Following are the different methods of starting of induction motor:-
1. Rotor resistance starting:- used mainly for the starting of the slip ring type 3 phase induction motor.
2. Direct on-line starter
3. Star-delta starter
4. Auto-transformer starter
These are the 3 starting methods used for the stating of the 3 phase squirrel cage motor. The starting current and the staring torque are two important factor for starting of three phase induction motor. It is necessary that the starting current should be kept low so that motor overheating avoided. The stating torque should must be 50% to 100% more than the load torque. Now let us discuss about the starting of induction motor
1. Rotor resistance stating:-
External resistance should be added to the rotor circuit so that the starting torque should be equal to the maximum torque of the motor. After that cutting out of resistance so that the torque should be maintained at maximum. The adding of the resistance also decreases the current, hence the stating torque is 2 times the full load current .
2. Direct on-line starting:-
This is the most important and inexpensive methods of starting of squirrel cage induction motor. Initially the motor is switched on directly to full load supply. initially current is very large but torque is 0.75 to 2 times of the full load torque. To avoid excessive supply voltage drop because of large starting current. To reduce starting current small cage motor are started at reduce supply voltage.
3.Star delta starting:-
This method is used when motor is connected in delta at normal condition. Both end of the motor are connected through 3-phase change over switch. For starting, the stator wingdings are connected in star and when the machine is the switch is thrown quickly to the running position. Thus connecting the motor to delta in normal operation.
4. Autotransformer stating:-
this method reduce starting voltage applied to the motor and therefore the starting current and torque. The principle is similar to star delta stating and there limitation are also same. the advantage is that adjustable value of current and torque can be obtained.