Page 80 - Industrial Power Engineering and Applications Handbook

Duties of induction motors 3/61
0.2

0.18

0.16

0.14

-
t O.I2
8 0.1
E
(I)
0.08

0.06
LL2.0
@m
- L1.5 0.04
Om
- BC,, .2
@m
.o
~Bc=l 0.02
om
%=0.75
ern
0
0 1'2 3 4 6 7 0 1 2 3 4 5 6 7
150'7,
~
Y
Only these portions
of curves are relevant
Only these portions
of curves are relevant
(a) For 1. s 200% I
(b) For > 200%
4
' I
Note For ease of illustration two graphs are drawn ($5 200% and 1, > 200% , For actual use, the relevant portions of the graphs (as marked)
alone must be drawn on one common graph. More points can be plotted in the required region for a closer setting of the relay.
Figure 3.13 Thermal curves to set the relay for over-current protection corresponding to different operating temperatures

Example 3.4 where Om = 115 - 40
(a) If a CMR 25 h.p. motor, with a thermal heating constant
of 1.5 hours reaches a maximum temperature of 115°C in = 75°C
continuous operation with an ambient temperature of 40°C, From (a) and (b)
then the half-hour rating P of this motor can be determined om = 0. (3
as below.
Compare the temperature rises which are proportional to
the losses at the two outputs and the losses are proportional to
the square of the load. Ignoring the mechanical losses then - e-% ) . (51'
Om for load 'P' = P' (a) or om = e,[i
and @(, ,ph,, for 25 h.p. = (25)' when run just for half an hour
or 1 = (1 - 0.716).
(b)
Since a 25 h.p. motor now operates only for half an hour 25
or P=-
210.284
= 47 h.p.