Find all your transformer-related terms in one place and save research time. Whether you want to verify definitions for compliance purposes or maintain consistent language across your organization’s technical documents, this transformer glossary serves as a reliable resource for all your needs.
A
Air-Cooled Transformer
An air-cooled transformer is an electrical device that uses natural air rather than liquid as its primary cooling medium, eliminating leaks and minimizing fire hazards.
Alternating Current (AC)
AC is an electric current that frequently reverses direction instead of flowing in one direction like a direct current (DC). The current’s magnitude changes constantly with time between positive and negative values, typically following a sinusoidal pattern.
Ambient Noise Level
Ambient noise level is the background sound pressure level at a transformer’s location — measured in decibels (dB) with a calibrated sound level meter — before energization. It establishes the baseline for compliance with regulatory noise levels.
Ambient Temperature
Ambient temperature is the normal temperature of the air surrounding a transformer, which impacts its cooling efficiency and operational capacity.
ANSI
ANSI, or the American National Standards Institute, is an organization that provides transformer manufacturing, testing and performance standards to ensure interoperability and reliability.
B
Basic Insulation Level (BIL)
BIL is a standardized measure of a transformer’s ability to withstand transient overvoltage events such as lightning strikes or switching surges without insulation failure. It establishes the required surge arrester ratings and is validated via impulse testing.
C
Core
The core is the laminated steel or iron magnetic circuit that provides a low-reluctance path for flux linkage between transformer windings. It forms magnetic coupling between the primary and secondary, directly impacting the transformer’s voltage transformation capability, efficiency and magnetic field strength.
Core Loss
Core or iron loss is the dissipated energy in the core due to eddy currents and hysteresis when a transformer is energized at rated voltage.
Core Saturation
Core saturation is a condition in which the core’s magnetic flux density reaches its maximum limit due to overvoltage, faults and inrush. It can potentially damage the windings and cause overheating and sharp current spikes.
Current Transformer (CT)
A current transformer steps down high primary currents to standardized, measurable levels like 1A or 5A for safety, accurate metering and system interoperability.
D
Delta
Delta is a transformer winding configuration that connects three coils end-to-end to form a closed loop, with each phase 120 degrees apart.
Delta-Delta
Delta-delta is a transformer with both primary and secondary windings connected in a delta configuration. There is no phase shift between the primary and secondary.
Delta-Wye
Delta-wye is a transformer configuration with delta primary and wye secondary windings. It transforms voltage with a 30-degree phase shift.
Drive Isolation Transformer
A drive isolation transformer powers variable frequency drives (VFDs) for improved insulation. It safeguards drives from transients and voltage spikes.
Dry Type Transformer
A transformer that uses air circulation for cooling — natural convection or forced by fans — with solid insulation instead of liquid, which reduces fire and leak risks.
Duty Cycle
Duty cycle is a transformer’s operational profile, defining the sequence, magnitude and duration of loads — including overloads and idle periods — it can handle without surpassing thermal limits.
E
Eddy Currents
Eddy currents are circulating currents induced within conductive materials, like a transformer’s core, by changing magnetic fields.
Effective Voltage or Current
Effective voltage is the DC equivalent of an AC waveform that provides the same power to a resistive load. It standardizes AC measurements for accurate power calculations.
Efficiency
Efficiency is the ratio of output power to input power, accounting for core and copper losses. It is expressed as a percentage and directly affects operational expenses.
Electrostatic Shield
An electrostatic shield is a grounded conductive layer between windings that blocks capacitive coupling of high-frequency noise. It prevents transient voltage propagation and minimizes electromagnetic interference (EMI) in sensitive equipment.
Electrostatic Shielding
Electrostatic shielding is a technique of leveraging grounded conductive shields between transformer windings to isolate circuits from electric field interference.
Exciting Current
Exciting current measures the electric flow that the primary winding draws when the secondary is open-circuited. It magnetizes the core and is expressed as a percentage of a winding’s rated current.
F
Fan-Cooled
Fan-cooled means using external or internal fans to force air over a transformer to dissipate heat.
Faraday Shield
The Faraday shield is a grounded, conductive barrier positioned between transformer windings that minimizes capacitive noise coupling. It has little to no effect on magnetic fields.
Faraday’s Law
A law that states that a changing magnetic field induces a voltage in a conductor. The magnitude of the induced voltage varies directly with the rate of magnetic flux changes and the number of turns.
Ferroresonance
Ferroresonance is a nonlinear resonance phenomenon in iron-core transformers that causes erratic voltages and currents.
Frequency
Frequency equals the number of cycles an alternating current makes per second, measured in Hz.
Full Capacity Above Nominal (FCAN) Taps
FCAN taps are transformer settings that provide rated output above the nominal input voltage, compensating for voltage declines in long-distance lines and preventing overloads.
Full Capacity Below Nominal (FCBN) Taps
FCBN taps are transformer settings that maintain rated output below the nominal input voltage, while still ensuring the transformer operates at its rated kVA capacity.
G
Ground
A ground is an accidental or intentional conducting connection between an electrical circuit and the earth or a common reference plane that ensures a safe path for fault currents, preventing electric shocks and equipment damage.
Ground Fault
A ground fault is an unintentional electrical path between a live conductor and a grounded surface.
Ground Fault Circuit Interrupter (GFCI)
A GFCI is a device that disconnects a circuit when it detects a current imbalance between hot and neutral conductors, protecting people from shock in wet or damp areas.
Grounded
Grounded refers to a system that has a neutral conductor connected to the earth or another conductor to limit voltage surges.
H
High-Voltage Windings
High-voltage windings are primary or secondary coils that operate at high voltages, and they are identified by the letter “H.”
Hertz (Hz)
Hertz is the unit of measurement for frequency, which is equal to one cycle per second.
Horse Power (HP)
HP is a unit that indicates mechanical power. 1HP ≈ 746W.
Hydroelectric
Hydroelectric means generating electricity using flowing water to drive turbines.
I
Impedance
Impedance is the total opposition to AC current (vector sum of resistance + reactance) represented as a percentage of the rated voltage at full load.
Inductance
Inductance refers to a conductor or coil that resists changes in current. It determines reactance and impedance.
Inductor
An inductor is a passive component that provides inductance.
Inrush Current
Inrush current is a temporary surge that occurs in a transformer when an electrical device first turns on.
J
Jack Pads
Jack pads are reinforced mounting points for leveling or lifting transformers to avoid structural damage during installation.
K
K-Factor
K-factor is a rating from one to 50, indicating a transformer’s ability to handle the heating effects of harmonic currents.
Kilovolt-Ampere (kVA)
kVA is a unit of apparent power. 1kVA = 1,000VA.
Kilowatt (kW)
kW is a unit of real power. 1kW = 1,000W.
Kilowatt-Hour (kWh)
kWh is a unit of energy. 1kWh = 1kW for 1 hour
L
Lamination
Lamination refers to thin, insulated layers of cold-rolled grain-oriented (CRGO) silicon steel stacked to create a transformer core.
Line or Line-to-Line Voltage
Line voltage is the root-mean-square voltage measured between any two phase conductors in a power system. It determines a transformer’s insulation class, BIL rating and primary windings specifications.
Linear Load
A load where the current waveform is sinusoidal and proportional to voltage.
Load
Load measures the electrical power level (kVA or VA) that devices or circuits connected to a transformer draw.
Load Losses
Load or copper losses happen due to resistance in a transformer’s windings.
Low-Voltage Windings
Low-voltage windings are transformer coils that operate at low voltages. They’re identified by the letter “X.”
M
Magnetic Shielding
Magnetic shielding is the process of diverting stray magnetic flux from sensitive areas using a high-permeability material.
Multiple Winding
A multiple winding comprises more than two electrically isolated windings. It provides voltage flexibility and isolates sensitive loads from noise.
N
Natural Convection
Natural convection is the process of cooling a transformer via the fluid movement of oil or air without mechanical fans.
NEMA Enclosure
Standardized housing ratings for environmental protection by the National Electrical Manufacturers Association (NEMA).
NEMA Standard
U.S. guidelines for electrical equipment to ensure interoperability and safety.
No-Load Current
The current drawn when a transformer is energized at the rated voltage with an open secondary. A high no-load current could indicate core damage.
No-Load Losses
No-load or excitation losses occur when a transformer is energized but not supplying a load, mainly due to core magnetization.
Nominal Voltage
Nominal voltage is the standard system voltage.
O
Oil-Immersed, Air-Cooled (OA) Transformer
An OA transformer uses oil to cool and insulate itself. The absorbed heat dissipates to the surrounding air through its external surfaces.
Ohm’s Law
Ohm’s Law states that the current traveling through a conductor between two points is directly proportional to the voltage across those two points. This relationship is expressed as: V = I×R (voltage = current × resistance).
Oscillation
Oscillation is the cyclic variation in voltage or current within a circuit due to resonance or switching.
Overload
Overload refers to a transformer’s operating above the rated capacity.
P
Padmount Transformer
A padmount transformer is a tamper-resistant, ground-mounted electrical transformer, generally part of an underground distribution system, that steps down high-voltage electricity to a voltage ideal for businesses, homes and industries. It can be dry type or oil-filled, depending on whether it’s needed for indoor or outdoor applications.
Parallel Operation
Parallel operation means running two or more transformers connected to the same primary and secondary circuits to share the load.
Peak Voltage
The maximum voltage attained during a single waveform cycle.
Phase
Phase is the angular separation between AC waveforms. Transformers can be single- or three-phase.
Power Factor
Power factor is the ratio of real power (kW) to the apparent power (kVA) in an AC circuit. A power factor of 1 indicates correct phase alignment between voltage and current, while a lower power factor means more current is required to deliver the same amount of useful work, increasing losses and decreasing the transformer’s effective capacity.
Primary Winding
The primary winding is a coil in a transformer connected to the input voltage source.
R
Rated Power
Rated power is the maximum power a transformer can safely deliver under normal operating conditions.
Ratio Test
A ratio test verifies the primary to secondary voltage ratio to ensure correct transformer design.
Reactance
Reactance is the opposition to AC flow due to inductance and capacitance, and is measured in ohms.
Rectifier
A rectifier is a device that converts AC to DC.
Regulation
Regulation is the percentage voltage drop from no-load to full-load
Resin-Filled Construction
Resin-filled construction is a dry type transformer with windings encapsulated in epoxy resin to protect against moisture and contaminants.
S
Secondary Windings
In a transformer, a secondary winding is a coil of wire that receives electrical energy from the primary winding through electromagnetic induction. It’s the output winding connected to the load, delivering transformed voltage or current.
Shielding
Shielding refers to a conductive barrier — a Faraday or electrostatic barrier — placed between windings. It minimizes EMI and radio frequency interference (RFI) for a cleaner output voltage and better signal integrity in sensitive applications.
Short Circuit
A short circuit is an unintended low-resistance path in an electric circuit that causes excessive current.
Single-Phase Transformer
A single-phase transformer is an electrical device with one primary and one secondary winding. The AC in the primary coil builds a varying magnetic field that induces a voltage in the secondary, transferring power.
Step-Down Transformer
A step-down transformer transfers energy from the high-voltage primary winding to the low-voltage secondary.
Step-Up Transformer
A step-up transformer transfers electric energy from the low-voltage primary winding to the high-voltage secondary.
T
Taps or Voltage Taps
Voltage taps are connections along a winding that enable voltage output adjustment to compensate for grid input variations.
Tap Changer
A tap changer adjusts a transformer’s output voltage by altering the number of turns in one winding.
Temperature Rise
Temperature rise is the allowable winding or oil temperature increase above the ambient temperature.
Test Potential
Test potential is the applied voltage during dielectric tests. It confirms insulation integrity according to the Institute of Electrical and Electronics Engineers (IEEE) standards.
Three-Phase Transformer
A three-phase transformer is an electrical device with three primary and secondary delta or wye windings.
Total Losses
Total losses equal load losses plus no-load losses in a transformer.
Transformer
A transformer transfers electrical energy between two or more circuits at the same frequency through electromagnetic induction. It increases or decreases the voltage in power systems.
Transformer Coils or Windings
Transformer windings or coils are generally copper or aluminum conductors wound around a core to create inductance.
Turns Ratio
Turns ratio is the ratio of the number of turns in the primary to the number of turns in the secondary.
V
Volt-Amperes (VA)
Volt-amperes, the unit of apparent power, equals voltage multiplied by current.
Voltage Ratio (of a Transformer)
Voltage ratio is the primary voltage to secondary voltage ratio, which is approximately equal to the turns ratio in an ideal transformer.
W
Weather Shield
A weather shield is a protective cover for outdoor transformer bushings.