Environmental Engineering Reference
In-Depth Information
þ
13,
þ
33 and
þ
17) is connected to a corresponding inverter totem pole mid-point as
shown in more detail in Figures 5.46 and 5.47. A (
) phase sign for the 4-pole
connection means that the current entering the connection point of the corresponding
branch (
þ
21 for -A,
þ
9 for -B and
þ
33 for -C) has the opposite direction from the
currents entering the other two branches belonging to the same phase.
Pole changing is performed by inverter control, by re-assigning coil strings
to different phases without the need of any mechanical contactors as shown in
Figures 5.45-5.47.
Indirect vector control, with a shaft encoder, is used for both motor and gen-
erator operation. Because the changeover from 12-pole to 4-pole is performed by
reconnecting the stator coils, the machine flux is reduced to zero during the
transfer.
When the ratio of two speeds is 1:3, 1:5, 1:7 etc. (1:odd number), the coils
usually have full pitch at lower polarity. When the ratio of two speeds is 1:2, 1:4,
1:6 etc. (1:even number), the coils must have shortened pitch at lower polarity. A
special case of PPM winding with speed ratio 1:2 is Dahlander connection, in
which the coil pitch at lower polarity is 50% shortened in order to give a full pitch
at higher polarity.
p
= 12
A
B
C
A
B
C
A
B
C
Phases:
p
= 4
Inverter
totem-pole
connection
points
A
-A
A
B
-B
B
C
-C
C
+
+
+
+
+
+
+
+
+
1
5
9
13
17
21
25
29
33
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
2
22
6
26
10
30
14
34
18
-
-
-
-
-
-
-
-
-
19
3
23
7
27
11
31
15
35
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
20
4
24
8
28
12
32
16
36
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
37
57
41
61
45
65
49
69
53
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
38
58
42
62
46
66
50
70
54
-
-
-
-
-
-
-
-
-
55
39
59
43
63
47
67
51
71
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
56
40
60
44
64
48
68
52
72
+
+
+
+
+
+
+
+
+
Figure 5.45 Winding connection for 4-pole and 12-pole configurations. The coil
polarity is shown in Figure 5.40. The coil connections and the
inverter connection points (indicated above) are fixed. Pole change
is performed by assigning coil strings to the appropriate phases,
through inverter control (see Figures 5.46 and 5.47.)
