Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

motor circuit
the three components of an electrical circuit are source, load, and interconnecting circuit conductors. A motor circuit is an electrical circuit designed to deliver power to a motor. It includes the over-current protective devices, controller, disconnect switch, circuit conductors, and the motor itself.
motor circuit protector (MCP)
a listed combination motor controller containing an adjustable instantaneous-trip circuit breaker and coordinated motor overload protection. MCPs can provide short-circuit and bolted ground-fault protection via the circuit breaker magnetic element, overload protection via the overload device, motor control, and disconnecting means all in one assembly.
motor control center (MCC)
an enclosure with one or more sections containing motor control units that have a common power bus.
motor control circuit
a circuit containing devices such as the start/stop switches, mainline coil, main-line sealing contacts, overload contacts, timers and timer contacts, limit switches, antiplugging devices, and anything else used to control devices in the motor circuit.
motor current signature
analysis the use of the currents of an electric machine to provide diagnostic or other information on the health of the machine, coupling, or load.
motor operated switch
a switch operated by a motor that is capable of being controlled from a remote location.
motor starter
an electric controller, either manual or automatic, for accelerating a motor from rest to normal speed and for stopping the motor.
motor-generator set
a set consisting of a motor mechanically coupled to and driving one or more generators. The set used to be employed for AC-to-DC or DC-to-AC power conversion or voltage level or frequency conversion. Solid-state conversion units are replacing motor-generator sets in most applications.
multiplex
(1) to use a single unit for multiple purposes, usually by time sharing or frequency sharing. multiplexer

(2) the armature winding of a commutated electrical machine in which multiple, identical coil windings are placed on the rotor. In general, the number of the "plex" describes the total number of parallel windings between brush positions and, thus, also the multiplier on the number of parallel paths between brushes that would be provided by a simplex winding. For example, a duplex winding will have twice as many parallel electrical paths between brushes as a simplex winding, a triplex winding will have three times the number of paths, etc. See simplex, duplex, reentrancy.
multiplexer
a combinational logic device with many input channels and usually one output, connecting one and only one input channel at a time to the output.
multiplexing
(1) the process of transmitting a large number of information units over a smaller number of channels or lines. For example, if we have N independent signals that we want to transmit, then without using a multiplexer we need N independent channels to do so. Using a multiplexer to control the flow of these signals in only one channel reduces the number of wires, thus decreasing cost and increasing efficiency. Multiplexing is the superimposition of multiple signals to make up one signal. This is done to make the transmission of the signals efficient. Signals are multiplexed at the sending end of communication systems, and demultiplexed at the receiving end, in order to obtain the original signals.

(2) of or being a communication system that can simultaneously transmit two or more messages on the same circuit or radio channel.
multispeed motor
a motor that can be operated at any one of two or more definite speeds. For DC and induction motors, the speed settings are practically independent of the load, although the speed may vary with load for certain types of motors. Multispeed induction motors typically have two or more sets of windings on the stator with a different number of poles, one of which is excited at any given time.
mutual coupling
electromagnetic interaction between the elements in a phased array antenna in which the radiation from one element causes distortion of the current distributions or aperture field distributions of the other elements. Feed network mutual coupling occurs when the amplitude and phase of the excitation at a feed point are altered by the presence of reflected waves in the network.
mutual exclusion
a synchronization problem requiring that two or more concurrent activities do not simultaneously access a shared resource.
mutual inductance
the property that exists between two current-carrying conductors when the magnetic lines of force from one link with those from another.
MVA interrupting rating
the interrupting rating of a device expressed in terms of megavolt-amperes. The conversion between fault volt-amps and fault current in three-phase systems is VA = 1.73 x operating RMS line-line voltage x largest phase fault current expressed in RMS symmetrical amperes. Power circuit breakers can have separate MVA and maximum current interrupting ratings, with adjustments when the circuit breaker is operated below rated voltage.
nano
prefix for metric unit that indicates division by one billion.
nanometer
a metric measure of distance equivalent to one billionth of a meter.
National Electrical Code (NEC)
a standard for electrical construction, published by the National Fire Protection Association (NFPA 70-1). The National Electrical Code is often adopted by local jurisdictions and used by their electrical inspectors.
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.
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