barrier.

    Cast-coil Transformer — A transformer with high-voltage coils cast in an epoxy resin. Usually used with 5 to 15 kV transformers.

    CE — Mark to indicate third-party approved or self-certification to specific requirements of the European community.

    Celsius — Celsius (centigrade): Metric temperature measure. °F = (1.8 x °C) + 32, °C = (°F-32) / 1.8

    Center tap — A reduced capacity tap at the mid-point of a winding. The center tap on three-phase delta-delta transformers is called a lighting tap. It provides 5% of the transformer's kVA for single-phase loads.

    Certified tests — Actual values taken during production tests and certified as applying to a given unit shipped on a specific order. Certified tests are serial number–specific.

    Copper Losses — See Load Losses.

    Core-Form Construction — A type of core construction where the winding materials completely enclose the core.

    Current Transformer — A transformer generally used in instrumentation circuits that measure or control current.

    Common mode — Electrical noise or voltage fluctuation that occurs between all of the line leads and the common ground, or between ground and line or neutral.

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    Compensated transformer — A transformer with a turns ratio that provides a higher than nameplate output (secondary) voltage at no load, and nameplate output (secondary) voltage at rated load. It is common for small transformers (2 kVA and less) to be compensated.

    Conductor losses — Losses (expressed in watts) in a transformer that are incidental to carrying a load: coil resistance, stray loss due to stray fluxes in the windings, core clamps, and the like, as well as circulating currents (if any) in parallel windings. Also called load losses.

    Continuous duty rating — The load that a transformer can handle indefinitely without exceeding its specified temperature rise.

    Core losses — Losses (expressed in watts) caused by magnetization of the core and its resistance to magnetic flux. Also called no-load losses or excitation losses. Core losses are always present when the transformer is energized.

    CSA — Canadian Standards Association. The Canadian equivalent of Underwriters Laboratories (UL).

    CSL3 — Candidate Standard Level 3 (CSL3) design criteria developed by the U.S. Department of Energy. This term is used when considering the maximum, practical efficiency of a transformer.

    cUL — Mark to indicate UL Certification to specific CSA Standards.

    Decibel — (dB) Unit of measure used to express the magnitude of a change in signal or sound level.

    Delta connection — 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. Sometimes referred to as three-wire.

    Delta Wye — A term or symbol indicating the primary connected in delta and the secondary in wye when pertaining to a three-phase transformer or transformer bank.

    Dielectric tests — Tests that consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdowns of insulating materials and spacings under normal conditions.

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    Distribution Transformers — Those rated 5 to 120 kV on the high-voltage side and normally used in secondary distribution systems. An aplicable standard is ANSI C-57.12.

    Dripproff — Constructed or protected so that successful operation is not interfered with by falling moisture or dirt. A transformer in which the transformer core and coils are not immersed in liquid.

    Dry-type transformer — A transformer in which the core and coils are in a gaseous or dry compound insulating medium. A transformer that is cooled by a medium other than a liquid, normally by the circulation of air.

    Eddy currents — The currents that are induced in the body of a conducting mass by the time variation of magnetic flux or varying magnetic field.

    Efficiency — The ratio of the power output from a transformer to the total power input. Typically expressed as a %.

    Electrostatic shield — Copper or other conducting sheet placed between primary and secondary windings, and grounded to reduce electrical interference and to provide additional protection from line-to-line or line-toground noise. Commonly referred to as 'Faraday shield.'

    Encapsulated transformer — A transformer with its coils either dipped or cast in an epoxy resin or other encapsulating substance.

    Enclosure — A surrounding case or housing used to protect the contained equipment against external conditions and prevent personnel from accidentally contacting live parts.

    Environmentally preferable product — A product that has a lesser or reduced negative effect on human health and the environment when compared to competing products that serve the same purpose. This comparison may consider raw materials acquisition, production, manufacturing, packaging, distribution, reuse, operation, maintenance and disposal of the product. This term includes recyclable products, recycled products and reusable products.

    EPACT — The Energy Policy Act of 1992 (EPAct) is an important piece of legislation for efficiency because it established minimum efficiency levels for dry-type distribution transformers manufactured or imported after December 2006. EPAct, which was based on NEMA standards, defined a number of terms, including what constitutes an energy-efficient transformer. The DOE issued a rule that defines these transformers and how manufacturers must comply. DOE EPAct rule (PDF): Energy Efficiency Program for Certain Commercial and Industrial Equipment: Test Procedures, Labeling, and the Certification Requirements for Electric Motors. Final Rule. 10-CFR Part 431.

    Excitation current — No load current. The current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is usually expressed in percent of the rated current of a winding in which it is measured. Also called magnetizing current.

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    FA — An ANSI cooling class designation indicating a forced air ventilated transformer, usually for dry type transformers and typically to increae the transformers and typically to increase the transformer's KVA rating above the natural ventilation or AA rating.

    Fan Cooled — Cooled mechanically to stay within rated temperature rise by addition of fans internally and/or externally. Normally used on large transformers only.

    FCAN — (Full Capacity Above Nominal) taps. Designates the transformer will deliver its rated kVA when connected to a voltage source which is higher than the rated primary voltage.

    FCBN — (Full Capacity Below Nominal) taps. Designates the transformer will deliver its rated kVA when connected to a voltage source which is lower than the rated primary voltage.

    FOA — An ANSI cooling class designation indicating forced oil cooling using pumps to circulate the oil for increased cooling capacity.

    FOW — An ANSI cooling class designation indicating forced oil water cooling using a separate water loop in the oil to take the heat to a remote heat exchanger. Typically used where air cooling is difficult such as underground.

    Frequency — On AC circuits, designates the number of times that polarity alternates from positive to negative and back again per second, such as 60 cycles per second. Typically measured in Hertz (Hz).

    Ground — Connecting one side of a circuit to the earth through low resistance or low impedance paths to help prevent transmitting electrical shock to personnel.

    Ground Strap — A Flat Strap of varying density, width and length to aid in the dissipation of High frequency noise, commonly generated by Switching Power Supplies, Lighting Ballasts, Inverters or Variable Frequency Drives.

    Harmonic — A sinusoidal waveform with a frequency that is an integral multiple of the fundamental frequency (60 Hz). 60 H3 fundamental, 120 H3 2nd harmonic, 180 H3 3rd harmonic, 240 H3 4th harmonic

    Harmonic distortion — Nonlinear distortion of a system characterized by the appearance of harmonic (non-sinusoidal) currents in the output, when the input is sinusoidal.

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    Harmonic distortion (total) — (THD) The square root of the sum of the squares of all harmonic currents present in a load, excluding the fundamental 60 Hz current. Usually expressed as a percent of the fundamental.

    High voltage windings — In a two-winding transformer, the winding intended to have the greater voltage. Usually marked with 'H' designations.

    HMT —  Harmonic Mitigating Transformer (HMT) is better able to handle the harmonic currents present in today's electrical power system. thereby increasing system capacity, reducing distortion throughout a facility, help to minimize downtime and 'mysterious' maintenance on equipment, and return the longevity of equipment life through reduced operational energy losses, thereby running cooler.

    Hp — Horsepower. The energy required to raise 33,000 pounds a distance of one foot in one minute. 1 hp is equal to 746 watts, or 0.746 kW.

    Hi-pot — A standard test on dry-type transformers consisting of extra-high potentials (voltages) connected to the windings. Used to check the integrity of insulation materials and clearances.

    Hottest-spot temperature — The highest temperature inside the transformer winding. Is greater than the measured average temperature of the coil conductors, when using the resistance change method.

    Hysteresis — The tendency of a magnetic substance to persist in any state of magnetization.

    Impedance — The retarding forces of current in an AC circuit; the current-limiting characteristics of a transformer. Symbol = Z

    Indoor transformer — A transformer that, because of its construction, is not suitable for outdoor service.

    Inductance — In electrical circuits, the opposition to a change in the flow of electrical current. Symbol = L

    Inducted potential test — A standard dielectric test of transformer insulation. Verifies the integrity of insulating materials and electrical clearances.

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    Inrush current — The initial high peak of current that occurs in the first few cycles of energization, which can be 30 to 40 times the rated current.

    Insulating transformer — Another term for an isolation transformer.

    Insulation — Material with a high electrical resistance.

    Insulation materials — Those materials used to insulate the transformer's electrical windings from each other and ground.

    Integral TVSS or SPD — Major Standard Change for Surge Protective Devices (formerly known as Transient Voltage Surge Suppressors). The primary safety standard for transient voltage surge suppressors (TVSS) has undergone major revisions in the past three years with mandatory compliance by manufacturers required by September 29, 2009. Even the name of the standard has changed from UL Standard for Safety for Transient Voltage Surge Suppressors, UL 1449 to UL Standard for Safety for Surge Protective Devices, UL 1449. This means that TVSS listed to the UL 1449 2nd Edition standard will no longer be able to be manufactured after September 29, 2009. All Surge Protective Devices must be designed, tested, manufactured and listed to the UL 1449 3rd Edition standard after this date.

    Isolation transformer — Transformers designed to provide electrical isolation between the primary and secondary windings without stepping voltage and current either up or down.

    K-factor — A common industry term for the amount of harmonics produced by a given load. The larger the K-factor, the more harmonics that are present. Also used to define a transformer's ability to withstand the additional heating generated by harmonic currents.

    kVA — Kilovolt-ampere. Designates the output that a transformer can deliver for a specified time at a rated secondary voltage and rated frequency without exceeding the specified temperature rise. When multiplied by the power factor, will give kilowatts or kW. 1000 VA = 1 kVA

    Lamination — Thin sheets of electrical steel used to construct the core of a transformer.

    Limiting temperature — The maximum temperature at which a component or material may be operated continuously with no sacrifice in normal life expectancy.

    Linear load — A load where the current waveform conforms to that of the applied voltage, or a load where a change in current is directly proportional to a change in applied voltage.

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    Liquid-immersed Transformer — A transformer with the core and coils immersed in liquid (as opposed to a dry-type transformer).

    Live part — Any component consisting of an electrically conductive material that can be energized under conditions of normal use.

    Load — The amount of electricity, in kVA or volt-amperes, supplied by the transformer. Loads are expressed as a function of the current flowing in the transformer, and not according to the watts consumed by the equipment the transformer feeds.

    Load losses — I2R losses in windings. Also see conductor losses.

    Low voltage winding — In a twowinding transformer, the winding intended to have the lesser voltage. Usually marked with 'X' designations.

    Mid-tap — See center tap.

    Moisture-resistant — Constructed or treated so as to reduce harm by exposure to a moist atmosphere.

    Natural-draft or Natural-draft Ventilated — An open transformer cooled by the draft created by the chimney effect of the heated air in its enclosure.

    Noise level — The relative intensity of sound, measured in decibels (dB). NEMA Standard ST-20 outlines the maximum allowable noise level for dry-type transformers.

    Nonlinear load — A load where the current waveform does not conform to that of the applied voltage, or where a change in current is not proportional to a change in applied voltage.

    Non-ventilated transformer — A transformer where the core and coil assembly is mounted inside an enclosure with no openings for ventilation. Also referred to as totally enclosed non-ventilated (TENV).

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    No load losses — Losses in a transformer that is excited at rated voltage and frequency but that is not supplying a load. No load losses include core losses, dielectric losses and conductor losses in the winding due to the exciting current. Also referred to as excitation losses.

    Overload capability — Short-term overload capacity is designed into transformers as required by ANSI. Continuous overload capacity is not deliberately designed into a transformer because the design objective is to be within the allowed winding temperature rise with nameplate loading.

    OA — An ANSI cooling class designation indicating an oil filled transformer.

    Parallel Operation — Single and three-phase transformers having appropriate terminals may be operated in parallel by connecting similarly-marked terminals, provided their ratios, voltages, resistances, reactances, and ground connections are designed to permit paralleled operation and provided their angular displacements are the same in the case of three-phase transformers.

    Percent IR — (% resistance) Voltage drop due to resistance at rated current in percent of rated voltage.

    Percent IX — (% reactance) Voltage drop due to reactance at rated current in percent of rated voltage.

    Percent IZ — (% impedance) Voltage drop due to impedance at rated current in percent of rated voltage.

    Phase — Type of AC electrical circuit; usually single-phase two- or three-wire, or three-phase three- or four-wire.

    Polarity test — A standard test on transformers to determine instantaneous direction of the voltages in the primary compared to the secondary.

    Primary taps — Taps added to the primary (input) winding. See Tap.

    Primary voltage — The input circuit voltage.

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    Primary Winding — The transformer winding located on the energy input (supply) side.

    Poly-phase — More than one phase.

    Potential (Voltage) Transformer — A transformer used in instrumentation circuits that measure or control voltage.

    Power factor — The cosine of the phase angle between a voltage and a current.

    Primary Taps — Taps added in the primary winding (see Tap).

    Rating — The output or input and any other characteristic, such as primary and secondary voltage, current, frequency, power factor and temperature rise assigned to the transformer by the manufacturer.

    Ratio test — A standard test of transformers to determine the ratio of the input (primary) voltage to the output (secondary) voltage.

    Reactance — The effect of inductive and capacitive components of a circuit producing other than unity power factor.

    Reactor — A single winding device with an air or iron core that produces a specific amount of inductive reactance into a circuit. Normally used to reduce of control current.

    Regulation — Usually expressed as the percent change in output voltage when the load goes from full load to no load.

    Sealed Transformer — A transformer completely sealed from outside atmosphere and usually contains an inert gas that is slightly pressurized.

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    Secondary Taps — Taps located in the secondary winding (see Tap).

    Secondary Winding — The transformer winding located on the energy output (load) side.

    Secondary Voltage Rating — Designates the load-circuit voltage for which the secondary winding (winding on the output side) is designed.

    Scott T connection — Connection for three-phase transformers. Instead of using three sets of coils for a three-phase load, the transformer uses only two sets of coils.

    Series/multiple winding — A winding consisting of two or more sections that can be connected for series operation or multiple (parallel) operation. Also called series-parallel winding.

    Shell-type Construction — A type of transformer construction where the core completely surrounds the coil.

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

    Sound levels — All transformers make some sound mainly due to the vibration generated in its core by alternating flux.

    Star connection — Same as a wye connection.

    Step-down transformer — A transformer where the input voltage is greater than the output voltage.

    Step-up transformer — A transformer where the input voltage is less than the output voltage.

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    T-Connection — Use of Scott Connection for three-phase operation. A connection brought out of a winding at some point between its extremities, usually to permit changing the voltage or current ratio.

    T-T connection — See Scott T connection.

    Tap — A connection brought out of a winding at some point between its extremities, usually to permit changing the voltage or current ratio. Taps are typically used to compensate for above or below rated input voltage, in order to provide the rated output voltage. See FCAN and FCBN.

    Temperature class — The maximum temperature that the insulation system of a transformer can continuously withstand. The common insulation classes are 105, 150, 180 (also 185) and 220.

    Temperature rise — The increase over ambient temperature of the windings due to energizing and loading the transformer.

    Total losses — The sum of the no-load losses and load losses.

    Totally enclosed non-ventilated enclosure — The core and coil assembly is installed inside an enclosure that has no ventilation to cool the transformer. The transformer relies on heat to radiate from the enclosure for cooling.

    Transformer — An electrical device, without continuously moving parts, which, by electro-magnetic induction, transforms energy from one or more circuits to other circuits at the same frequency, usually with changed values of voltage and current.

    Transformer tests — Per NEMA ST-20, routine transformer production tests are performed on each transformer prior to shipment. These tests are: Ratio tests on the rated voltage connection; Polarity and Phase Relation tests on the rated connection; No-Load and Excitation Current tests at rated voltage on the rated voltage connection and Applied Potential and Induced Potential tests. Special tests include sound level testing.

    Transverse mode — Electrical noise or voltage disturbance that occurs between phase and neutral, or from spurious signals across metallic hot line and the neutral conductor.

    Turns ratio — The ratio of the number of turns in the high voltage winding to that in the low voltage winding.

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    Typical test data — Tests that were performed on similar units that were previously manufactured and tested.

    UL — (Underwriters Laboratories) An independent safety testing organization.

    Universal taps — A combination of six primary voltage taps consisting of 2 at +2-1/2% FCAN and 4 at -2-1/2% FCBN.

    Volt-amperes — Circuit volts multiplied by circuit amperes.

    Voltage Ratio — The ratio of the RMS primary terminal voltage to the RMS secondary terminal voltage under specified conditions of load.

    Voltage Regulation — The change in secondary voltage that occurs when the load is reduced from rated value to zero, with the values of all other quantities remaining unchanged. The regulation may be expressed in percent (or per unit) on the basis of the rated secondary voltage at full load.

    Watt — A unit of electrical power when the current in a circuit is one ampere and the voltage is one volt.

    Winding Losses — See Load Losses.

    Winding Voltage Rating — Designates the voltage for which the winding is designed.

    Wye connection — A standard three-wire transformer connection with similar ends of single-phase coils connected together. The common point forms the electrical neutral point and may be grounded. Also referred to as three-phase four-wire. To obtain the line-to-neutral voltage, divide the line voltage by √3(1.732).


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