Page 719 - Basic Electrical Engineering

Example 8.11 A four-pole 50 Hz, three-phase induction motor has rotor
resistance of 0.5 Ω phase. The maximum torque occurs at a speed or 1470

rpm. Calculate the ratio of starting torque to maximum torque.

Solution:

The synchronous speed,

Rotor speed, N = 1200 rpm
r

Slip,

At this slip, torque is maximum. Condition for maximum torque is R =
2
SX .
20

Thus,

For constant supply voltage, the expression for torque, T is given as

Value of maximimum torque T is
m

At starting, S = 1, Starting torque T is
st