Difference between Locked Rotor Current and Starting Current

Locked Rotor current and Starting current of induction motor seems to be the same thing at first glance but it is not so. These are two different terms having different meaning and significance. In this post we will discuss the difference between the locked current and starting current of induction motor.

Locked rotor current is basically the current drawn by the motor at its rated voltage when its rotor is kept stationary or in other words rotor is not spinning or rotating. So when we start a motor, its rotor is already at rest. This means, starting current and locked rotor current should be same. Isn’t it? No, it is not so. How?

See, motor can be started using one of the various starting methods. If the motor is started using Direct Online (DOL) method, then the voltage applied at its terminal will be rated voltage of motor. As the rotor is at rest during starting and voltage is rated voltage, the starting current will be equal to locked rotor current. But if any other starting method viz. Star Delta / Soft Start is used, then motor will be started at a lower voltage (lower than the rated voltage), hence starting current will be less than the locked rotor current.

 

Another important difference between them is that locked rotor can be anytime during the operation of the motor. For example, consider a motor running normally under its rated load. A sudden increase in load beyond its rating will cause increase in motor current and hence increased motor torque. But if the load torque requirement is more than the motor torque, the motor current will further increase to increase its torque and will reach maximum (equal to pull out torque). If still the load torque requirement is more than the pull out torque, the motor speed will decrease to zero. This is the case of locked rotor. There are various other causes like jamming of motor bearing, load jamming, single phasing of motor etc.

Thus locked rotor current can be drawn anytime depending on when the rotor is being halted or stalled while the starting current is only taken during the motor starting.

Locked rotor current should not persist for a long time else it may lead to insulation failure due to overheating or may lead to buring of stator / rotor.