Glossary of Transformer Terms

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 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.

Efficiency- The percentage of input power available for use by the load.

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 - 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.

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 force 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.

Ground Fault- Any undesirable current path 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.

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 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.

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.

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.

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(DT)- 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 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).

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.