Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

magneto-hydrodynamic (MHD) generator
a heat-to-electricity conversion device with an intermediate kinetic energy stage. In the MHD generator, a partially conducting gas is heated by a fuel-fired source or a nuclear reactor to convert the heat energy to kinetic energy, and then passed between the poles of an electromagnet, which converts some of the kinetic energy to electrical energy. The electrical energy is collected through a pair of electrodes situated in the gas channel.
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.
magnetomotive force (MMF)
a magnetic circuit term referring to that phenomenon that pushes magnetic flux through the reluctance of the circuit path. MMF is analogous to the concept of electromotive force (voltage) in an electric circuit. For a magnetic core with a single coil of N turns, carrying current I, the MMF is NI, with units of amperes (sometimes expressed as ampere-turns).
magnetoresistance
the change in electrical resistance in a conducting element experiencing a change in applied magnetic field. This is most pronounced when the magnetic field is perpendicular to current flow.
magnetostriction
a change in the length of a ferromagnetic material as the flux changes under the influence of an applied magnetic field, or resulting from domain formation after cooling from above Curie temperature. In an AC device, the steel in the core expands and contracts twice each cycle, creating audible noise (e.g., transformer hum).
magnetostrictive smart material
one of a class of materials with self-adaptively modifiable elastic properties in response to a magnetic field applied in proportion to sensed stress-strain information.
magnetotransport
motion of electrons or holes in a conducting material in the presence of an applied magnetic field.
magnetron
any arrangement of magnets in a sputter deposition or etch system that provides the magnetic field required to trap electrons in closed loops near the cathode, thus enhancing deposition/etch rates.
main switch
a switch which controls all power to a building's wiring or other electric installation.
mains voltage
European term for the voltage at the secondary of the distribution transformer.
maintainability
the probability that an inoperable system will be restored to an operational state within the time t.
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.