Difference between Locked Rotor Current and Starting Current

An induction motor motor takes about 5 to 7 times current of its rated full load current when it is started. The motor is at rest and maximum emf is induced in the rotor winding and rotor draws large current. The motor current gradually reduce with acceleration of the motor.The rotor induced voltage is reduced to minimum after motor attains the full rated speed.

The current drawn by the motor starting from its rest position is called starting current. The current drawn by the motor accelerating from rest position when rated voltage is applied to stator is called the locked rotor current. Therefore, in some situations the starting current and locked current is the same. However, it is not essential every time the starting current locked current is the same. Let us discuss how the locked current and starting current are related?

The induction motor is started by a motor starter. The widely used starters are direct online starter and star delta starter. In DOL starter, rated voltage is applied to the stator when motor accelerate from its rest position. Yes, in this case we can say the starting current and the locked current is the same because it is fulfilling following conditions.

  1. Motor is at rest

  2. Rated voltage applied to stator

Now, take the case of star delta starter. The motor is started in star when motor is at its rest position and after accelerating to some speed the motor winding is automatically connected in the delta configuration to accelerate the motor to its rated speed. As you know the voltage in star configuration is 58.7 % of the line voltage. Therefore, the starting current is low as compared to when the motor is run in star. Can we say the starting current in star is equal to the locked rotor current? absolutely NO. Here, we are not applying full rated motor voltage to stator. Therefore, the starting current is much less than the locked rotor current.

When motor is running at its rated speed and driving a mechanical equipment. The load on the motor can be suddenly increase up to that extent that motor in not capable to run the load anymore. In following condition the motor may come to stall if ;

  1. Jamming

  2. Bearing cease

  3. Sudden increase of load due to process variation

In these condition, motor tries to maintain its torque equilibrium with the load torque by drawing additional current from the source and thus the stator current increases. If motor can not drive the equipment the motor will come to stall. The current drawn by the motor in this condition is called locked rotor current. Thus, motor can draw locked rotor current intermittently after running from rest to full speed if condition of jamming happens.

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