Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

(1) signal or data that is sent back to a commanding unit from a control process output for use as input in subsequent operations. In a closed-loop system, it is the part of the system that brings back information on the process condition under control.

(2) the provision of a path from the output to the input of a system, such that the output may be made a function of both the input and the previous outputs of the system.

(3) the technique of sampling the output of an amplifier and using that information to modify the amplifier input signal. A portion of the output is "fed back" to the input. Positive feedback occurs when the output is added to the input; negative feedback occurs when the output is subtracted from the input. Negative feedback, invented by communications engineer Harold Black in 1928, usually results in a gain - bandwidth tradeoff: decreasing and stabilizing the amplifier gain, while increasing the bandwidth. According to Norbert Wiener, feedback is a method of controlling a system by reinserting into it the results of its past performance.
flexible AC transmission system
a transmission scheme in which each power line is maintained at its optimal impedance, generally by means of thyristor-controlled series compensators.
fractional horsepower
National Electrical Manufacturers Association (NEMA) classification describing any "motor built in a frame smaller than that having a continuous rating of 1 horsepower, open type, at 1700 to 1800 rpm."
gas capacitor
a capacitor whose dielectric is composed of a high-pressure gas, often nitrogen or an electronegative gas such as sulfur hexaflouride.
high rupturing capacity (HRC)
a term used to denote fuses having a high interrupting rating. Most low-voltage HRC-type fuses have an interrupting rating of 200 kA RMS symmetrical.
homopolar machine

See homopolar generator
induction furnace
a method of smelting or heating metals with eddy currents induced by a high-frequency coil surrounding the crucible.
induction machine
classification of any of a variety of electrical machines in which an AC current in the stator coils is used to produce a rotating magnetic flux that, by Faraday action, induces an AC voltage in a set of coils (the induction coils) on the machine's rotor. The rotor coils are shorted to cause a second AC current to flow in the rotor coils, which produces, in turn, a second rotating flux. The interaction of the rotor-and stator-produced fluxes creates torque.
instantaneous contact
the contacts of a contactor or relay that open or close with no time delay.
interior permanent magnet machine
a permanent magnet machine in which the permanent magnets are buried in the rotor iron. With this construction the airgap is small, allowing the airgap flux to be reduced by current control. This allows an operating mode in which the top speed of the motor can be as much as five times the base speed, and is one of the main features of this motor.
interrupting capacity

See interrupting rating
jumping jack
a voltmeter.
leakage reactance
the amount of inductive reactance associated with leakage flux. The leakage flux is the flux which traverses in paths farther from the designated paths such as the magnetic core in transformers and the air gap in electric machines and constitutes the non-useful flux. The electric circuit symbol of leakage reactance is Xl . It is a function of the leakage inductance and the frequency of operation. Higher values of leakage reactance affect the regulation and efficiency of the system. Xl is expressed in ohms.
light loss factor (LLF)
the ratio of the illumination when it reaches its lowest level at the task just before corrective action is taken, to the initial level if none of the contributing loss factors were considered.
lighting effectiveness factor (LEF)
the ratio of equivalent sphere illumination to ordinary measured or calculated illumination.
line outage distribution factor
a ratio used in contingency analysis. Given two parallel lines in a power system called x and y, assume that line y is removed from service. The line outage distribution factor of line x for the outage of line y is the ratio of the change in power flow on line x to the flow on line y before the outage.
line-connected reactor

See shunt reactor
loss factor
the product of the dielectric constant and the power factor.
magnetic actuator
any device using a magnetic field to apply a force.
magnetohydrodynamic MHD machine
a form of electric machine in which a stream of electrically conductive gas or liquid is passed through pairs of orthogonally positioned magnetic poles and electrodes. In an MHD generator, the fluid is forced by the prime mover to produce a DC across the electrodes. In the MHD motor, a current across the electrodes through the fluid forces the stream to flow.