Alternating Current (AC) An electrical current flow of continuously changing polarity, which rises to a maximum voltage in the other direction before changing polarity once again. This pattern is referred to as a sinusoidal wave and the number of cyucles 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 isolation.

Core The ferrous center part of a transformer or inductor used to increase the strength of the magnetic field.

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.

Delta A standard three phase connection with the ends of each phase winding connected 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 Shielding Placed between windings (usually the primary and secondaries) to provide maximum isolation Additional Electrostatic Shields can be placed between secondary windings as required. Shielding is normally connected to the transformer’s core (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 (Iex) The current drawn by a transformer at nominal input voltage in its unloaded (open-circuit) condition.

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. *See Electrostatic Shielding.

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 resistors, inductors, or capacitors which offers comparatively little opposition to certain frequencies or direct current, while blocking or attenuating other frequencies.

Flux The lines of force of a magnetic field.

Ground Fault Any undesirable current path from a current carrying conductor to ground.

Impedance (z) Forces, including resistance and capacitive or inductive reactance, which resist current flow in A.C. circuits.

Inductance (L) 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 a DC 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.

Isolation Transformer A transformer with primary and secondary windings physically separated by design to permit magnetic coupling between isolated circuits while minimizing electrostatic coupling.

Linear Load A load in which the relationship between current and voltage is directly proportional.

Mangetic Shielding Conductive material placed around a transformer’s coils to attenuate stray magnetic fields.

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.

Non-Linear Load A load in which the relationship between current and voltage is not directly proportional.

Ohm’s Law The relationship between voltage (EMF), 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.

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

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 power.

Rated Power The total output power available from all secondary windings, expressed in Voltamperes (VA) or Kilovoltamperes (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.

Reactor Also called a line reactor, it is an electronic component made up of at least one inductor element wired between a power source and a electric load. Often used to oppose quick changes in current.

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 or open-circuit and full load conditions.

Scott -T Transformer A transformer most commonly used to transfer old 2-phase power into 3-phase, or 3-phase power into 2-phase.

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 Power (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.

Taps (or Voltage Taps) Additional connections to a 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 maximum 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 insure adequate insulation performance. Normally applied between a winding and all other windings, and between a winding and ground. Also known as the Dielectric Withstanding Voltage and the Hipot Voltage.

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 A static electrical device, which, by electromagnetic induction, regenerates A.C. 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).

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.

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 polyphase 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.

Zigzag Transformer Transformers most commonly used for grounding purposes where one is not available. Also used to create a vector phase shift, trapping triplen harmonic currents (3rd, 9th, 15th, and 21st).