Starting torque of an induction motor is very less compared to its rated torque. The torque of an induction motor depends on the rotor resistance, rotor reactance, and on the slip of the motor.
Starting Torque of the induction Motor
The starting torque of the induction motor can be expressed by the following mathematical formula. If the synchronous speed of the motor is ns, and the torque delivery of the motor is T, the input power to the rotor is given by;
Pr = 2πnsT —————(1)
The slip power is equal to the copper loss in the rotor circuit.
The slip Power
= s x Rotor input Power
= s x 2πnsT ————(2)
The slip power is equal to the copper loss in the rotor.
From equation(4) it is clear that the torque of the induction motor is proportional to the rotor resistance and inversely proportional to the reactance of the rotor. When an induction motor is started, the slip is unity and the slip and the rotor reactance start getting decreased with an increase in the speed of the motor. The starting torque of the motor is given by,
The rotor resistance does not depend on the slip of the motor. The rotor reactance depends on the slip. At the start, the rotor reactance is large because the slip of the motor at the start is equal to unity. The large rotor reactance at the start makes the starting torque of the motor poor. The motor draws about 5 to 6 times the current of its full load current and produces very less torque. Most of the current is reactive in nature and does not contribute to the production of torque. That is why the starting torque of the squirrel cage induction motor is poor.
The slip ring motor can produce more torque than the squirrel cage induction motor. The additional resistance can be added to the rotor winding of the slip ring induction motor. The starting current gets reduced and starting torque of the slip ring induction motor improves. When the motor attains its rated speed the rotor resistance is short-circuited and the motor functions like a squirrel cage induction motor.
The starting torque of the squirrel cage induction motor can be improved if the rotor resistance is increased. The double cage squirrel cage induction motor is used for the application which requires higher starting torque. At the start, the current flows through the outer cage winding of the motor which has higher resistance than the inner cage winding resistance. The current starts shifting from the upper or outer cage to the inner cage when the motor starts accelerating towards its base speed. Most of the current flow through the inner cage when the motor is running at its rated speed.
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