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The starting torque of a slip ring induction motor is maximum when rotor resistance/phase is ________ rotor reactance/phase.
less than
equal to
not equal to
more than
equal to
The starting torque of a three-phase slip ring induction motor is maximized when the external resistance inserted into the rotor circuit makes the total rotor resistance per phase equal to the rotor leakage reactance per phase at standstill.
The starting torque of a three-phase slip ring induction motor is maximized when the external resistance inserted into the rotor circuit makes the total rotor resistance per phase equal to the rotor leakage reactance per phase at standstill.
TsтАЛ=╧ЙsтАЛ3тАЛтЛЕR22тАЛ+X2s2тАЛV12тАЛR2тАЛтАЛ тАФ Expression for starting torque
R2тАЛ=X2sтАЛ тАФ Condition for maximum starting torque
The torque produced is proportional to the real power transferred across the air gap. By matching the rotor resistance R2тАЛ to the standstill rotor reactance X2тАЛ, the phase angle of the rotor current is optimized to produce maximum torque. Increasing external resistance allows the motor to achieve this condition at starting, providing high starting torque for heavy industrial loads.
Slip ring induction motors allow external resistance to be added to the rotor circuit.
Higher starting resistance reduces the starting current and increases the starting torque.
The slip at which maximum torque occurs is directly proportional to rotor resistance.
This method is specifically applicable to wound rotor motors, not squirrel cage motors.
High starting torque for high-inertia loads.
Reduced starting current, preventing voltage drops in the supply system.
Increased construction cost due to slip rings and brushes.
Higher maintenance requirement compared to squirrel cage motors.
Cranes and hoists
Elevators and lifts
Compressors
At standstill, the slip s=1, thus the effective reactance is X2sтАЛ=2╧Аf2тАЛL2тАЛ.
Option A is incorrect as torque would be lower due to phase mismatch.
Option C is incorrect as general values do not ensure peak performance.
B is correct тАФ The starting torque is maximized when the total rotor resistance per phase equals the standstill rotor reactance per phase.
In exams, remember that for a squirrel cage motor, the resistance is fixed, so you cannot adjust the starting torque. Always check if the question specifies 'slip ring' or 'wound rotor'.
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