Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

power factor correction
the addition of reactive load to bring the combined power factor nearer unity. Since most industrial loads are inductive, capacitors are often employed as passive devices for power factor correction.
a figure of merit that represents the ratio of stored to dissipated energy per cycle.
a channel within a building which holds bare or insulated conductors.
reactance grounded
an electrical system in which the neutral is intentionally grounded through a reactance. Frequently used in the neutral of generators and transformers to limit the magnitude of line to ground fault currents.
reactive compensation
process of counteracting the reactive component of a device by means of capacitors and inductors. Both series and shunt compensation are prevalent.
reactive load
a load that is purely capacitive or inductive.
reactive power
(1) electrical energy per unit time that is alternately stored, then released. For example, reactive power is associated with a capacitor charging and discharging as it operates on an AC system. Symbolized by Q, with units of volt-amperes reactive (VAR), it is the imaginary part of the complex power.

(2) the power consumed by the reactive part of the load impedance, calculated by multiplying the line current by the voltage across the reactive portion of the load. The units are vars (volt-ampere reactive) or kilovars.
a container where the nuclear reaction takes place. The reactor converts the nuclear energy to heat.
salient-pole rotormachine
AC motor/generator design in which the rotor is constructed of outward-projecting pole pieces mounted on a shaft-mounted central spider assembly. Spider assemblies are typically spoked. Pole pieces are built up from laminated sheets, which are bolted together between a pole shoe on the outer end and dovetail fixture on the inner end. The dovetails are keyed into slots on the spider to mount the pole pieces to the rotor. Rotor windings are generally constructed from preformed, insulated coils that are fit over the pole pieces during assembly. Salient rotors are typically low-speed designs with short axial length and large diameter.
shunt capacitor
a capacitor or group of capacitors which are placed across an electric power line to provide a voltage increase or to improve the power factor of the circuit. A switchable shunt may be disconnected from the circuit when conditions warrant, while a fixed shunt is permanently connected to the power line.
shunt DC machine
a DC machine with the field winding connected in shunt with the armature. In shunt generators, residual magnetism must be present in the machine iron in order to initiate the generation process. These machines are also known as self-excited, since they supply their own excitation.
shunt reactor
a reactor intended for connection in shunt to an electric system to draw inductive current.
single machine infinite bus system
a model of a power system consisting of a single generator working into an infinite bus which represents the remainder of the system.
slip-ring contact
a rotating, brush-contacted ring electrode connected to one end of a coil in an AC generator.
subtractive polarity
polarity designation of a transformer in which terminals of the same polarity on the low-and high-voltage coils are physically opposite each other on the transformer casing. With subtractive polarity, a short between two adjacent terminals results in the difference of the two coil voltages appearing between the remaining terminals.
Subtractive polarity is generally used on transformers larger than 500 kVA and higher than 34.5 kV. Smaller units use additive polarity.
See additive polarity
switched reluctance machine
a doubly salient, singly excited electrical machine that contains a different number of poles on the stator and rotor. Since there are a different number of poles on the rotor and stator, only one stator phase can be aligned at a time with the rotor.

When operated as a motor, the stator phases are sequentially switched on and off to pull the rotor into alignment with them. This requires knowledge of the rotor position to properly excite the stator phases. The switched reluctance machine can also operate as a generator. In this case the stator windings are charged with a current as the rotor comes into alignment. When the current reaches a determined level, the windings are reconnected to send current out of the machine.
As the rotor is driven, the inductance drops, causing an increase in current.

This type of machine requires an external capacitor bank, switches and diodes in each phase, and a sophisticated control system to operate.
synchronous machine
an AC electrical machine that is capable of delivering torque only at one specific speed (ns), which is determined by the frequency of the AC system (f) and the number of poles (P) in the machine. The relationship between synchronous speed and the other variables is

ns = 120f/P
synchronous reactance
the inductive reactance of the armature windings in synchronous machines under steady-state conditions. Designated by the symbol Xs,expressed in ohms per phase, the synchronous reactance is a function of the stator inductance and the frequency of the stator currents.
synchronous reluctance machine
a type of synchronous machine that has no rotor winding. The rotor consists of salient poles, which causes the reluctance to vary as a function of position around the airgap. When operated as a motor, a rotating magnetic field is established by the stator windings that causes a reluctance torque on the rotor as the path of lowest permeability stays aligned with the peak of the stator flux wave.
a metal frame which holds a lightning arrester and a cut-out to the top of a utility pole.