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Starting Torque of a Slip Ring Motor

9.4. Starting Torque of a Slip Ring Motor. In a slip ring motor the torque is increased by improving its power factor by adding external resistance in the rotor circuit from the star- connected rheostat; as the motor gains speed the rheostat resistance is gradually cut out. This additional resistance, however, increases the rotor impedance and so reduces the rotor current. At first, the effect of improved power factor predominates the current-decreasing effect of impedance, hence starting torque is increased. But after a certain point, the effect of increased impedance predominates the effect of improved power factor and so the torque starts decreasing.