Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

permanent magnet brushless DC machine
a machine that is similar in structure to a permanent magnet synchronous machine, containing armature windings on the stator and permanent magnets on the rotor. The permanent magnet brushless DC machine, however, is characterized by a trapezoidal flux density distribution in the airgap instead of the sinusoidal distribution of the synchronous machine. In operation, a DC voltage is applied sequentially to the stator coils to create a rotating field that pulls the rotor with it. To correctly operate, the brushless DC machine requires sensors to determine the rotor position so that the proper stator phases may be excited.
permanent magnet DC machine
a DC machine in which the field excitation in the stator is provided by permanent magnets instead of electromagnets.
permanent magnet DC motor

See permanent magnet DC machine
permanent magnet machine
a machine that uses permanent magnets to establish the field. In DC machines, the permanent magnets are placed on the stator, while on AC synchronous machines they are placed on the rotor.
permanent magnet stepper motor
a stepper motor that has a permanent magnet assembly on the rotor.
permanent magnet synchronous machine
a polyphase AC motor with rotor mounted permanent magnets and sinusoidal distribution of stator phase windings. The field windings in the rotor are replaced by permanent magnets to provide the field excitation in these machines.
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
(1) a notion used extensively in interpreting complex quantities such as Fourier series, Fourier transforms etc. Given a complex number c = x+iy = r cos θ + ir sin θ, then r represents the magnitude of c and  the phase.

(2) a horizontal translation parameter of the signal. Given a sinusoidal signal s(t) = Asin(2πft + θ), then f represents the frequency (in Hz) of s and θ the phase.

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.
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