Environmental Engineering Reference
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over a pole pitch with boundary conditions of 0 mmf at the q -axis of mmf. When
this procedure is followed, the resulting model is obtained.
R sy /4
R s t
R g
R rt
R ry /4
F m
F sm
F rm
F rm
Rotor
end ring
leakage
Stator end
winding
leakage
Stator slot
and diff.
leakage
Rotor slot
and diff.
leakage
F sm
Figure 5.37 IM non-linear model, per pole
This analysis illustrates that the IM is capable of very high torque density if
overdriven well into magnetic saturation. The downside of doing this is that effi-
ciency in the low speed, high torque regime is very low, of the order of 35%.
However, because the M/G is used only transiently at these conditions (engine
cranking), it is a very practical approach to meeting strict package limitations with
a rugged electric machine.
5.4.2 Winding reconfiguration
Expanding the constant power speed range of an IM has traditionally been
accomplished using mechanical contactors. Everyday examples of such approaches
include multi-speed ceiling fans that have separate stator windings for each pole
number. The industrial machine tool industry uses this technique for high speed
spindle applications for which CPSRs > 10:1 are required. In some spindle appli-
cations, speed ratios of up to 30:1 are necessary [28] with low speed for ferrous
metal cutting and high speed for aluminium alloys.
Conventional means of winding changeover have been delta-wye switching to
realize a sqrt(3):1 speed change or to use series-parallel winding reconnection to
realize a 2:1 speed ratio. The series-parallel winding change is most often used in
industrial drives, especially for spindle applications, and it does have merit in
hybrid propulsion systems. Figure 5.38 illustrates the technique employed. Stator
coils in all phase belts can be tapped windings or series-parallel changeover.
Figure 5.38 shows series-parallel changeover for which stator currents can
actually be increased in the high speed, parallel coil configuration. With tapped
stator winding, the low speed and high speed powers are different:
2
P H
P L ¼
N L
N H
ð
5
:
26
Þ
I sH
I sL ¼
N L
N H
where N x refers to the stator coil number of turns in low or high speed modes.
The issue with arrangements as shown in Figure 5.38 is that mechanical con-
tactors for doing winding changeover are typically bulky and not robust enough for
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