Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

equivalent resistance
the resistance of the windings of an electromagnetic machine reflected to one side (component) of the machine.
See equivalent impedance
equivalent source
fictitious source used in the equivalence theorem.
equivalent sphere illumination (ESI)
the level of sphere illumination that would produce task visibility equivalent to that produced by a specific lighting environment.
equivalent system dynamics
a dynamical system model resulting from substituting the equivalent control into the plant's modeling equation. The equivalent system's trajectory is confined to a surface that is parallel to the sliding surface if the system's initial condition is off the sliding surface. If the initial condition is on the sliding surface, then the equivalent system's trajectory will stay on the sliding surface.
four-quadrant operation
(1) a signed representation of electrical or mechanical variables in the phase plane in order to situate the different modes for energy transfer. This term can be used both for power electronics and electrical machines. For electrical variables, the four-quadrant operation is defined by the voltage-current (or current-voltage) characteristic with the two variables expressed as instantaneous or mean values. For mechanical variables, the term is defined by the torque-speed (or speed-torque) characteristic with the same time-domain representation as previously explained. The energy transfer is defined with electrical power in the voltage-current curve and with mechanical power in the torque-speed curve. The four-quadrant operation is related to reversible power in electromechanical systems.
For example, in the case of power electronics, the four-quadrant operation is defined only with electrical variables to visualize the way of energy transfer in a static converter. For electrical machines, the same operation can
be defined with electrical variables at the input (motor) or at the output (generator) and also with mechanical variables at the output (motor) or at the input (generator).

(2) the four combinations of forward/reverse rotation and forward/reverse torque of which a regenerative drive is capable. These are: motoring: forward rotation/forward torque; regeneration: forward rotation/reverse torque; motor: reverse rotation/reverse torque; and regeneration: reverse rotation/forward torque. four-wave mixing a nonlinear optical
phenomenon in which four optical beams interact inside nonlinear media or photorefractive crystals. When four beams of coherent electromagnetic radiation intersect inside a nonlinear or photorefractive medium, they will, in general, form six interference patterns and induce six volume refractive index gratings in the medium. The presence of the index gratings will affect the propagation of these four beams. This may lead to energy coupling. The coupling of the four optical
beams is referred to as four-wave mixing. In one of the most useful four-wave mixing configurations, the four beams form two pairs of counterpropagating beams. In this particular configuration, some of the refractive index gratings are identical in their grating wavevectors. This leads to the generation of phase conjugate waves. Four-wave mixing
is a convenient method for the generation of phase conjugated waves.
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.
full-load torque
the torque of a motor that is producing rated power at rated speed.
fundamental frequency
for a continuous time periodic signal, f(t) with fundamental period T (seconds), ω0π/T (rad/sec).
See periodic signal
globally asymptotically stable equilibrium
an asymptotically stable equilibrium (see the definition) with a region of attraction (see the definition) equal to
gravitational torque
torque that depends upon the position of the robot in the gravitational field.
harmonic frequency
integral multiples of fundamental frequency. For example, for a 60-Hz supply, the harmonic frequencies are 120, 180, 240, 300, ....
hysteresis torque coupling
a magnetic drive in which the magnetizing stator magnet drives a rotor of hysteresis material through the complete hysteresis cycle once per rotation, resulting in a constant torque characteristic irrespective of relative speed.
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.
load torque
the resisting torque applied at the motor shaft by the mechanical load that counterbalances the shaft torque generated by the motor and available at the shaft.