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further interesting that rare earth magnets in the flux squeeze design do not fare
significantly better than ferrite magnets. The rare earth magnet design gained only
10% higher power density. The ferrite magnet design, however, must be cooled so
that the rotor temperature does not exceed 30 C of ambient since its temperature
coefficient of remanence is 0.17%/ C.
Compared to a conventional synchronous machine, SPM design in this case,
the flux squeeze IPM has higher specific output over the range of 1.3-4.2 kW when
the speed regime ranges from 10 to 100 krpm in aerospace applications.
0.8
0.9
1.0
1.1
Vol. ratio
100
80
60
40
20
10
0.25
1
2345678
Power (kW)
1.3 kW
4.2 kW
Figure 5.25 Constant volume contours of flux squeeze IPM/SPM designs
The flux squeeze design has an advantage over the SPM in terms of specific
power density only for small size machines, 1.3-4.2 kW, as shown in Figure 5.25,
and for high speeds. The design size of larger machines is restricted more by
winding temperature than by airgap flux density, so that conventional large IPM
designs tend to have higher power density than SPM designs.
Comparisons of motor performance based on machine volume are common in
the automotive and aerospace industry. This is because package volume is gen-
erally very restricted and costly in both industries. Other investigators have used
various techniques to compare various electric machines for specific power output
[21]. Comparisons of performance based on flux-mmf diagrams have also come to
the same conclusions as those stated above regarding the IPM in contrast to SPM
and other machines. In the analysis and design experiments performed by the
authors of Reference 21, all the machines studied were designed to occupy the
same volume so that valid comparisons of specific power, torque and torque ripple
could be realized. Figure 5.26 illustrates the relative ranking of the machine types
studied thus far in this chapter. The basis for comparison is that the machines fit the
standard D132 IM frame size, airgaps are identical at 0.5 mm, slot fills are held
fixed at 40% and total copper losses are fixed at 634 W (115 C rise and 7.5 kW
continuous power).
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