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Magnetic Field
The space in which a magnetic force exists.
Maximum Operating Voltage
This is the maximum 50 to 60 Hz AC voltage the unit can
sustain without damage or failure of the suppressor.
Measured Limiting (used to be known as "let-through") Voltage
This is the maximum voltage measured across the terminals of
the suppressor during the time the testing voltages were applied to the unit..
Mega--(M)
A metric prefix meaning 1,000,000 or 106.
Mega-ohm
A unit of electrical resistance equal to
one million ohms.
Mega-ohmmeter
An instrument for measuring
extremely high resistance.
Megger
A test instrument used to measure insulation resistance
and other high resistances. It is a portable hand operated d-c generator used as
an ohmmeter.
Mercury Lamps
An electric discharge lamp in which
the major portion of the radiation is produced by the excitation of mercury
atoms.
Metal Halide Lamps
A discharge lamp in which the
light is produced by the radiation from the mixture of metallic vapor and the
products of disassociation.
Metal Oxide Varistor-(MOV)
A MOV is a voltage
sensitive breakdown device which is commonly used to limit overvoltage
conditions (electrical surges) on power and data lines. When the applied voltage
exceeds the breakdown point, the resistance of the MOV decreases from a very
high level (thousands of ohms) to a very low level (a few ohms). The actual
resistance of the device is a function of the rate of applied voltage and
current.
Micro--(U)
A metric prefix meaning one millionth of a
unit or 10-6.
Micron
A metric term meaning one millionth of a
meter.
Milli--(m)
A metric prefix meaning one thousandth
of a unit or 10-3
Motor, Shunt- Wound
This type of motor runs
practically constant speed, regardless of the load. It is the type generally
used in commercial practice and is usually recommended where starting conditions
are not usually severe. Speed of the shunt-wound motors may be regulated in two
ways: first, by inserting resistance in series with the armature, thus
decreasing speed: and second, by inserting resistance in the field circuit, the
speed will vary with each change in load: in the latter, the speeds is
practically constant for any setting of the controller. This latter is the most
generally used for adjustable-speed service, as in the case of machine tools.
Motor, DC, Series- Wound
This type of motor speed
varies automatically with the load, increasing as the load decreases. Use of
series motor is generally limited to case where a heavy power demand is
necessary to bring the machine up to speed, as in the case of certain elevator
and hoist installations, for steelcars, etc. Series-wound motors should never be
used where the motor can be started without load, since they will race to a
dangerous degree.
Motor, DC, Compound- Wound
A combination of the
shunt wound and series wound type, which combines the characteristics of both.
Varying the combination of the two windings may vary characteristics. These
motors are generally used where severe starting conditions are met and constant
speed is required at the same time.
Motor, Squirrel-Cage-Induction
The most simple
and reliable of all electric motors. Essentially a constant speed machine, which
is adaptable for users under all but the most severe starting conditions.
Requires little attention as there is no commutator or slip rings, yet operates
with good efficiency.
Motor, Wound-Rotor (Slip Ring) Induction
Used for
constant speed-service requiring a heavier starting torque than is obtainable
with squirrel cage type. Because of its lower starting current, this type is
frequently used instead of the squirrel-cage type in larger sizes. These motors
are also used for varying-speed-service. Speed varies with this load, so that
they should not be used where constant speed at each adjustment is required, as
for machine tools.
Motor, Single-Phase Induction
This motor is used
mostly in small sizes, where polyphase current is not available. Characteristics
are not as good as the polyphase motor and for size larger that 10 HP, the line
disturbance is likely to be objectionable. These motors are commonly used for
light starting and for running loads up to 1/3 HP Capacitor and repulsion types
provide greater torque and are built in sizes up to 10 HP.
Motor, Synchronous
Run at constant speed fixed by
frequency of the system. Require direct current for excitation and have low
starting torque. For large motor-generators sets, frequency changes, air
compressors and similar apparatus which permits starting under a light load, for
which they are generally used. These motors are used with considerable
advantage, particularly on large power systems, because of their inherent
ability to improve the power factor of the system.
MTBF
(Mean Time Between Failure) the probable
length of time that a component taken from a particular batch will survive if
operated under the same conditions as a sample from the same batch.
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