Induction Motor Protection (Part3)
Electrical Faults (Part2)
This protection is also called short circuit protection. At the time of such a fault, the current increases by 8 to 10 times the full load current of the motor. Attached armature type relay unit is connected in each phase with a current setting of 4-5 times the full load current. This is because starting current can be 4-5 times full load current.
Hence to operate the relay only under fault condition such a setting is necessary. Such a protection is shown in the Fig.2.
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| Fig. 2 Phase and earth fault protection |
The phase faults can cause burn out of coils and stampings and hence motor should be disconnected as quickly as possible when fault occurs. Fast over current relays also are used to provide phase fault protection.
As mentioned above to avoid relay functioning during starting, the short circuit protection current setting must be just above the maximum starting current of the motor.
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