What is Thermal Overload

Thermal over loading of electrical motor or over heating problem of electric motor and the necessity of motor thermal overload protection . Whenever we think about the overheating of a motor, the first thing strikes in our mind is over loading. Due to mechanical over loading of the motor draws higher current from the supply which leads to excessive over heating of the motor. The motor can also be excessively over heated if the rotor is mechanically locked i.e. becomes stationary by any external mechanical force. In this situation the motor will draw excessively high current from the supply which also leads to thermal over loading of electrical motor or excessive over heating problem. Another cause of overheating is low supply voltage. As the power id drawn by the motor from the supply depends upon the loading condition of the motor, for lower supply voltage, motor will draw higher current from mains to maintain required torque. Single phasing also causes thermal over loading of motor. When one phase of the supply is out of service, the remaining two phases draw higher current to maintain required load torque and this leads to overheating of the motor. Unbalance condition between three phases of supply also causes over heating of the motor winding, as because unbalance system results to negative sequence current in the stator winding. Again, due to sudden loss and reestablish of supply voltage may cause excessive heating of the motor. Since due to sudden loss of supply voltage, the motor is de-accelerated and due to sudden reestablishment of voltage the motor is accelerated to achieve its rated speed and hence for that motor draws higher current form the supply. As the thermal over loading or over heating of the motor may lead to insulation failure and damage of winding, hence for proper motor thermal overload protection , the motor should be protected against the following conditions

1. Mechanical over loading,

2. Stalling of motor shaft,

3. Low supply voltage,

4. Single phasing of supply mains,

5. Unbalancing of supply mains,

6. Sudden Loss and rebuilding of supply voltage.

The thermal overload relay has three bimetal pole, or strip, relays that are equipped with bimetal tripping elements. The current for the motor runs through these three elements. The tripping elements and poles are heated directly by the electrical current or indirectly by the current by passing through the insulated heating winding that forms part of the protective housing around the pole, or strip. When too much electrical current passes through the pole, it causes it to overheat. As the pole overheats, it begins to bend. This causes the tripping elements to engage, releasing the relay and causing the contacts to switch positions, shutting off the electrical supply to the motor. Because they are designed to help avoid overheating and causing damage to the motor, the temperature readings given by the thermal overload relay might be hotter than the motor actually is. The relay works to shut off the excess electrical current before it reaches the motor which results in the temperature discrepancy.