Thermal Protection in Motors

A bimetallic overload Relay
Thermal Protection is an important protection in motors.  Motors can get heated due to overloading, high ambient temperature, variations in power quality, etc.  Thermal overload can result in stator overheating, faulty operation and in some extreme cases even fire.  Hence, all motors need to be fitted with protection against thermal overload.

Thermal overload protections can be classified into three types  viz. Bimetallic, Magnetic and temperature sensing protection.

Bimetallic Protection
In bimetallic protections, a strip of two metals which are attached to one another is used.  The motor current is made to pass through the strips.  As current passes through the strips, the strips heat up and expand.  Since, the strip is made up of two different metals and these metals have different rates of expansion, the strip bends in one direction.  When the temperature of the strip reaches a particular value, it activates a mechanism which trips the motor.   This kind of protection is widely used and is simple in construction.    However, this method is not suitable in applications which require frequent starting and stopping of the motor.

The bimetallic protection gets reset faster than the motor cooling temperature and it may thus permit the motor to be started again when the motor has not sufficiently cooled from the thermal overload.  

Magnetic Protection
This consists of a magnetic element whose field strength is a function of the motor current.  When the motor current exceeds a preset value, the electromagnet inside the relay operates and trips the motor.  The downside of this kind of protection is that it does not take into account ambient operating conditions such as temperature and ventilation which play an important role in the temperature rise of motors.  

Temperature based thermal overload protection
This is method of protection that is relatively new.  This method involves actually measuring the temperature of the motor and those of the winding hotspots using a temperature sensor such as the RTD (Resistance Temperature Detector).  This method uses direct temperature sensing and is the most reliable and accurate, though, it is more expensive.