Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

permanent split-capacitor (PSC) motor
an induction motor that operates from a single-phase supply. The motor contains two phase windings in quadrature; however, one of them has a capacitor in series with it to create a phase shift between the winding currents. Both windings and the capacitor operate continuously so the machine acts like a two-phase machine when running at its operating speed, producing less vibration and noise than a single-phase motor. Since the capacitor runs continuously, it is sized smaller than the capacitor used in a capacitor-start induction motor (CSIM). Thus, the PSC motor produces a lower starting torque than the CSIM.
permeability
tensor relationship between the magnetic field vector and the magnetic flux density vector in a medium with no hysteresis; flux density divided by the magnetic field in scalar media. Permeability indicates the ease with which a magnetic material can be magnetized. An electromagnet with a higher permeable core material will produce a stronger magnetic field than one with a lower permeable core material.
Permeability is analogous to conductance, when describing electron flow through a material.
See reluctance
permeance
the magnetic analog for conductance, indicating the ease with which magnetic flux will follow a certain path, which can be approximated by calculations based purely on magnetic circuit geometry.
permeance coefficient
the slope of the load line for a magnetic circuit, determined solely by physical geometry of the magnet and permeable materials around it; the ratio of magnetic induction (B) and applied field (H ) at the operating point.
permittivity

See electric permittivity
persistent current
a current circulating in a closed structure without applied potential. Examples are the supercurrent in a superconducting magnet and the current in a closed mesoscopic ring in a magnetic field.
phase angle meter
meter used to measure the phase angle difference between two AC quantities. In power systems, typical meters use perpendicular moving coils to measure the phase angle between an AC current and an AC voltage. More accurate devices typically measure the time interval between zero crossings of the two input signals.
phase comparator
often referred to as a phase detector; a three-port device that produces an analog output proportional to the phase difference between its two inputs.

Since both inputs are periodic, the relative output voltage (or current) as a function of input phase difference (i.e., the transfer function) is also periodic; the shape of the transfer function (sometimes called the "output characteristic" of the phase detector) depends upon the particular technique used to accomplish the phase detection. These include sinusoidal, triangula, and sawtooth shape factors. Analog/digital implementation, required linearity, and range of input phase difference are primary factors in determining a suitable output characteristic for a specific phase comparator application.
phase comparison relay
a phase comparison relay is a protective relay used on transmission lines which operates by comparing phase angles of signals generated at opposite ends of the line. They employ a dedicated communications channel to make the comparison. The signals compared are typically corresponding phase currents or sequence currents.
phase constant
a constant, which is generally complex, that is important in the study of electromagnetic waves. The phase constant is equal to the frequency of excitation of the wave times 2 times pi times the square root of the product of the permeability and the permittivity of the medium that the wave is traveling in. Also called propagation constant or wavenumber.
phase control
a method for controlling the amount of power delivered to a load by varying the delay angle. This controls the delay between the instant when the voltage across the power semiconductor goes positive and the actual start in conduction of the device.
phase delay
the difference in the absolute angles between a point on a wavefront at the device output and the corresponding point on the incident input wavefront, expressed in seconds or degrees. The delay can exceed 360 degrees.
phase detector gain
the ratio of the DC output voltage of the phase detector to the input phase difference. This is usually expressed in units of volts per radian.
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
phase shift
a time displacement of a waveform with respect to another waveform of the same frequency.
phase shifter
a device that changes the phase angle between two buses in a power system. Conventional phase shifters are special autotransformers in with each phase voltage in connected in series with a variable component of voltage from another phase.

By adjusting the variable component, the phase angle can be changed. Newer phase shifters are built with power electronic devices. Phase shifters are often used in antenna arrays.
phase-to-phase fault
a fault with two transmission lines being short circuited.
phonon
a quantized packet of energy associated with material lattice vibrations that have been excited by an incident photon.
photodetector
device capable of producing or modifying an electrical signal in proportion to the amount of light falling on the active area of the device.
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