Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

manually-controlled shunt capacitors
a bank of shunt capacitors that are controlled via SCADA signals from an operating center as opposed to local automatic control by voltage sensing.
National Electrical Manufacturers Association (NEMA)
an electrical trade association that establishes standards for electrical equipment. In the case of electric motors, NEMA establishes standard frame sizes, starting torque, starting current, and other quantities for a given horsepower machine.
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
normally closed contact
contact of a contactor that is closed when the coil of the contactor is deenergized and opened when the coil is energized.
normally open contact
a contact that is open under normal operating conditions and closes when an action is initiated in its controller. For a contact that is part of a relay, the contact remains open when the relay is deenergized and closes when the relay is energized.
nuclear reaction
a reaction which causes changes in the nucleus of an atom, thus changing elements to another element or isotope, usually with the release of energy.
nuclear reactor
(1) an apparatus designed to facilitate, contain, and control a nuclear chain reaction.

(2) any heat-producing array of fissile radioactive materials constructed so as to produce a controlled chain-reaction.
null a point on the radiation pattern that corresponds to zero or minimum values.
ohmic contact
a heavily doped and/or low barrier height metal to semiconductor interface or contact that has a very low resistance relative to the remainder of the device, such that the device performance is not significantly degraded. At lower doping levels, the ohmic contact is described by Ohm's Law, while at higher doping levels, tunneling dominates.
parasitic capacitance
the generally undesirable and not-designed-for capacitance between two conductors in proximity of one another.
parasitic reactance
the generally undesirable and not-designed-for reactance associated with one or more conductors in a circuit.
permanent magnet AC motor
a generic term used to describe both permanent magnet synchronous motors and brushless DC motors.
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 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 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.
pitch factor
in an electric machine, the ratio of the fractional pitch in electrical degrees to the full pitch, also in electrical degrees.
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.
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 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.