Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.


See alternating current
AC circuit
electrical network in which the voltage polarity and directions of current flow change continuously, and often periodically.

Thus, such networks contain alternating currents as opposed to direct currents, thereby giving rise to the term.
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.
AC motor
an electromechanical system that converts alternating current electrical power into mechanical power.
AC/AC converter
a power electronics device in which an AC input voltage of some magnitude, frequency, and number of phases is changed to an AC output with changes to any of the previously mentioned parameters.

AC/AC converters usually rectify the input source to a DC voltage and then invert the DC voltage to the desired AC voltage.
AC/DC converter

See rectifier
AC-DC integrated system
a power system containing both AC and DC transmission lines.
aluminum cable, steel-reinforced. A kind of overhead electric power conductor made up of a central stranded steel cable overlaid with strands of aluminum.
active device
a device that can convert energy from a DC bias source to a signal at an RF frequency. Active devices are required in oscillators and amplifiers.
active filter
(1) a filter that has an energy gain greater than one, that is, a filter that outputs more energy than it absorbs.
(2) a form of power electronic converter designed to effectively cancel harmonic currents by injecting currents that are equal and opposite to, or 180. out of phase with, the target harmonics. Active filters allow the output current to be controlled and provide stable operation against AC source impedance variations without interfering with the system impedance.
The main type of active filter is the series type in which a voltage is added in series with an existing bus voltage. The other type is the parallel type in which a current is injected into the bus and cancels the line current harmonics.
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.
air capacitor

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

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.
the maximum current which can be safely carried by a conductor under specified conditions.
armature reaction
(1) in DC machines, a distortion of the field flux caused by the flux created by the armature current. Armature reaction in a DC machine causes lower flux at one pole-tip and higher flux at the other, which may lead to magnetic saturation. It also shifts the neutral axis, causing sparking on the commutator.

(2) in AC synchronous machines, a voltage drop caused by the armature current. In the steady state model of the synchronous machine, the armature reaction is accounted for by a component of the synchronous reactance. armature voltage control a method of controlling the speed of a DC motor by varying the voltage applied to the armature while keeping the voltage applied to the field circuit constant.
asynchronous AC systems
AC systems either with different operating frequencies or that are not in synchronism.
asynchronous machine

See induction machine