The voltage is induced when the rotor conductor cuts the magnetic flux. The current starts flowing in the rotor conductor. Due to an interaction of the rotor current and the main flux, the torque is produced.
What happens at the starting of the motor? The voltage induced in the rotor depends on the relative speed of the synchronous speed of the rotation magnetic field and the speed of the rotor. At start, the rotor is at standstill so its speed is equal to zero. At starting, the difference between the speed of the synchronous speed of the rotating magnetic field and the speed of the rotor is maximum.
The difference between the synchronous speed and the rotor speed is called slip of the motor. The slip of the motor is;.
As the speed of the rotor is zero at start,the rotor conductor will cut the maximum flux and the maximum voltage will be induced in the rotor. As the motor starts accelerating the speed of the rotor will align in the direction of synchronous speed of the motor and the slip will get reduced. This is cannot be changed. To reduce starting current, you have to reduce applied voltage as per equation number.
Induction Motor consists of stator and rotor. Stator acts as primary of the transformer and the rotor acts as secondary of the transformer. In this, rotor conductor short-circuited. In a three-phase Induction motor the magnitude of the induced emf depends on the slip of the induction motor.
Here the magnitude of the rotor current is depending upon the magnitude of the rotor emf. The rotor current in the running condition will be. Here you can see when the slip is high the rotor current will be very high. At the same time, The rotor emf is equal to slip times of stator applied voltage.
Hence at the start, the speed of the induction motor Is zero and the slip will be maximum. So that the magnitude of rotor induced emf is very large at the start. But As per rotor construction, the conduction in the rotors are short-circuited. As the motor speed — and, therefore, back EMF — increases, the current further decreases, until normal operating speed and normal operating current are reached. Locked rotor current is the current a motor will draw when the rotor is blocked or has not yet started to move.
The U. Department of Energy explains the difference between inrush current and locked rotor current this way:. Locked rotor current is the root-mean-square RMS current that is established following the peak inrush; the current remains near the locked rotor value during acceleration until the motor approaches its operating speed.
The terms inrush current and starting current are often used to mean locked rotor current. High inrush current can cause nuisance tripping of protective devices or motor damage.
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