Electrical transformer glossary
Technical glossary of the key terms relating to electrical transformers. Written by the ABL Transfo engineers, a French manufacturer since 1993.
Understanding the vocabulary of the electrical transformer
Whether you are an electrician, a design office or a knowledgeable individual, this glossary gives you access to precise definitions of the technical terms encountered in the sizing and use of electrical transformers. Each term comes with a practical note drawn from our experience as a manufacturer.
Apparent power (kVA)
The rated power (S = U·I) that sizes the transformer.
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ProtectionBimetallic strip
Thermal protection device with automatic tripping
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ComponentsCoil
Inductive component made of windings of conductive wire
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ProtectionCooling method (ONAN/AN/AF)
The method of dissipating heat (air or oil, natural or forced).
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ComponentsCurrent density
The amps per mm² in the conductor, key to sizing the windings.
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ProtectionDielectric strength
The withstand voltage of an insulator before breakdown.
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Losses & efficiencyEddy currents
The currents induced in the core, limited by the lamination of the sheets.
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Losses & efficiencyEfficiency
The ratio of useful power to absorbed power, often above 95 %.
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Electrical engineeringElectromagnetic induction
Faraday's law: the physical principle that makes every transformer work.
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Electrical engineeringElectromotive force (EMF)
The voltage induced in a coil by the change of magnetic flux.
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Electrical engineeringFrequency (50 / 60 Hz)
The network frequency, which directly affects flux and losses.
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ProtectionGalvanic isolation
The total electrical separation between the primary and the secondary.
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Losses & efficiencyHysteresis losses
The energy dissipated at each magnetisation cycle of the core.
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Electrical engineeringImpedance
Opposition of a circuit to alternating current
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Electrical engineeringInrush current
The current peak at switch-on, up to 8 to 15 times the rated current.
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Power qualityK-factor
The oversizing of a transformer intended for harmonic loads.
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Electrical engineeringLeakage inductance
The uncoupled flux between windings, the source of the impedance.
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Losses & efficiencyLoad losses (copper losses)
The Joule losses in the windings, which vary with the load.
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ComponentsMagnetic core
Central element that channels the magnetic flux
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MagnetismMagnetic flux
The magnetic field channelled by the core, the carrier of coupling between windings.
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MagnetismMagnetic flux density (Tesla)
The flux density (B) that sets the operating point of the core.
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MagnetismMagnetic permeability
A material's ability to conduct and channel magnetic flux.
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MagnetismMagnetic saturation
The limit beyond which the core can no longer channel the flux.
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MagnetismMagnetising current
The no-load current that establishes the flux in the core.
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Losses & efficiencyNo-load losses (iron losses)
The constant losses in the core: hysteresis and eddy currents.
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Electrical engineeringPower factor (cos φ)
The voltage/current phase shift, the link between apparent and active power.
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Electrical engineeringPrimary current
Current flowing in the primary winding
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Electrical engineeringPrimary voltage
Voltage applied to the input winding
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MagnetismReluctance
The "resistance" of the magnetic circuit to the passage of flux.
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Electrical engineeringSecondary current
Current flowing in the secondary winding
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Electrical engineeringSecondary voltage
Voltage induced at the transformer output
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ComponentsTapping (tap)
The ±2.5 / ±5 % taps to adjust the voltage to the real network.
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Power qualityTHD (total harmonic distortion)
The overall indicator of the harmonic pollution of an electrical signal.
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ProtectionThermal insulation class (B/F/H)
The maximum permissible temperature of the insulation (130 / 155 / 180 °C).
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ComponentsThree-phase vector group (Dyn11, Yd…)
The star/delta connection of the windings and the clock index.
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ComponentsTurn (number of turns)
One turn of winding; the N1/N2 ratio sets the transformation.
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Electrical engineeringTurns ratio
The ratio n = U1/U2 = N1/N2 between primary and secondary.
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Electrical engineeringVoltage regulation
The voltage difference between no-load and rated-load operation.
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Is a term missing from this glossary?
This glossary is regularly expanded. For any technical question about transformers, our engineers will reply within 24 hours.
