Methods of Improving Starting Torque

Methods of Improving Starting Torque:

• Starting torque of induction motor can be increased by increasing the rotor resistance.This can be done by reducing cross sectional area of rotor bars and by changing the conductor material. However, This will reduce the efficiency of motor. This problem can be overcomed by using different design for rotor which gives high starting torque without compromising the efficiency. Following two types of rotor design's are used. 

1). Double cage rotors. 

2). Deep bar rotors. 

1). Double cage rotors:

• In this type of rotor construction, rotor winding has two cages. As shown in the above Fig, the outer cage uses bars of small cross section, thus it gives high resistance. As this cage is near to the air gap, its leakage reactance is small. The inner cage has bars of larger cross section so it offers less resistance, but as its is away from the air gap it offers high leakage reactance.

• In this type of motors, division of current between the outer and inner cage is determined by the impedance of each cage. At starting the frequency of rotor current is same as supply, hence reactance of cages is much higher than resistance so most of the current flows through outer cage (due to its lower reactance). Thus at starting we get high staring torque and low starting current (as outer cage has high resistance).

• Under load conditions, as the motor runs near the synchronous speed, rotor frequency is very-small so reactance's of two cages are negligible, hence the curent division between two cages is determined by the resistance and most of the curent is carried by inner cage (out to its small resistance). This gives good efficiency under running condition.

2). Deep bar rotors:

• Above shows shapes of deep bar and slot used in this type of rotors.

• At the starting due to high leakage (as frequency is high), current is mainly confined to the upper portion of the conductor and hence gives high resistance which results in the high starting torque. Under running condition, the current is distributed uniformly which offers less resistance which results in good efficiency.