5.3.1 Width of coil
B
coilThe width of coil Bcoilillustrated in Fig. 5.5 is calculated based on the equivalent magnetic network of PMAFM-ISA and the specified maximum allowable flux density, which determines the area ratio between the copper coil and the SMC stator bar.
The PMAFM-ISA with different Bcoilare listed in Table 5.4, while the limiting characteristic curves are illustrated in Fig. 5.12. Considering the magnetization curve of the SMC Somaloy 700HR3P illustrated in Fig. 3.11 and the necessary mechanical strength, the Bcoilis adjusted to 3.82 mm.
Version Coil width Flux density at no-load
Basic design 4.14 mm 1.39 T
PMAFM-ISA 1 3.82 mm 1.33 T
PMAFM-ISA 2 3.47 mm 1.27 T
PMAFM-ISA 3 3.09 mm 1.21 T
Table 5.4. PMAFM-ISA design with different coil width Bcoil
PMAFM-ISA 3 PMAFM-ISA 2 PMAFM-ISA 1 Basic design
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
150 180 210 240
180 190 200 210 220 230
Figure 5.12. Limiting characteristic curves of PMAFM-ISA designs with different Bcoil
5.3 Further optimization of PMAFM-ISA with 3D FEM
5.3.2 Skewing of PM
A significant disadvantage of the DLCW with elementary winding2A1 is the large torque ripple. In order to reduce the torque ripple of PMAFM-ISA 1, the skewing of PM is one of the most effective method. There are many different ways to skew PM and two of them are achieved by rotating the end edges of PM with the opposite and the same directions respectively, as illustrated in Fig. 5.13.
θ
θ (a) Skewing method 1
θ
θ
(b) Skewing method 2
Figure 5.13. Different skewing methods of PM
When the PMAFM-ISA designs with differently skewed PM are fed with the maximum current, the torque ripples are listed in Table 5.5.
Version Skewing Skewing Max. Torque method angleθ rippleΔT
PMAFM-ISA 1 - - 54.18 N m
PMAFM-ISA 4 1 12° 16.65 N m
PMAFM-ISA 5 1 15° 13.62 N m
PMAFM-ISA 6 2 15° 14.44 N m
PMAFM-ISA 7 2 18° 10.78 N m
Table 5.5. PMAFM-ISA designs with differently skewed PM
It should be noted that the skewing angleθfor the first method should be limited to avoid the overlapping of the adjacent PMs. In order to achieve an smaller torque ripple less than 11 N m, the PM is skewed with the second method and the skewing angle equals 18°. However, it should be noted that a larger skewing
angle leads to considerable deterioration of the electromagnetic properties of PMAFM-ISA. Therefore, a larger skewing angle is not regarded any more.
5.3.3 Slot opening factor
α
slotSubsequently, the PMAFM-ISA design 7 is further optimized by varying the slot opening factorαslotin terms of the limiting characteristic curves. The designs with differentαslotare listed in Table 5.6, while the limiting characteristic curves are illustrated in Fig. 5.14.
Version Slot opening factor Max. Torque rippleΔT
PMAFM-ISA 7 0.7 10.78 N m
PMAFM-ISA 8 1.0 13.85 N m
PMAFM-ISA 9 0.4 17.23 N m
Table 5.6. PMAFM-ISA designs with different slot opening factorαslot
PMAFM-ISA 9 PMAFM-ISA 8 PMAFM-ISA 7
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
100 150 200 250
150 170 190 210 230
Figure 5.14. Limiting characteristic curves of PMAFM-ISA designs with differentαslot
It can be noted that the limiting characteristic curve of PMAFM-ISA rises with higher slot opening factor. In addition, a slot opening factor equal to 1 implies there is no lateral extrusion of the stator shoe, which simplifies the manufac-turing process considering the brittle mechanical property of SMC. Due to the
5.3 Further optimization of PMAFM-ISA with 3D FEM
above consideration, the slot opening factor αslot is determined as 1. On the other side, the disadvantage, namely the slightly increased torque ripple, has to be accepted.
5.3.4 Pole arc to pole pitch ratio
α
pmBased on the specified geometrical parameters, the PM form including the thick-ness Lpm and the ratio of PM arc to pole pitchαpmare varied. Hereby, the PM mass should remain unchanged. The investigated PMAFM-ISA designs with different PM forms and the torque ripple are listed in Table 5.7, while the limit-ing characteristic curves are illustrated in Fig. 5.15.
Version PM Thickness Ratioαpm
Max.
Torque rippleΔT PMAFM-ISA 8 2.2 mm 2/3 13.85 N m
PMAFM-ISA 10 1.96 mm 0.75 14.38 N m
PMAFM-ISA 12 2.5 mm 0.59 17.82 N m
Table 5.7. PMAFM-ISA designs with different pole arc to pole pitch ratioαpm
PMAFM-ISA 12 PMAFM-ISA 10 PMAFM-ISA 8
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
140 160 180 200 220 240
170 190 210 230
Figure 5.15. Limiting characteristic curves of PMAFM-ISA depending onαpm
From the above 3D FEM results, it is clear that a broad and thin PM leads to improvement of the limiting characteristic curve. In comparison, a thicker and
narrower PM results in higher torque ripple. Based on the above analysis, the PM form remains unchanged. It should be noted that the PM demagnetization of the ultimate PMAFM-ISA design must be carefully examined due to the small PM thickness, especially at high temperature.
5.3.5 Thickness of stator shoe
L
shoeThickness of stator shoe is another important geometric parameter. The PMAFM-ISA designs with different thickness of stator shoe are listed in Table 5.8 and the limiting characteristic curves are illustrated in Fig. 5.16.
Version Stator shoe thickness
PMAFM-ISA 8 5 mm
PMAFM-ISA 13 0 mm
PMAFM-ISA 14 2 mm
PMAFM-ISA 15 8 mm
Table 5.8. PMAFM-ISA designs with different thickness of stator shoe Lshoe
PMAFM-ISA 15 PMAFM-ISA 14 PMAFM-ISA 13 PMAFM-ISA 8
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
140 160 180 200 220 240
170 190 210 230
Figure 5.16. Limiting characteristic curves of PMAFM-ISA depending on Lshoe
It can be seen that the PMAFM-ISA performance becomes better with decreas-ing thickness of stator shoe. However, the thickness of the stator shoe Lshoe remains unchanged considering that a stator shoe is extremely helpful in the
5.3 Further optimization of PMAFM-ISA with 3D FEM
winding process and a minimum thickness of 5 mm is necessary due to the brit-tle mechanical property of SMC.
5.3.6 PM form
For the current optimal design PMAFM-ISA 8, there is an extremely small arc at the inner radius of PM, which can result in great manufacturing difficulty. For this reason, the form is slightly changed by cutting away the PM tips to achieve a flat bottom, as illustrated in Fig. 5.17.
Figure 5.17. Changing of PM form to achieve a flat bottom
Because the PM mass should be kept as constant, the PM thickness Lpmor the pole arc to pole pitch ratioαpmneed to be changed. The PMAFM-ISA designs with the same PM mass and different PM form are listed in Table 5.9, while the limiting characteristic curves are illustrated in Fig. 5.18.
Version PM Thickness Ratioαpm
Max.
Torque rippleΔT
PMAFM-ISA 8 2.2 mm 2/3 13.85 N m
PMAFM-ISA 16 2.3 mm 2/3 11.41 N m
PMAFM-ISA 17 2.2 mm 0.7 10.46 N m
Table 5.9. PMAFM-ISA designs with different PM forms
PMAFM-ISA 17 PMAFM-ISA 16 PMAFM-ISA 8
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
140 160 180 200 220 240
180 190 200 210 220 230
Figure 5.18. Limiting characteristic curves of PMAFM-ISA depending on PM forms
It can be noted that the PMAFM-ISA 17 with broader PM has better perfor-mance by observing the limiting characteristic curve. In addition, the torque ripple is further reduced to less than 11 N m. As a result, the PMAFM-ISA de-sign 17 is considered as the most promising dede-sign temporarily.
5.3.7 SMC material
For the above optimization, the SMC material Somaloy 700HR3P is used to for the stator segments and rotor yoke. However, as mentioned in chapter 3, the components manufactured from this material cannot be machined because of the lack of binder material. For the ideal case, the necessary components should be directly pressed. Due to the unacceptable high costs of press system, the stator segments and rotor yoke for the prototype should be milled from the standard cylinder. For this reason, it is necessary to compare the PMAFM-ISA designs with components made from the other manufacturable SMC materials. The de-signs with different SMC material are listed in Table 5.10 and their limiting characteristic curves are illustrated in Fig. 5.19.
It can be noted the performance of the PMAFM-ISA design 17 declines slightly as the SMC material with binder material shows higher losses and lower permeability. In addition, there is no obvious difference between the PMAFM-ISA made from the SMC Siron®S300b and Siron®S400b.
5.3 Further optimization of PMAFM-ISA with 3D FEM
Version Utilized SMC Material PMAFM-ISA 17 Somaloy 700HR3P PMAFM-ISA 18 Siron®S300b PMAFM-ISA 19 Siron®S400b PMAFM-ISA 20 Siron®STestb
Table 5.10. PMAFM-ISA designs with different SMC material
PMAFM-ISA 20 PMAFM-ISA 19 PMAFM-ISA 18 PMAFM-ISA 17
Rotational speed n / min−1
TorqueT/Nm
0 1000 2000 3000 4000 5000 3500 4000
140 160 180 200 220 240
180 190 200 210 220 230
Figure 5.19. Limiting characteristic curves of PMAFM-ISA depending on SMC material
In comparison, the PMAFM-ISA design based on the material Siron®STestb has better performance in terms of the limiting characteristic curves. Therefore, the PMAFM-ISA design 20 is determined temporarily as the best design.
5.3.8 Segmentation of PM
Eddy current losses in PM can significantly influence performance of electrical machine and are one of the most important thermal sources of PMAFM-ISA, where PMs are mounted on the surface of the rotor yoke [S7]. One of the most effective methods to reduce the PM eddy current losses is the segmentation of PM, which can be either segmented along the radial direction, the circumferen-tial direction or a combination of both directions, namely the 2D segmentation.
The eddy current losses in all PMs of PMAFM-ISA designs with differently seg-mentation of PM at no-load and 60 min−1are listed in Table 5.11. It is apparent that the PM segmentation along the radial direction leads to a more even area of PM segments and can achieve less losses. Compared with the direction of seg-mentation, the number of PM segments or rather the area of per segment is more dominant. For instance, a PM consisting of 6 segments can generate nearly only half of the PM eddy current losses compared with PMAFM-ISA design 20. The informative efficiency map of PMAFM-ISA 25 is illustrated in Fig. 5.20.
Version Segmentation Number of
PM losses method segments
PMAFM-ISA 20 - - 15.65 W
PMAFM-ISA 21 circumferential 2 12.17 W
PMAFM-ISA 22 circumferential 3 9.46 W
PMAFM-ISA 23 radial 2 10.70 W
PMAFM-ISA 24 2D 4 8.84 W
PMAFM-ISA 25 2D 6 7.28 W
Table 5.11. PMAFM-ISA designs with different PM segmentation
With the help of PM segmentation, the efficiency of PMAFM-ISA 25 is ob-viously improved and the peak efficiency increases more than 1.5 % compared with PMAFM-ISA 20. In addition, the reduction of the peak torque at low speed area becomes much smaller and the speed-torque-curve becomes much flatter.