The resistance of squirrel cage rotor is fixed and small as compared to reactance. Reactance of rotor is high as frequency of a current is equals to supply frequency. Hance at starting power factor is poor. starting current is although very high but lags by very large angle behind E2. So the torque is only 1.5 times the full load torque but starting current is 5 to 7 times the full load current. Hence squirrel cage induction motor is not suitable for load which required high starting torque.
where as in slip ring induction motor the starting torque of motor increases by improving the power factor of motor. In this we connect the start connected rheostat to the rotor circuit. So the current decreases but torque increases. When motor catches speed this external resistance gradually cut off and all the slip rings are short circuit by copper bar and the brushes are lifted up and work as squirrel cage induction motor.
Impedance traingle of rotor circuit of induction motor.
Let E2 = rotar EMF per phase at stand still
R2 = rotar resistance per phase
X2 = rota reactance per per phase at stand still
Z2 = √( R2² + X2²) rotar impedance per phase at stand Still
Rotor current,
I2 = E2 / √( R2² + X2²)
power factor,
cos∅2 = R2 / √( R2² + X2²)
T = K1 E₂ R2/( R2² + X2²)
Rotor torque
= K1 * E2 / √( R2² + X2²) * E2 * R2 / √( R2² + X2²)
= K1 E₂² R2 /(R2² + X2²)
if we increase the resistance of rotor circuit then we can improve the power factor.
E2 rotor induced EMF
I2 rotor current
R2 rotor resistance per phase
X2 rotor reactance per phase at stand still
from the above equation it is clear that if we increases external resistance R2 then torque will increase but are two total current will decreases and power factor also improves.
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