• 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 Squirrel cage induction motor:
• The resistance of a Squirrel cage induction motor is fixed and small when compared to the reactance of the rotor at standstill condition.
• At this state the frequency of the rotor current is equal to the supply frequency.
• The starting current I2 is high (5 to 7 times the full load current) and lags the voltage E2 by a large angle (almost 90), with the results that the starting torque per ampere is very poor.
• The starting torque is roughly 1.5 times the full load torque.
• Hence the motors are not useful where the motor has to start against heavy loads.
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
• 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 Squirrel cage induction motor:
• The resistance of a Squirrel cage induction motor is fixed and small when compared to the reactance of the rotor at standstill condition.
• At this state the frequency of the rotor current is equal to the supply frequency.
• The starting current I2 is high (5 to 7 times the full load current) and lags the voltage E2 by a large angle (almost 90), with the results that the starting torque per ampere is very poor.
• The starting torque is roughly 1.5 times the full load torque.
• Hence the motors are not useful where the motor has to start against heavy loads.
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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