Theoretical concept on Starting of Induction motor.

The induction motor is most used for its simplicity and robustness. It is used in mostly domestic purpose and in some cases in industries too. Now other than its advantages, it has a limitation on starting condition. The limitation is that – induction motor starts by itself but draws more than 5 to 7 times current of full load. This high initial current flow has severe impact on motor and circuitry.

Adding external resistance to rotor circuit to improve starting of induction motor

Now adding resistance with rotor circuit would improve the situation. But at running condition this resistance is not acceptable. So we can add resistance while starting and remove it while the motor is running. The phase wound rotor induction motor has this flexibility to add and discard external resistance.
But the squirrel cage induction motor which is most used and popular does not have the flexibility to add external resistance to its rotor circuit. This is due to the fact that in squirrel cage rotor – the copper bar used instead of coil is short circuited permanently.

Adding external resistance to rotor circuit is only possible with phase wound rotor, not possible with squirrel cage rotor.

Reduced voltage concept to improve starting of induction motor

So as adding an external resistance to rotor circuit is not possible for squirrel cage induction motor another method called “Reduced voltage” is used to improve the inrush of starting current. The terminal voltage is reduced so that the starting current is low. As I=V/R, R being fixed when voltage is low the current will be less. But this method has some disadvantages like by lowering the starting current , the starting torque also becomes low. So some calculation is required for specific machines to choose the reduced voltage level. Many popular motor starting method is on the base of reduced voltage concept- like Star Delta starter, Auto transformer starter, reactor starter etc.

Controlling the speed by Frequency converter to improve starting of induction motor

The frequency converter provides ac current of low frequency so that the motor starts at low speed and runs at low speed for a while to be stable enough. Then the speed of motor is increased step by step to rated speed. This is done on the basis of a formula – N=(120f)/p. Here N=speed, f= frequency of current, p= number of pole of the motor. As the pole of the machine is fixed the only variable here is the frequency.

As the motor runs at low speed there will be low torque thus the starting current will be low too. The only disadvantage of using frequency converter is the costing. Frequency converter is a costing
starting of an induction motor

Typical Induction motor. [Photo courtesy- Wikipedia.]
Typical Induction motor. [Photo courtesy- Wikipedia.]

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