Page 692 - Basic Electrical Engineering

When value of R equals the value of X , i.e., when R = 8 Ω, and X = 8
20
2
2
20
Ω, maximum torque will occur at S = 1, i.e., at starting.
Example 8.2 A four-pole, three-phase induction motor is supplied with 400
V, 50 Hz supply. The rotor-circuit resistance is 2 Ω and standstill rotor-circuit

reactance is 8 Ω. Calculate the speed at which maximum torque will be
developed.

Solution:

Given, f = 50 Hz, P = 4. R = 2 Ω, X = 8 Ω
2
20

Condition for maximum torque is given by

R = SX 20
2

substituting values

at this slip, rotor speed, N = (1 − S)N s
r
= (1 − 0.25)1500

= 1125 rpm

The value of maximum torque

We will put the condition for maximum torque, i.e., R = S X in the torque
20
2
equation as

R = SX , Maximum torque T is
m
2
20
putting