Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

definite-purpose motor
any motor design, listed and offered in standard ratings with standard operating characteristics, with special mechanical features for use under service conditions other than usual or for use on a particular type of application.
differential protection
a protective relaying scheme in which the currents entering and leaving the protected line or device are compared.
differential protection unit
a protective unit based on the difference of currents flowing in and out of a protected zone.
differentially compounded
a compound machine in which the flux produced by the MMF of the shunt field winding and the flux produced by the MMF of the series field winding oppose each other. Most often obtained by incorrectly connecting the machine, the differentially compounded machine may demonstrate very erratic behavior.
direct-axis subtransient open-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that immediately following a disturbance, during which the effects of all amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient open-circuit time constant of a machine with a single d-axis amortisseur windings is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is very small and the detailed expression simplified.
direct-axis transient open-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, during which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the transient open-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the amortisseur winding resistance is infinite and the detailed expression simplified.
directional power relay
a protective relay that operates for power flow in a given direction. Applications are in cases where normal power flow is in one direction, including anti-motoring protection on a turbine-generator and fault backfeed protection on parallel step-down transformers.
dispatch
the determination of the power output of each plant in an electric power system.
dissipation factor (DF)
the ratio of the effective series resistance of a capacitor to its reactance at a specified frequency measured in percentage.
See loss tangent
dissipation power
(1) ratio of real power (in phase power) to reaction power (shifted 90. out of phase).
(2) the ratio of the imaginary to real parts of the complex permittivity, expressed as a dimensionless ratio.
diversity path
a form of diversity in which multiple copies of the signal are created via different paths from the transmitter to receiver.
dropout
equipment misoperation due to an interruption, noise, or sag.
dropout current
the current at which a magnetically-operated device will revert to its de-energized position.
dropout voltage
the voltage level where proper equipment operation is hindered.
duplex
a method of winding the armature of a commutated electric machine such that the number of parallel electrical paths between brushes is double that provided by a simplex winding. Duplex windings are constructed by placing consecutive coils in alternate coil slots and continuing the winding twice around the rotor, filling the empty slots on the second pass. The result is two complete, identical windings between brush positions rather than the one winding that is produced when coils are placed in adjacent slots.
See simplex
dust-ignition-proof machine
a machine designed with a casing or specialized enclosure to safely contain any internal ignition or flammable substances or components, and prevent them from igniting external flammables such as explosive gases, vapors, and dust particles.
economic dispatch
a generation scheme in which units are utilized such that the greatest profit is generated for the utility.
electric permittivity
tensor relationship between the electric field vector and the electric displacement vector in a medium with no hysteresis; displacement divided by the electric field in scalar media.
electric polarization vector
an auxiliary vector in electromagnetics that accounts for the creation of atomic dipoles in a dielectric material due to an applied electric field. Macroscopically, the electric polarization vector is equal to the average number of electric dipole moments per unit volume. Mathematically, P = D - Є0E, where D is the electric flux density, E is the electric field intensity, and Є0 is the free space permittivity. SI units are coulombs per square meter.
electric susceptibility
tensor relationship between the electric field vector and the electric polarization vector in a medium with no hysteresis. It is the polarization divided by the permittivity of free space and the electric
field in scalar media.
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