Stalling in Induction Motors, its effects and prevention

Stalling of the motor refers to a condition, where the motor is unable to rotate.  This condition can be caused either due to any obstruction in the load or due to any problem with the motor such as bearing seizure, etc.

This condition is also known as locked rotor.  

When the speed of the rotor decreases to a very low value or stops completely due to stalling, the slip of the induction motor increases.  This causes higher voltage and consequently higher current to be induced in the rotor windings.  

The stator currents also increase.  The equivalent of the motor stalled condition is that of a transformer whose secondary is short circuited. 

The high current drawn will cause damage to the windings and cause the rotor to heat up.

Stall protection devices work by monitoring the motor current and the speed.  If the motor draws higher current at a preset low speed, the relay is activated.