Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

See angle of arrival
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.

arc lamp
lamp made by driving a high current across a gap between two electrodes. Some types operate in air consuming the electrode, for example, a carbon arc in which the electrode material is made as a rod and fed into the discharge to replace what is consumed. Others operate in a vacuum envelop that reduces the electrode consumption.
arc resistance
period of time that the surface of an insulating material can be submitted to the action of an electrical arc without becoming conductive.
arcing fault
See arcing ground
arcing ground
a ground fault on a power line which alternately clears and restrikes, causing high, repetitive voltage surges.
arm pin
a pin insulator

the magnetic circuit of a rotating electrical machine, including the main current carrying winding, in which an alternating voltage is induced by the magnetic field.
armature circuit
components of the machine that carry armature current. For example, in a DC machine the armature circuit could consist of the armature windings, brushes, series field winding, compensating windings, interpoles, starting resistor(s), main-line contacts, and overload sensor.
armature current limiting
a condition wherein the stator currents are clamped at the maximum allowable limit due to excessive heating of the stator.

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.

armature winding
an arrangement of coils carrying the main current, typically wound on the stator of a synchronous machine or the rotor of a DC machine, in which an alternating voltage is induced by the magnetic field.
armless construction
a method of distribution line construction, often used for aesthetic purposes, in which pin insulators are mounted on steel brackets bolted directly to a utility pole without the use of a crossarm.
arrester discharge current
the current in an arrester during a surge.

arrester discharge voltage
the voltage in an arrester during a surge.

artificial dielectric
a dielectric material that has been modified to alter its properties. Common modifications include micro-machining to remove material from the substrate under planar patch antenna to improve radiation properties and the fabrication of periodic arrays of holes to realize guiding or photonic bandgap structures.