Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

AC coupling
A method of connecting two circuits that allows displacement current to flow while preventing conductive currents.
Reactive impedance devices (e.g., capacitors and inductive transformers) are used to provide continuity of alternating current flow between two circuits while simultaneously blocking the flow of direct current.
active power

See real power
active power line conditioner

a device which senses disturbances on a power line and injects compensating voltages or currents to restore the line's proper waveform.

adaptive control
a control methodology in which control parameters are continuously and automatically adjusted in response to be measured/estimated process variables to achieve near-optimum system performance.
additive polarity
polarity designation of a transformer in which terminals of the same polarity on the low-and high-voltage coils are physically adjacent to each other on the transformer casing. With additive polarity, a short between two adjacent terminals results in the sum of the two coil voltages appearing between the remaining terminals. Additive polarity is generally used for transformers up to 500kVA and 34.5kV. Larger units use subtractive polarity.
See subtractive polarity
adjustable-speed drive

See variable speed drive
air capacitor

a fixed or variable capacitor in which air is the dielectric material between the capacitor's plates.

air-gap line
the line that is obtained by continuing the linear portion of the saturation curve of a synchronous machine or a DC machine. The figure shows a plot of generated voltage vs. field current at constant machine speed. Initially, an increase in field current yields a linear increase in the generated voltage, but as the iron becomes saturated, the voltage rolls off. The air-gap line gives the voltage that would be obtained without saturation.
air-gap voltage
the internal voltage of a synchronous machine that is generated by the air gap flux. Also referred to as the voltage behind leakage reactance.
ambient temperature
the temperature of the air or liquid surrounding any electrical part or device. Usually refers to the effect of such temperature in aiding or retarding removal of heat by radiation and convection from the part or device in question.
the maximum current which can be safely carried by a conductor under specified conditions.
ampere interrupting rating
the interrupting rating of a device expressed in amps (often rms symmetrical amps).
See MVA interrupting rating
a special generator that acts like a DC power amplifier by using compensation coils and a short circuit across its brushes to precisely and fastly control high powers with low level control signals.
a particle having the same mass as a given fundamental particle, but whose other properties, while having the same magnitude, may be of opposite sign. Each particle has a partner called an antiparticle. For example, electrical charge in the case of the electron and positron, magnetic moment in the case of the neutron and antineutron. On collision a particle and its antiparticle may mutually annihilate with the emission of radiation. Some properties of the antiparticle will be identical in magnitude but opposite in sign to the particle it is paired with.
symmetry created by simultaneously mirroring an object in both the X and Y axes.
aperiodic waveform
a phrase is used to describe a waveform that does not repeat itself in a uniform, periodic manner. Compare with periodic waveform.
(1) an opening to a cavity, or wave-guide, from which radiation is either received or transmitted. Typically used as antenna or a coupling element.
(2) a physical space available for beam to occupy in a device. Aperture limitations are the physical size of the vacuum chamber; a magnetic field anomaly may deflect the beam so that the full available aperture cannot be used.
aperture antenna
an antenna with a physical opening, hole, or slit.
apparent power
(1) in an AC system, the product of voltage, Eand current, I. Apparent power (or total power) is composed of two mutually independent components -- an active component (real power), and a reactive component (imaginary power). Apparent power is denoted by S, and has the unit of voltamperes.
(2) the scalar product of the voltage and current delivered to the load. It can also be expressed as the vector S = P + jQ, where P = real power and Q = reactive power.
arc fault interrupter
the mechanism that breaks the fault current arc in a power circuit breaker.