Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

polyphase system
electrical system that has more than one phase, which are separated by angles of 360=n, where n is the number of phases. For example, three phase systems are polyphase systems where the three phases are separated by 120 electrical degrees from each other. A six-phase system is a polyphase system where each successive phase is separated by 60 electrical degrees from the other.
position sensor
a device used to detect the position of the rotor with respect to the stator. The most commonly used position sensors for electric motors are Hall effect devices, encoders, and resolvers with resolver-to-digital converters.
position servo
a servo where mechanical shaft position is the controlled parameter.
See servo
positioner
a mechanical device used to move an antenna or target to a desired position for measurement purposes. Positioners can be single or multi-axis and are usually controlled by computers and automated measurement equipment.
positive sequence the set of balanced normal (abc) sequence components used in symmetrical components. Balanced load currents, for example, are strictly positive sequence.
positive transition angle
the angular portion of the time-based output signal (in degrees) that has a positive slope. This quantity could be loosely interpreted as the "leading edge" angle.
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.
post insulator
an electrical insulator which is supported by its firmly-bolted base, either in an upright position or cantilevered out horizontally from a utility tower.
pot head
a fork-shaped transition between a three-phase buried cable and an overhead three-phase electric power line.
potential
an auxiliary scalar or vector field that mathematically simplifies the solution process associated with vector boundary value problems. See also Hertzian potential.
potential coil
a long, finely wound, straight coil, similar in operation to a Chat-tuck coil, that is used with a fluxmeter to measure magnetic potential difference between a point in a magnetic field and a flux-free point in space.
potential source rectifier exciter
a source of energy for the field winding of a synchronous machine obtained from a rectified stationary AC potential source. The AC potential can be obtained from the machine phase voltages, or from an auxiliary source. The components of the exciter are the potential source transformer and the rectifiers (including possible gate-circuitry).
potential transformer (PT)
a device which measures the instantaneous voltage of an electric power line and transmits a signal proportional to this voltage to the system's instruments.
See voltage transformer
Potier reactance
the leakage reactance obtained in a particular manner from a test on a synchronous machine at full load with a power factor of zero lagging. The test requires little power but supplies the excitation for short circuit and for normal rated voltage both at full-load current at zero power factor. The Potier reactance is determined
by a graphical manner from the open circuit characteristic and the short circuit point for full-load current.
power
(1) a measurable quantity that is the time rate of increase or decrease in energy. Units are in watts.

(2) ratio of energy transferred from one form to another (i.e., heat, radio waves, light, etc.) to the time required for the transfer, expressed in watts.
power angle
the angular displacement of the rotor from the stator rotating magnetic field while the machine is on load. The power angle is also the angle between the terminal voltage Vt of a synchronous machine and the generated voltage Eg or Em, respectively, for a generator or motor. This angle denoted by δ is also referenced as power angle or torque angle or the load angle in a synchronous machine. It signifies the limits of the machine to remain in synchronism.
See torque angle
power angle curve
a curve shown the relationship between the active power output of a generator and its power angle.
power conditioner
a device designed to suppress some or all electrical disturbances including overvoltages, undervoltages, voltage spikes, harmonics, and electromagnetic interference (EMI). Example power conditioners are active filters for the reduction of harmonics, metal-oxide varistors (MOVs) and isolation transformers for the protection against voltage spikes, and EMI filters.
power disturbance
a variation of the nominal value of the voltage or current.
power divider
passive electronic circuit consisting of one input and two or more outputs. A signal applied to the input is divided into equiphase output signals, generally of equal amplitude.
power factor
in an AC system, the ratio of the (active component) real power P to the apparent power S; it is given by the cosine of the angle subtended by S on the real, P axis. See also apparent power, real power, reactive power.
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