Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

dimensionless ratio of the average over any period of time (T = 1/frequency) of the ratio of the maximum energy stored (Umax) to the power absorbed or dissipated (Pabsorbed = Pin - Pout) in a passive component or circuit, including external loading effects, expressed as a dimensionless ratio. For most applications, the higher the Q, the better the part.
locked-rotor torque
the torque produced in an induction motor when the rotor is locked and rated AC voltage is applied to the stator.
lower frequency band edge
the lower cutoff frequency where the amplitude is equal to the maximum attenuation loss across the band.
magnetic torque coupling
any device utilizing a magnetic field to transmit torque.
negative sequence
the set of balanced but reverse sequence (acb) components used in symmetrical component analysis. Normal load currents contain no negative sequence current.
negative sequence overcurrent relay
a protective relay that senses and operates on negative sequence overcurrent. Typical applications include the sensing of unbalanced faults and the protection of synchronous and induction machines from rotor overheating.
negative-sequence impedance
the impedance offered by a circuit when negative-sequence currents alone flow through it, expressed in ohms. The impedance is complex, with its real part being the circuit resistance and imaginary part, which is a function of frequency and inductance referenced as negative-sequence reactance, also expressed in ohms.
negative-sequence reactance
inductive reactance offered by a circuit for the flow of negative-sequence currents alone. Expressed in ohms, the inductive reactance is a function of frequency and the inductance of the circuit
to negative-sequence current flow.
See negative-sequence impedance
phase sequence
describes the rotational orientation of the voltage phasors in a 3phase electrical power system.
A positive phase sequence, designated by the nomenclature ABC, indicates a 3-phase connection in which the B phase voltage lags the A phase voltage by 120 degrees, and the C phase voltage leads the A phase voltage by 120 degrees.
A negative phase sequence, designated by ACB, reverses this relationship so that the B phase leads the A phase, and the C phase lags the A phase.
See phase sequence indicator
phase sequence indicator
device used to detect the phase sequence of a 3-phase electrical power system.
See phase sequence
positive-sequence reactance
the inductive reactance offered by a circuit to the flow of positive-sequence currents alone. The positive-sequence reactance is a function of the operating frequency of the circuit and the inductance of the circuit to positive-sequence currents.
power quality
(1) the concept of maintaining appropriate voltage and current waveforms and frequency in transmission, distribution, and generation systems, and usually taken to mean undistorted and balanced waveforms.

(2) a measure of an electric supply to meet the needs of a given electrical equipment application. As delivered by the utility, power quality is the faithfulness of the line voltage to maintain a sinusoidal waveform at rated voltage and frequency.
pull-in torque
the amount of torque needed to change a synchronous motor's operation from induction to synchronous when self-started.
pull-out torque
the maximum value of torque that an AC motor can deliver. An induction motor operating at the pull-out torque will operate at maximum slip, and loading it beyond the pull-out torque will cause the motor to stall. Synchronous motors remain at synchronous speed up to the pull-out torque. Exceeding the pull-out torque for a synchronous machine will lead to pole slipping and destruction of the machine.
pull-up torque
the minimum torque generated by an AC motor as the rotor accelerates from rest to the speed of breakdown torque. For an induction motor, this value usually is less than the locked-rotor torque, and thus establishes the maximum load that can be started.
q
unit of electric charge. q = 1.602 x 10-19 coulombs.
Q-factor
a figure of merit that represents the ratio of stored to dissipated energy per cycle.
reluctance torque
the type of torque a reluctance machine's operation is based upon.

A reluctance torque is produced in a magnetic material in the presence of an external magnetic field, which makes it to line up with the external magnetic field. An induced field due to fringing flux develops a torque that eventually twists the magnetic material around to align itself with the external field.
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.
secondary system of equations
a system of algebraic and differential equations obtained from the primary system of equations by transformation of network variables.