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,
Control Devices of Electric Motor
a) Mechanically operated switches, such as door switches, limit switches on cranes, hoists or planning 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 equipment, 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.
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.
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