Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

maintenance
the changes made on a system to fix errors, to support new requirements, or to make it more efficient.
major hysteresis loop
for a magnetic material, the loop generated as intrinsic or magnetic induction (Bi or B) is plotted with respect to applied field (H) when the material is driven from positive saturation to negative saturation and back, showing the lag of induction with respect to applied field.
male connector
a connector presenting pins to be inserted into a corresponding female connector that presents receptacles.
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.
master control relay (MCR)
used in programmable logic controllers to secure entire programs, or just certain rungs of a program. An MCR will override any timer condition, whether it be time-on or time-off, and place all contacts in the program to a safe position whenever conditions warrant.
matched load
load that does not reflect any energy back into the transmission line. It could be a load equal to the characteristic impedance of the transmission line or a structure with electromagnetic absorbing properties.
maximum excitation limiter
a controller that is used to limit the maximum amount of field current, or over-excitation, at a synchronous generator. This excitation limit is set by rotor winding heating limit.
maximum transducer power gain
maximum value of transducer power gain a circuit or device exhibits; occurs when the input and output ports of the circuit are terminated with simultaneous conjugate match conditions. The transducer power gain is defined as the ratio of power delivered to a load to the power available from the source.
Maxwell
is best known as the greatest theoretical physicist of the 19th century. It was Maxwell who discovered, among other things, that light consisted of waves. He developed the fundamental equations describing electromagnetic fields in his work, A Dynamical Theory of the Electromagnetic Field, published in 1864. Maxwell also gave us the mathematical foundation for the kinetic theory of gases. Maxwell's life was cut short by cancer, and thus he was unable to see his greatest theoretical propositions proven by experiment.
MCM
a unit of area used to specify the cross-sectional area of a wire, equal to 1000 circular mils.
messenger cable
a fully-insulated three-phase aerial cable in which three individual insulated conductors are carried on insulated looms hung upon a bare messenger wire. Such cable is used frequently in distribution work.
messenger wire
a grounded wire which is used to structurally support an aerial cable.
metadyne
a DC machine with more than two brush sets per pair of poles. The additional brushes are located in the direct axis for the armature MMF to provide most of the excitation for higher gains.
metal halide
molecule formed by the reaction of metals and halogen atoms.
metal-insulator-metal (MIM)
capacitor a capacitor, which has a thin insulator layer between two metal electrodes. Generally, this capacitor is fabricated in semiconductor process, and this insulator layer provides high capacitance. Two extreme behaviors of a capacitor are that it will act as an open circuit to low frequencies or DC (zero frequency), and as a short frequency at a sufficiently high frequency (how high is determined by the capacitor value).
See thin film capacitor
MHz
egahertz, or millions of operations per second.
molded case circuit breaker
a low voltage air circuit breaker that includes thermal and/or magnetic overcurrent sensing which directly trips the breaker. The molded case circuit breaker is nearly always manually closed, opened, and reset.
momentary interruption
a loss of voltage of less than 0.1 pu for a time period of 0.5 cycles to 3 seconds.
momentary monitoring the duration at supply frequency from 30 cycles to 3 seconds.
momentary overvoltage
an increase in voltage above the system's specified upper limit for more than a few seconds. Generally a rather loosely-defined term.
motor
an electromechanical device that converts electrical energy from a DC or an AC source into mechanical energy, usually in the form of rotary motion.
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