Control Devices of Electric Motor
Control Devices of Electric Motor | There are so many types and designs that it is impossible to
mention all of them. The following are just a few examples,
a)
Mechanically operated switches, such as door switches, limit switches on
cranes, hoists or planing machines, float switches operating on rise and fall
of liquid level, etc., many of these control devices are suitable to operate
the circuits required directly. A separate relay or contactor will only be
required if the current or voltage to be controlled is beyond the capacity of
the control switch or the control equipment is too far away.
b)
Electrically operated switches, such as overload relays, reverse power
relay, battery cut-in and cut-out relays. In these cases the control device
itself functions as a relay.
c) Pressure operated diaphragm switches, such as pressure switches on air vessels to control operation of compressors,
oil pressure switches o diesel engines to stop them in the events of failure of
lubrication and water pressure switches to give alarm indication to staff on
failure of circulating water supply to engines, etc.
d) Thermostats operation on a variation of temperature, such as mercury-thermometer type thermostats to start and stop air
conditioning equipments, sealed bellows type thermostats for giving alarm to
the operators of diesel engines, if the lubricating oil temperature rises
unduly, embedded type thermocouples in large turbo-generators to give audible
and visible warning and to shunt down the set, if necessary, when the
temperature of the winding rises unduly, and galvanometer type relays arranged
in wheatstone bridge circuit for maintaining temperature constant in oil-fired
furnaces.
e) Photo electric relay operating on various of light intensity, such as for switching on street lights, flood lights, etc. in the
evenings as soon as it is dark enough, for burglar alarms if any one interrupts
a beam of light focused on the photoelectric cell, or to detect smoke and give
fire signals.
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Control Devices of Electric Motor
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f) Electronic relays: There
is no limit to what an electric relay can do. It can be made to fulfill any requirement
beyond the capacity of any other equipment. For instance, it can automatically
maintain the thickness of a sheet of paper or metal within very close limits,
or count accurately moving objects flying past an electric eye at thousands per
minute, etc. its applications are far too many to be dealt with in the present
volume.
Final
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