Question

List the factors that determine, the starting torque of the three- phase induction motor.

How does this torque generally compare with the value of the rated torque?

Starting torque is given by

Where

E2 = rotor emf / phase at standstill

R2 = rotor resistance / phase

Ns = synchronous speed

X2 = rotor reactance / phase at stand still

Factor on which it depends are

Voltage ( E2 α V)

Therefore

Torque is very sensitive to voltage 4 change in 5% of supply voltage will produce a change of approximately 10% in the rotor torque.

Rotor reactance (X2) / phase [ X2 = 2 π f L] varies with frequency.

Frequency of supply

Resistance of rotor phase.

The starting torque is normally 1.5 times the full load / rotor torque.