Every motor must be protected from all possible faults to ensure prolonged and safe operation as well as time loss caused due to breakdown. Almost all the industries rely on the electric motor to control its processes and production. Hence it is necessary to make the motor fail-safe.
The time taken by them to open the contactor during overloads is specified by the trip class. It is commonly classified into Class 10, Class 20, Class 30 and Class 5. The OLR trips in 10 seconds, 20 seconds, 30 seconds and 5 seconds respectively at 600% of full load current to the motor. Class 10 and Class 20 are very commonly used ones. Class 30 overload relay are used for protection of motors driving high inertia loads and Class 5 relay are used for the motors requiring very fast tripping.
| 1.058 * Ir | 1.2 * Ir | 1.5 * Ir | 7.2 * Ir | |
|---|---|---|---|---|
| Class | Time to trip from a cold start | |||
| 10A | >hr | <2hr | <2min | 2s |
| 10 | >2hr | <2hr | <4min | 2s |
| 20 | >2hr | <2hr | 8min | 2s |
| 30 | >2hr | <2hr | <12min | 2s |
As discussed above, there are three major conditions for overload trips:
Apart from these, there may be some additional protection feature available. This varies from one manufacturer to the other.
During normal operation, the current flowing through each pole of an overload relay to the motor remains the same at a time. If anyone of the phase is interrupted, the current through the other two phases rises to 1.73 times the normal value. Hence the overload relay gets heated up and it trips. Phase failure is also known as single phasing of motoror phase loss.
Overload relay cannot protect against short circuits. They should always be used with short circuit protection devices. Otherwise, any short circuits in the motor can potentially damage them. They can protect against overloads, phase loss, and phase imbalance, but not short circuits.
Overload relay is a device that can protect a motor from overloads, phase failure and phase imbalances. Based on the principle of operation they are classified into thermal and electronic overload relay.Thermal OLRis based on the principle of deformation of a bimetallic strip on heating and the electronic overload relay is a microprocessor-based device.
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