Auto Transformer starting of squirrel cage rotor Induction motor.

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.

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.

The current on the primary of the transformer, and consequently on the line, results to be reduced by k2. Consequently to the voltage reduction by factor k, also the inrush torque is reduced by k2 compared to starting at full voltage. When the motor has reached approximately 80% to 90% of its steady-state speed, the contactor KY is opened and the motor continues to be supplied at reduced voltage by the inductance of the auto-transformer windings. At this point, the contactor KL is closed and the contactor KA is opened so that the motor is supplied directly from the network.

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.

auto transformer starting of squirrel cage rotor induction motor
auto transformer starting of squirrel cage rotor induction motor

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