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In the protection of motors there are two types of overcurrent relays used to protect a motor against overload
- -Magnetic Relay
- -Thermal Relay
Why are HRC fuses used in motor circuits to provide back-up protection?
HRC fuses are used as back-up protection against short-circuit faults in the motor or wiring
A squirrel cage motor runs at its full load of 15A and it draws six times this value when starting. What type of fuse would you use for this motor circuit?
How much current would this fuse be expected to carry in the initial start-up stage?
A magnetic overload protection would be used in this situation. This type of protection can delay the tripping action so that the motor can start properly without tripping
- -15A x (6 times start up current)
- 15 x 6 = 90A
Motors do not have protection against mechanical stresses. To ensure a motor will not fail during operation several precautions can be used
State these precautions:
-The most common way to monitor mechanical stress is by regular crack testing of the shaft when then motor is nit running and so electrical protection systems for this condition are not common
-Another but more expensive way is to install protection devices to monitor stresses directly. Specialised relays exist to monitor shaft distortion and trip the motor if it reaches a pre-set value
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