1. Adviser: Ming-Shyan Wang Student: Cian-Yong Fong Student ID: MA120215
A Permanent Magnet Integrated Starter Generator for Electric Vehicle Onboard Range Extender Application
Can-Fei Wang , Meng-Jia Jin , Jian-Xin Shen , and Cheng Yuan
Department of Electrical Engineering, Zhejiang University, Hangzhou , China
Wujiang Nanyuan Electric Appliance Co., Ltd., Suzhou , China
IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 4, APRIL
Adviser: Ming-Shyan Wang
Student: Cian-Yong Fong
Student ID: MA120215
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2. Robot and Servo Drive Lab.
Outline
1.Abstract
2.Introduction
3. Proposed Machine Structures
4. Eddy Current Losses
a. Sleeve on SPM Rotor
b. Short Circuit Rings Around Magnets on SPM Rotor
c. Axial Segmental Sleeves on SPM Rotor
d. Short Circuit Rings Around Magnets on IPM Rotor
5. Experiment Results
6. Cost Evaluation
7. Conclusions
8. References
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Abstract
This paper deals with a permanent magnet (PM) brushless machine used for integrated starter generator (ISG) in an electric vehicle (EV) onboard range extender (ORX).
ISGs with both surface-mounted PM (SPM) and interior PM (IPM) rotors have been developed for comparatively analysis.
Some techniques for eddy current loss reduction are discussed, while influence of the rotor protecting sleeve material and thickness, axial segmental sleeves, and short circuit rings (ShCRs) around each magnet are particularly investigated.
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4. Robot and Servo Drive Lab.
Introduction
In this paper, finite element method (FEM) is used for design and analysis of the ISG. Different stator-slot/rotor-pole combinations and rotor structures are analyzed. Especially, the eddy current loss and fixation method of magnets are comparatively studied for both surface-mounted PM (SPM) and interior PM (IPM) types.
Some different techniques for the magnet fixation with low eddy current loss are analyzed. Subsequently, two 12-slot/10-pole ISG prototypes, with SPM and IPM rotors, respectively, are built for experimental validation.
Furthermore, the cost is also evaluated, taking both material and manufacture cost into account. In conclusion, both SPM and IPM ISGs satisfy the requirements, while the IPM ISG has a slightly lower efficiency as well as lower cost.
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5. Proposed Machine Structures
TABLE II
KEY PARAMETERS OF THE PROPOSED MACHINE
Fig. 1. ISG configurations, (a) SPM, and (b) IPM.
Optimizations based on FEM have been carried out. The proposed ISG machines with both SPM and IPM rotors are shown in Fig. 1 and the key parameters are listed in Table II.In the IPM rotor the laminated core can enclose the magnets by themselves, while in the SPM rotor an additional sleeve is need. The sleeve has to be dealt with cautiously in both electromagnetic design and manufacture process.
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6. Proposed Machine Structures
Fig. 6. Short circuit rings (ShCR) around magnets in IPM.
To reduce the flux leakage, thinner bridge is preferred. However, the bridge should be thick enough to guarantee the mechanical strength, considering the significant centrifugal force when the machine operates at high speed. Usually, magnetic saturation occurs on the bridge rather than on the rib, which is between the two adjacent poles to support the bridge.
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7. Sleeve on SPM Rotor
A major consideration for the sleeve is its eddy current loss, which is associated with the electrical conductivity. Different nonmagnetic materials are comparatively analyzed, including copper, stainless steel and fiberglass. Their electrical conductivity is listed in Table III. Their skin depth when the machine runs at the maximum speed of 4500 rpm is also given in Table III.
TABLE III
PHYSICAL PARAMETERS OF SLEEVE MATERIALS
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8. Robot and Servo Drive Lab.
Sleeve on SPM Rotor
Fig. 2. Eddy current losses in both magnets and different sleeves.
Obviously, fiberglass sleeve is the best in terms of minimum eddy current loss. However, the thermal expansion of fiberglass is quite different from that of the magnets and rotor core. Due to the considerable temperature rise of the rotor when running at high speed, the fiberglass might be broken as a result of the different thermal expansion. Thus, fiberglass is not preferred to serve as the sleeve.
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9. Sleeve on SPM Rotor
TABLE III
PHYSICAL PARAMETERS OF SLEEVE MATERIALS
Fig. 2. Eddy current losses in both magnets and different sleeves.
This is related to the material skin depth. As can be seen from Table III, the skin depth of both copper and stainless steel is much larger than the investigated sleeve thickness in Fig. 2, hence, eddy current exists in the
whole thickness of the sleeve. Therefore, the thinner the sleeve is, the lower the eddy current loss.
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10. Short Circuit Rings Around Magnets on SPM Rotor
Fig. 3. Short circuit rings (ShCR) around magnets in SPM.
In it is proposed to add a short circuit ring (ShCR) around each magnet, as shown in Fig. 3, so as to reduce the eddy current loss in the rotor. Such ShCRs are also examined in this project. Theoretically, the induced currents caused by the flux linkage variation in the ShCR would build an inverted magnetic field to prevent the flux variation. And, the decreased flux variation would also result in eddy current loss reduction in the magnets.
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11. Short Circuit Rings Around Magnets on SPM Rotor
Fig. 4. Comparison of losses with or without sleeve or ShCR in SPM rotor.
When both “sleeve and ShCR” are employed, their influence on the eddy current loss cancels each other. Apparently, the ShCR plays a remarkable role to reduce the eddy current loss. However, it suffers from a considerable ohmic loss due to its extremely large short circuit current. This is also evident in
Fig. 4. Therefore, considering the overall losses and the rotor mechanical strength, the stainless sleeve is essential while the ShCR is not employed.
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12. Axial Segmental Sleeves on SPM Rotor
Fig. 5. Prototype SPM rotor, (a) without sleeve, and (b) with two axially segmental sleeves.
As analyzed earlier, the thinner sleeve produces the less eddy current loss. However, the radial thickness of the sleeve has to be large enough to contain the magnets safely at hgih rotary speed. By other means, the sleeve could be separated into some shorter sleeves in axial direction.
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13. Short Circuit Rings Around Magnets on IPM Rotor
Fig. 7. Comparison of losses with or without ShCR in IPM rotor.
Fig. 8. Prototype IPM machine, (a) IPM rotor, and (b) IPM rotor and nonoverlapping winding stator.
Clearly, by using the ShCR, the eddy current loss is drastically decreased by 90%, viz., from 202W to 18W. However, much more extra ohmic loss exists in the ShCR, so that the total loss of the case “with ShCR” is about 65% higher
than that with the “original” IPM rotor. Therefore, the ShCR is not adopted in this application.
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Experiment Results
Fig. 9. Prototype ISG, (a) ISG alone, and (b) ICE-ISG unit.
Fig. 10. Phase current during operations in (a) starter mode, and (b) generator mode.
The speed in Fig. 10(a) is about 340 rpm, and only two phases of currents are illustrated. Variation of current amplitude reveals that the ICE shaft torque changes in a rotation cycle. When the speed was above 800 rpm, the ISG was turned off and the ICE was run by fuel. The ISG did not work as a generator until the ICE run over the speed of 4000 rpm. Fig. 10(b) demonstrates the phase current waveform at 4800 rpm with a peak value of 47 A.
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15. Cost Evaluation
The main cost saving for the IPM rotor is the magnet cost. Although more magnets are employed in the IPM rotorthan in the SPM rotor, the IPM rotor uses block magnets while the SPM rotor uses tile-shaped magnets, and the former are 20% cheaper per unit weight than the latter. Moreover, the sleeve also increases the cost of the SPM rotor.
TABLE IV
COST COMPARISON (UNIT: CNY)
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Conclusions
It has also been observed that the thinner the sleeve is, the less eddy current loss exists as long as the skin depth is larger than the sleeve thickness.
Furthermore, axially segmenting the sleeve can reduce the eddy current loss effectively, while the short circuit rings (ShCRs) around magnets can reduce the eddy current loss but meanwhile bring significant ohmic loss.
Both SPM and IPM machines can satisfy the requirements, while the IPM machine has a relatively lower cost as well as lower efficiency.
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References
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References
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