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Motor Protection Relay for High Voltage Induction Motor

Published on 24/2/2012 & updated on 31/8/2018
Above 90% of motors used in an industry are induction motors, because they are cheap, robust and easy to maintain. For higher HP (>250HP) motors we prefer high voltage, because it will reduce operating current and the size of the motor.

Why We Require Protection of Motors?

To understand this we need to know the cost associated with the failure of motor, i.e.

The basic function of a protective relay is to identify the fault and isolate faulty part from the healthy part of the system. This will improve the reliability of power system. For protection of motor, we have to identify the various causes of failure and to address the same. The various causes of failure are as below

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Motor Protection Relay for High Voltage Induction Motor

Brief descriptions of the different failures are given below:

All motor protection relays operate on the basis of current taken by the motor. Motor protection relay is used for high voltage area having the following features

For setting of the relay we require the CT ratio and full load current of the motor. The setting of different element is listed below

The schematic diagram to connect a motor protection relay is as below Modern digital motor protection relays are having some extra features, i.e. protection against no load running of a motor and thermal protection. In case of no load running, the relay senses the motor current. If it is less than the specified value then it will trip the motor. We can also connect the temperature probe to the relay, which will monitor the bearing and winding temperature and trip the motor if it exceeds the specified value of temperature.

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