Motor Protection : Over temperature

Motor temperature is one of the factor that we should monitor and make sure it running in a normal range.It is closely related with winding temperature.If temperature is highly from standard value,it can cause a winding insulator damage or weak.

From that damage,it will make the internal short circuit between winding phase and can cause a serious damage for an electric motor.

What is the main reason for the motor overheat or over temperature?Normally it cause due to a high ambient temperature,under sizing motor capacity,not suitable voltage rated,running in overload and bearing jammed.

To avoid issued for overheat or over temperature,we can install the overheat protection device that can prevent the motor from damage or winding burn.This device is common used for high priority motor in operation to avoid major breakdown.

How Over Temperature /Overheat protection works?

The important component for over temperature protection from type of semiconductor device is called ” Thermistor” is used because this material have the unique property.

It have the comparatively resistance in low value at normal temperature and constant upto some value and has a large positive temperature co-efficient of resistance that can be controlled the range of temperature.

Another good feature is that its power carrying capacity is self-limiting since the device has the tendency to increase its resistance as the temperature rises.


Basic Operation For Over Temperature Protection


During starting sufficient current will flow through the CR relay to actuate it as the temperature and resistance of the thermistors are low.

When the START-push button is pressed the primary winding of control transformer T1 gets energised and therefore the secondary winding sends direct current to CR relay through diodes D1 and D2.

With the closing of CR, contact CR1 closes and hence coil M gets energised. The three main contacts close and the
motor is energised.

Whenever there is excessive motor temperature due to some reason, the thermistor resistance increases and causes reduction in the current flowing through CR relay coil.Thus the CR relay drops, which in turn drops contactor M and therefore the motor stops.

The motor can be started again only when temperature and hence resistance of the thermistors drop to low values.For this, the relay is adjusted such that its pick up current is more than twice its drop out current.


  1. Should the choice of a motor or generator be influenced by the temperature of where you live? For instance, I’ve heard diesel can operate poorly in cooler climates. Does ambient temperature ever effect the efficiency of electricity?

  2. Energy consumption by electrical motors accounts for a major portion of the energy usage of a typical facility. However, the power dissipated by motor starters themselves can be significant, and can range from a few watts to multiple watts depending on the type of motor starter used.

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