Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

direct-axis subtransient short-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 short-circuited. The interval characterized is that immediately following a disturbance, during which the effects of amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient short-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the largest root of the numerator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is 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.
direct-axis transient short-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings short-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, in which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the short-circuit transient time constant is obtained by taking the reciprocal of the smallest root of the numerator 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 overcurrent relay
an over-current relay that operates only for overcurrents flowing in the tripping direction. Direction sensing is typically done with respect to a voltage or current signal, which is not affected by fault location.
dropout current
the current at which a magnetically-operated device will revert to its de-energized position.
eddy current
a circulating current in magnetic materials that is produced as a result of time-varying flux passing through a metallic magnetic material.
eddy current brake
a braking device in which energy is dissipated as heat by generating eddy currents.
eddy current drive
a magnetic drive coupled by eddy currents induced in an electrically conducting member by a rotating permanent magnet, resulting in a torque that is linearly proportional to the slip speed.
eddy current loss
the energy wasted in sustaining undesirable eddy currents in an electrical conductor.
electric current density
a source vector in electromagnetics that quantifies the amount of electric charge crossing some cross-sectional area per unit time. The direction of the electric current density is in the direction of electric charge motion. SI units are amperes per square meter.
equivalent circuit
a combination of electric circuit elements chosen to represent the performance of a machine or device by establishing the same relationships for voltage, current, and power.
equivalent current
a theoretical current used to obtain the scattered field from a surface or discontinuity. The equivalent current is formulated to represent the actual physical currents so as to result in an equivalent scattered field.
exciting current
the current drawn by a transformer primary with its secondary open circuited. It is the vector sum of the core loss current Ic and the magnetizing branch current Im . The exciting current Ie is also the current measured in the open circuit test on a transformer. The exciting current is calculated as the ratio of the primary induced EMF and the impedance of the tank circuit. On load, it is equal to the difference between the primary and reflected secondary currents of the transformer.
fault secure
pertaining to a circuit, with respect to a set of faults, if and only if for any fault in this set, and any valid input code the output is a non-code or correct code (the output is never an invalid code). The circuit
is considered to operate properly if the output is a code word.
feeder circuit
an electrical circuit designed to deliver power from the service equipment or separately derived system to the branch circuit panelboard(s) on a facility. For large systems, there may be more than one level of feeder circuits. See also branch circuit.
field circuit
a set of windings that produces a magnetic field so that the electromagnetics induction can take place in electric machines.
field current control
a method of controlling the speed of a DC motor by varying the field resistance, thus producing a change in the field current.
fuse cutout
a primary distribution voltage level fuse that employs a replaceable fusible link and provides a means of disconnect. The interrupting rating of a fuse cutout can be somewhat lower than that of a power fuse, however.
gas circuit breaker
a circuit breaker in which the arc between the contacts is extinguished by immersion in or a blast of an electronegative gas.
ground current
the current that flows in a power system in a loop involving earth and (in some usages) other paths apart from the three phases.
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