Environmental Engineering Reference
In-Depth Information
S
4
N
4
t
p
135
7
9 1 315
17
19
21
slot #
N
20
S
20
t
p
,
2
Figure 5.48 PPM windings: 4-pole (top) and 20-pole (bottom)
For the 20-pole connection shown in Figure 5.48 (bottom), the phase belt is
equal to one and the pole pitch is equal to two. Pole areas in this connection are
denoted by
N
20
and
S
20
.
Speed ratio (1:even number) - shortened winding pitch at lower polarity.
Assume that an ac machine has to operate in 4-pole and in 16-pole connection
(Figure 5.49).
●
The double layer winding for PPM in this case will have 6 phases at 4 poles
(
m
4
¼
6) and 3 phases at 16 poles (
m
16
¼
3). Phase belt at 4 poles is
q
4
¼
2, and at
16 poles
q
16
¼
1. Coil pitch expressed in number of teeth is
y
¼
9, and the winding
is placed in 48 slots.
Again, as shown in Figure 5.48, only the coils belonging to the same phase are
shown in Figure 5.49 for the case when the speed ratio is an even number.
PPM is the most generic method of arbitrary pole number change in an IM.
When the IM is constructed having toroidal windings, the ability to redefine phases
electronically makes this scheme even more flexible. However, there are issues
with a PPM machine just as discussed for the 2:1 electronic pole change method in
the previous section. The machine must be designed for flux patterns of the lowest
pole number, or some compromise between the low and high pole numbers,
otherwise the machine will be undersized magnetically for the low pole number and
oversized magnetically for the high pole number. Figure 5.50 illustrates the flux
patterns in a PPM machine that executes a 3:1 pole change electronically by
inverter phase group control in the most general sense [31].
It must be pointed out that current research into PPM is focused on minimization
of the current sensor requirements of high phase order systems. Recall that if the
