Page 72 - Industrial Power Engineering and Applications Handbook

Duties of induction motors 3/53
3.3.5 Continuous duty with intermittent periodic The recommended values of CDF are I5%, 25%. 40%
loading (S,) and 60%:
N
This is a sequence of identical duty cycles, each consisting CDF = -
of a period of operation at constant load and a period of N+V
operation at no-load. The repeat load and no-load periods where
are just adequate to attain thermal equilibrium during N = operation under rated conditions
one duty cycle. There is no rest and de-energizing period, V = operation on no-load and
(Figure 3.6). Unless otherwise specified, the duration of 0, = temperature rise attained during one duty cycle
the duty cycle will be IO minutes. = corresponding to the no-load heating

The designation in this case will be expressed as
One cycle
r V-i S6 - 40% CDF
P-&-+
Likely applications are conveyor belts and machine tools.
3.3.6 Continuous duty with start and brake (S,)
This is a sequence of identical duty cycles, each consisting
of a period of start, a period of operation at constant load
and a period of electric braking. The start, operating and
braking periods are just adequate to attain thermal
equilibrium during one duty cycle. There is no rest and
de-energizing periods (Figure 3.7):
Time - 0 CDF = D+N+F
D+N+F='

et - (Corresponding to no-load heating) where
D = period of starting
N = operation under rated conditions
Figure 3.6 Continuous duty with intermittent periodic loading, S, F = electric braking and
One cycle

t

Load - -u
0
-1
Starting
losses -
No load
losses

f

s
c
$3
I l l ?c
c
Corresponding
to no-load
/ heating
-6
Amb. temp.
- (0= 0)
Time -

Figure 3.7 Continuous duty with starts and brakes, S,