Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

angular frequency
the rate of change of the phase of a wave in radians per second.
automatic frequency control (AFC)
(1) an automatic feedback control system that is used to maintain active power balance by means of the speed governor system. In an interconnected system, scheduled power interchanges are maintained by means of controlling area generations.
(2) electronic circuitry used to keep the received signal properly placed within the desired IF frequency range. In televisions, the AFC circuitry is also called the AFT or "automatic fine tuning" section. The AFC circuit will generate an error signal if the input frequency to the IF drifts above or below the IF frequency. The error signal is fed back to vary the local oscillator frequency in the tuner section. See also automatic fine tuning (AFT).
diversity frequency
a method for increasing the reliability of digital communications in which multiple copies of the signal, or other types of redundant information, are transmitted. Frequency diversity implies that the received signal occupies a much wider bandwidth than the minimum bandwidth needed to carry the information.
electron collision frequency
the average number of collisions per second an electron has with heavy particles in a medium such as plasma.
frequency
the repetition rate of a periodic signal used to represent or process a communication signal. Frequency is expressed in units of hertz (Hz). 1 Hz represents one cycle per second, 1 MHz represents one million cycles per second, and 1 GHz represents one billion cycles per second.
frequency converter
an equipment or circuit that converts an RF signal to an intermediate (IF) signal in receivers. It converts an IF signal to an RF signal in transmitters.
frequency regulation
the change in the frequency of an unloaded generator with respect to its frequency in a fully-loaded state. Typically applied to small, isolated power systems such as emergency power units.
frequency relay
a protective relay which monitors the frequency of the electric power system.
frequency resolution
a measure of the ability of a system to resolve different frequencies in a signal. As the frequency resolution increases, more finely-spaced frequency components can be resolved. The time resolution of a system is roughly inversely proportional to the frequency resolution; the uncertainty principle places a lower bound on the time-frequency resolution product.
frequency variation
a change in the electric supply frequency.
fundamental frequency
for a continuous time periodic signal, f(t) with fundamental period T (seconds), ω0π/T (rad/sec).
See periodic signal
harmonic frequency
integral multiples of fundamental frequency. For example, for a 60-Hz supply, the harmonic frequencies are 120, 180, 240, 300, ....
instantaneous frequency
(1) the time rate of change of the angle of an angle-modulated wave.

(2) the frequency of radiation at some chosen instant of time; the rate of change of phase in radians per second, divided by 2π.
load frequency control
the purpose of load frequency control is to maintain the power system frequency at its nominal value while maintaining the correct outputs on individual generators to satisfy the loading on the system. As the load varies, the inputs to the generator prime movers must be controlled to keep the generation in balance with the loads.
lower frequency band edge
the lower cutoff frequency where the amplitude is equal to the maximum attenuation loss across the band.
resonant frequency
(1) a frequency at which the input impedance of an device is nonreactive, since the capacitive and inductive stored energy cancel each other.

(2) an oscillation frequency of the modes of a resonator.
slip frequency
the frequency of the rotor induced currents in an induction machine. Denoted by fsl, the slip frequency is given by slip - stator frequency (fs) and is the prime frequency used in slip frequency control of induction machines.
ultra-high frequency (UHF)
electromagnetic spectrum with frequencies between 300 MHz and 3000 MHz or wavelengths between 10 cm and 100 cm. Also called as decimetric waves.
underfrequency relay
a protection device that curtails loads in an area that is deficient in generation. Lower generation compared to load demands give rise to lower frequency and a frequency threshold can be used by the relay to initiate load shedding in order to balance generation and demand.
variable frequency drive
electric drive system in which the speed of the motor can be varied by varying the frequency of the input power.
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