Page 73 - Industrial Power Engineering and Applications Handbook

3/54 Industrial Power Engineering and Applications Handbook

6, = temperature rise attained during one duty cycle D+NI
= corresponding to the no-load heating. CDF =
D+NI +F, +N2 +F2 +N3
For this duty cycle also, the abbreviation is followed by at speed Nr, for load PI
the indication of number of cycles per hour and the FI.
For example, for 300 c/h and FI 2.5
S, - 300 ch FI - 2.5
at speed Nr2 for load P2
Likely applications are machine tools.
3.3.7 Continuous duty with periodic speed
changes (Ss)
at speed Nr3 for load P3
This is a sequence of identical duty cycles, each consisting
of a period of operation at constant load, corresponding where
to a determined speed of rotation, followed immediately F,, F2 = changeover of speed by acceleration.
by a period of operation at another load, corresponding D = electrical braking, Nr3 to Nr,
to another speed of rotation, say, by change of number of N,, N2, N3 = operation under rated conditions and
poles. The operating periods are just adequate to attain 0, = temperature reached during one duty cycle
thermal equilibrium during one duty cycle. There is no = corresponding to the heating under rated
rest and de-energizing period (Figure 3.8): conditions (PI as in Figure 3.8)

One cycle
4
I 1

Speed change-over 'I
(braking or
acceleration) losses
No load
t
I I
P
w

2

I- Corresponding
- to load P,