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
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.