Auto transformer starting of squirrel cage induction motor is used to make the starting of induction motor safe by lowering the inrush starting current in induction motor. Auto Transformer starting of squirrel cage induction motor works on the principle of reduced voltage to the stator terminal, that is lowering the supply voltage to the stator. So the voltage is selectively transform low at the time of stating and then make the voltage normal when the motor is running normal.
1.Why the starting current of induction motor is very high compare to running condition.
2.Theoretical concept on Starting of Induction motor.
Operating principle of Auto Transformer
The reduction of the supply voltage is obtained by means of a fixed tap auto-transformer or of a more expensive multiple tap auto-transformer, or even by means of a variable voltage transformer.
During starting with auto-transfomer the motor is connected to one tap of the auto transformer (magnetic only circuit-breaker closed, KA closed, KY closed) which reduces the network voltage by the factor “k” and which draws in the motor a current reduced by the same factor compared to the current that the motor would absorb when supplied directly at the full rated voltage.
Starting with auto-transfomers is to be considered quite expensive, certainly more than Y/Δ starting, and is applied to medium/high power squirrel-cage motors with high inertia.