Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.

active RC filter
an electronic circuit made up of resistors, capacitors, and operational amplifiers that provide well-controlled linear frequency-dependent functions, e.g., low-, high-, and bandpass filters.

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.
See analog-to-digital converter
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
the reciprocal of the impedance of an electric circuit.

air capacitor

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

air circuit breaker
a power circuit breaker where the power contacts operate in air. Some versions employ an air blast to extend and clear the arc on contact opening, while others employ arc chutes with magnetic or thermal assists.
air core transformer
two or more coils placed so that they are linked by the same flux with an air core. With an air core the flux is not confined.
air terminal
a lightning rod; any device which extends upward into the air from a structure for purposes of lightning protection.
air-blast circuit breaker
a circuit breaker in which the arc which forms between the contacts on opening is extinguished with a blast of high-pressure air.
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.
alley arm
a crossarm meant for use in an alleyway or other confined area in which poles must be placed close to buildings.

See crossarm
alternating current (AC)
a periodic current the average value of which over a period is zero.

alternating current machine
an electromechanical system that either converts alternating current electrical power into mechanical power (AC motor), or converts mechanical power into alternating current electrical power (AC generator, or alternator). Some AC machines are designed to perform either of these functions, depending on the energy source to the dynamo.

alternator-rectifier exciter
a source of field current of a synchronous machine derived from the rectified output voltage of an alternator. The components of the exciter consist of the alternator and the power rectifier (including possible gate circuitry), exclusive of all input control elements. The rectifier circuits may be stationary, or rotate with the alternator, which may be driven by a motor, prime mover, or by the shaft of the synchronous machine.

ambient field
the background magnetic field level existing in the environment, without contribution from specific magnetic field sources.