The torque developed in the squirrel cage induction motor with auto-starter is _______.

Autotransformer Starting of induction motor

When the full supply voltage per phase (V₁) is applied to the motor with direct switching, then the starting line current drawn by the motor from the supply line is given by,

\(I'_{stl}={V_1\over Z}\)

Now, with the autotransformer starting, if tapping of the autotransformer with transformation ratio 'k' is used, then the voltage available across the secondary winding terminals of the autotransformer is kV₁. Hence, the starting current of the motor with autotransformer starting is given by:

\(I_{stm}={kV_1\over Z}\)

If the no-load current of the transformer is neglected, then, the ratio of currents in a transformer is inversely proportional to the voltage ratio:

\(V_1I'_{stl}=kV_1I_{stm }\)

\(I'_{stl}=k({kV_1\over Z})\)

\(I'_{stl}={k^2V_1\over Z}\)

\(I'_{stl}={k^2}I_{DOL}\)

∵ T α I

Therefore, the torque developed in the squirrel cage induction motor with auto-starter is K2 × torque with direct switching.