Page 689 - Basic Electrical Engineering

The value of S varies from 0 to 1. Slip is 0 if the rotor is able to rotate at
synchronous speed, N . At the moment of start, rotor speed N is zero, and
s
r
hence slip is 1 or 100 per cent. Rotor resistance R is much smaller than the
2
standstill rotor reactance, X .
20
Using the expression for torque, we can draw the torque versus slip (or
speed) characteristic of the three-phase induction motor.

8.10 STARTING TORQUE

At start, rotor speed N = 0, therefore,
r

From the torque equation with S = 1

starting torque,

The value of starting torque will depend upon the rotor circuit parameters,
i.e., R and X .
20
2

The value of X is generally higher than R . Let us assume R = 1 Ω and X 20
2
2
20
= 8 Ω.

Let us, for the sake of interest, study the value of T if R is increased (which
2
s
can be done by including an extra resistance in the rotor circuit while starting
the motor).

With R = 1 and X = 8, and using K to be having a value, say 100
2
20