Page 806 - Industrial Power Engineering and Applications Handbook

23/762 Industrial Power Engineering and Applications Handbook
its excitation voltage, the magnitude of which depends
upon the p.f. of the system, will appear across the motor
terminals or any other appliances connected on the same
circuit. The motor, after disconnection from supply, will
receive the self-excitation voltage from the capacitor and
V, > ES (1.R ignored) while running may act as a generator, giving rise to
voltages at the motor terminals considerably higher than
(a) Receiving end voltage rises with leading p.f.'s
the system voltage itself. The solution to this problem is
to select a capacitor with its capacitive current slightly
less than the magnetizing current, I,,,, of the motor, say,
90% of it. See also Figure 1.15.
At voltages lower than rated, the no-load current, In,,
and the magnetizing current, Z,, of the motor is low and
rises with the voltage. At loads lower than rated, although
the p.f. will diminish sharply as discussed in Section 1.8,
the reactive component I, and the no-load current Ink,
I V, < E, (/.R ignored)
i.e. component OA of Figure 1.16, will remain the same.
(b) Receiving end voltage diminishes with lagging p.f.'s If these facts are not borne in mind when selecting the
capacitor rating, particularly when the p.f. of the motor
is assumed to be lower than the rated p.f. at full load,
Figure 23.30 then at certain loads and voltages it is possible that the

Table 23.7 Recommended capacitor ratings for direct switching with induction motors, to improve power factor to 0.95 or better at
-
all loads
Motor Capacitor rating in kVAr ut a motor rp.m Motor Capacitor ruting in KVAr at a motor r.p.m.
H. p: H. p:
3000 1500 1000 750 600 500 3000 1500 1000 750 600 500
rp.m. %p.m. rp.m. rp.m. rp.m. rp.m rp.m. rp.m. rp.m. rp.m. rp.m. rp.m
__-~
2.5 1 I 1.5 2 2.5 2.5 105 22 24 27 29 36 41
5 2 2 2.5 3.5 4 4 110 23 25 28 30 38 43
7.5 2.5 3 3.5 4.5 5 5.5 115 24 26 29 31 39 44
10 3 4 4.5 5.5 6 6.5 120 25 27 30 32 40 46
12.5 3.5 4.5 5 6.5 7.5 8 125 26 28 31 33 41 47
15 4 5 6 7.5 8.5 9 130 27 29 32 34 43 49
17.5 4.5 5.5 6.5 8 10 10.5 135 28 30 33 35 44 50
20 5 6 7 9 I1 12 140 29 31 34 36 46 52
22.5 5.5 6.5 8 10 12 13 145 30 32 35 37 47 54
25 6 I 9 10.5 13 14.5 1 50 31 33 36 38 48 55
__
27.5 6.5 7.5 9.5 11.5 14 16 155 32 34 37 39 49 56
30 7 8 10 12 15 17 160 33 35 38 40 50 57
32.5 7.5 8.5 I1 13 16 18 165 34 36 39 41 51 59
35 8 9 11.5 13.5 17 19 170 35 37 40 42 53 60
37.5 8.5 9.5 12 14 18 20 175 36 38 41 43 54 61
40 9 10 13 15 19 21 180 37 39 42 44 55 62
42.5 9.5 11 14 16 20 22 185 38 40 43 45 56 63
45 10 11.5 14.5 16.5 21 23 190 38 40 43 45 58 65
47.5 10.5 12 15 17 22 24 195 39 41 44 46 59 66
50 11 12.5 16 18 23 25 200 40 42 45 47 60 61
55 12 13.5 17 19 24 26 205 41 43 46 48 61 68
60 13 14.5 18 20 26 28 210 42 44 47 49 61 69
65 14 15.5 19 21 27 29 215 42 44 47 49 62 70
70 15 16.5 20 22 28 31 220 43 45 48 50 63 71
75 16 17 21 23 29 32 225 44 46 49 51 64 72
80 17 19 22 24 30 34 230 45 47 50 52 65 73
85 18 20 23 25 31 35 235 46 48 51 53 65 74
90 19 21 24 26 33 37 240 46 48 51 53 66 75
95 20 22 25 27 34 38 245 41 49 52 54 67 75
100 21 23 26 28 35 40 250 48 50 53 55 68 76