Handbook of Electrical Engineering

102 HANDBOOK OF ELECTRICAL ENGINEERING

of the rotor bars.

The functionsR 2 (s)andX 2 (s)can be approximated by the following simple linear expressions:-

R 2 (s)=(R 21 −R 20 )s+R 20

and
X 2 (s)=(X 21 −X 20 )s+X 20

Where the suffix 1 refers to the standstill value, and suffix 0 to the full-load value.

The ratio of the standstill values ofR 2 (s)andX 2 (s)to their full-load values are called the
‘deep-bar factors’ which are:-

Deep-bar resistance factor=ur 2 =

R 21

R 20

> 1. 0

Deep-bar reactance factor=ux 2 =

X 21

X 20

< 1. 0

The values of these factors vary with the kW rating and number of poles for the motor, and from one
manufacturer to another. Figures 5.2 and 5.3 show the variations in the deep-bar factors for a range
of motor ratings from 11 kW to 11 MW, taken from a small sample of typical oil industry two-pole
and four-pole motors.

Figure 5.2 Approximate deep-bar resistance and reactance factor curves for two-pole motors rated from 10 kW
to 10 MW.