The Torque Slip Characteristic of an Induction Motor & The Starting and Maximum Torque on the curve.


•    Torque developed by the motor at the instant of starting is called starting torque.
•    In some cases it is greater than the normal torque (running torque), where as in some cases it is somewhat less.

Starting Torque of a Slip ring induction motor:
•    The external resistance in the rotor circuit from the star connected rheostat, the rheostat resistance being progressively cut out from the motor gathers speed.

•     Additional external resistance increases the rotor impedance and so reduces the rotor current.

•    It improves the power factor, hence starting torque is increased. After certain stage the effect of increased impedance predominates the effect of improved power factor and so the torque starts decreasing.

Condition for maximum starting torque:
    The condition for maximum starting torque is,

        Rotor resistance = Rotor reactance


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