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International Journal of Energy Science (IJES) Volume 4 Issue 1, February 2014 doi: 10.14355/ijes.2014.0401.05

Study on PMSMs with wide Flux-weakening Speed Range for New Energy Electric Vehicles Bingyi Zhang1, Qingxu Li2, Hao Liu3, Guihong Feng4 School of Electrical Engineering, Shenyang University of Technology No.111, Shenliao West Road, Shenyang, 110870, P.R.CHINA zby541108@vip.sina.com; 2zhenliqingxu@163.com; 3416520056@qq.com; 4fenggh@sut.edu.cn

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Abstract This paper analyzes the permanent magnet synchronous motor used for electric vehicle, and the influence of the builtin “V� type size parameters on direct axis inductance, quadrature axis inductance as well as PM flux linkage. The research is about the change rule of permanent magnet torque and reluctance torque. In the view of the optimal driving system, it summarizes the design method for rotor magnetic circuit structure of PMSM used for electric vehicle, and analyzes the influence on Flux Weakening ability of the main electromagnetic parameters. It also includes the design of two sets of prototype of 25 kW with different rotor structure. Experimental results of the speed regulation characteristic curve and efficiency MAP verify the feasibility of the design rules proposed in this paper. Keywords New Energy Electric Vehicle; PMSM; Speed Adjusting with Fluxweakening

Introduction As the main drive motor of Electric Vehicle, PMSM has become one of the main core power systems because of its high torque density, high efficiency, high power factor, excellent characteristics under the circumstances of low-speed driving and the wide range of speed regulation. Flux-weakening control is needed upon rated speed. If the highest operating speed is not changed, the flux-weakening range will be enhanced when the rated speed is reduced. Therefore, the research of Flux Weakening character has great significance. Flux weakening property is directly related to the magnetic circuit structure of PMSM and Control Method. In order to broaden the range of speed, usually, these two aspects above are taken into consideration. Ai-meng Wang compared with the inductance and Flux Weakening character of 5 kinds of traditional magnetic circuit structure, the PMSM with "V" type structure can be better. Lixiang Chen analyzed one type of magnetic circuit structure with built-in 20

permanent magnet sectioned. This structure can effectively increase the direct axis inductance and improve weak magnetic properties. However, this only analyzes the built-in radial "-" structure. In conclusion, in the aspect of magnetic circuit structure improvements, many methods have been put forward. But when flux weakening can be realized, there will be some new defects or complicated structure against batch manufacturing. There is little literature involved of the built-in "V" type magnetic circuit structure, which is commonly used in the industry. This paper mainly studies the characteristics of speed adjusting with flux weakening about the built-in "V" structure. Analyze the influence of adjacent pole spacing and pole center implant depth on direct axis inductance, quadrate axis inductance, salient pole rate, permanent magnet magnetic chain, permanent magnet torque and reluctance torque. Design rules proposed in this paper. Design and manufacture two set of prototype which has the same rated parameters but different rotor structure parameters. The rated power is 25 kW. Get the characteristic curve of torque-speed and efficiency map graph that can verify the feasibility of the design rules summarized in this paper. Structure Parameter Analysis of the Built-in "V" Type The built-in "V" type structure is relatively simple compared with other structure such as the built-in "w" type and so on. Therefore, it becomes the first choice of a lot of prototype. As is shown in Fig. 1. Rotor core

Slot of permanent magnet

FIG. 1 THE BUILT-IN V TYPE STRUCTURE


International Journal of Energy Science (IJES) Volume 4 Issue 1, February 2014

Usually, adjustable range of spacing between permanent-magnet slot and outer edge of rotor is quite small. Among them, the parameter “b” will also have an impact on the direct axis inductance. Its principle is similar to the built-in "-" type structure as is shown in the paper of Lixiang Chen, Zhaoyu Zhang and Renyuan Tang. There mainly is direct axis inductance; quadrate axis inductance and PM flux linkage which has directly effect on characteristics of flux weakening. So the parameter above is analyzed firstly. In order to analyze the relationship between the magnetic circuit change, relevant parameters and weak magnetic property, in this paper, two parameters are defined as follows in the form of relative value. tj (1) t* j = τ (2) h * = h1 / h 2 Per-unit value of spacing between the two adjacent magnetic poles is shown in (1); Per-unit value of implant depth of the pole center is shown in the paper. “ t j ” is the actual value of spacing between the two adjacent magnetic poles, t j = τ − τ ⋅ α p “ α p ” is pole arc coefficient. “ τ ”is pole distance; Impact Analysis of the Spacing between the Two Adjacent Magnetic Poles on Flux Weakening Change the adjacent pole spacing, and get different magnetic circuit structure. Extract the fundamental component of air gap flux density in 2D finite element model in no load, equivalent direct axis state and quadrate axis state. The change rule of direct axis inductance, quadrate axis inductance and salient pole rate can be calculated from the data above in the paper of Qian-fan Zhang, Hong-xing Wu, Shu-kang Cheng. As is shown in Fig. 2 and Fig. 3.

FIG. 2 INDUCTANCE

FIG. 3 SALIENT POLE RATE

It is known that the direct axis inductance increase a little with the increase of the adjacent pole spacing from the overall trend. But the quadrate axis inductance increases more. The main factor having impact on direct axis inductance is the magnetic resistance characteristics near the axis of each pole.

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Along with the adjacent pole spacing increases, the quadrate axis reluctance reduce greatly, so the quadrate axis inductance increases obviously. Using the parametric scanning function of ANSOFT software, change the rotor position, and get the characteristic curve of torque and angle. As is shown in Fig. 4. After the data processing, the Fig. 5 can be gotten.

FIG. 4 RESULTANT TORQUE FIG. 5 TORQUE COMPONENT

When the current of external controller is constant, reluctance torque depends mainly on the difference of direct axis inductance and quadrate axis inductance. If the adjacent pole spacing is too big, growth rate of quadrate axis inductance will be greater than the direct axis inductance. It also leads to a substantial increase of reluctance torque. Permanent magnet torque only depends on the magnetic chain of permanent magnet and quadrate axis current. The magnetic chain of PM reduces due to the increasing of adjacent pole spacing. So, on the whole, permanent magnet torque decline. But because of the influence of higher order harmonics, harmonics of permanent magnet torque increase lead to small increase of permanent magnet synthetic torque when the adjacent pole spacing is too big. Impact Analysis of the Implant Depth of the Pole Center on Flux Weakening In the guarantee constant adjacent pole spacing, change implant depth of pole center, and get change rules of inductance as is shown in Fig. 6. Sensitivity of direct axis inductance to pole center implant depth is low. In most of the interval, direct axis inductance changes little until depth of per-unit is close to 2. And then it appears to drop considerably. The quadrate axis magnetic circuit mainly locates in physical axis of adjacent poles. Most of the quadrate axis magnetic passes through radial inside of the "V" type permanent magnet. If the implant depth increases, the distance between permanent magnet and inner diameter of rotor will decrease. And then local saturation may emerge. The permanent magnet torque also presents the linear 21


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International Journal of Energy Science (IJES) Volume 4 Issue 1, February 2014

trend of increase. And reluctance torque generally decreases, as is shown in Fig. 7. Electromagnetic torque of synthesis also increases.

FIG. 6 INDUCTANCE

FIG. 7 TORQUE COMPONENT

The back EMF is the induced voltage generated by permanent magnet at the rated speed. In this paper, it is expressed as “e0”. In order to reflect the influence of electromagnetic design scheme on flux weakening more intuitively, the back EMF is taken to express the magnetic field strength. When the controller capacity, maximum inverter voltage and peak current is not changed, the back EMF and direct axis inductance will determine the highest ideal speed in (10). u lim n max = e 0 2π p (10) Ld i lim − nN 60 Usually, the back EMF is greater than the demagnetization component. So the characteristics of flux weakening more depend on rational design of the back EMF. Type (11) can be deduced from type (10). e0 nN 2π p 1 (11) n L i = + u lim n max 60 N d lim u lim TAB 1 APPROXIMATE RELATIONSHIP BETWEEN FLUX WEAKENING RATIO AND THE BACK EMF

A. Change implant depth of the poles center pole center implant depth (per-unit value) 2.489 1.432 0.672 0.384

direct axis inductance Ld(mH) 0.111 0.179 0.182 0.176

per-unit value ratio of flux of back EMF weakening (nmax/nN) /(E0/UN) 0.988 1.322 0.886 1.948 0.786 2.453 0.723 2.800

size of magnetic circuit and calculate parameters above. Different inductance and magnetic chain of permanent magnet can be acquired. The relationship between the back EMF and the ratio of flux weakening is shown in Tab 1. According to Tab 1, along with the back EMF reduction, the ratio of flux weakening grows higher, and increasing of direct axis inductance further improves flux weakening ability. The maximum ideal speed expressed in (10) is under no-load condition. However, there must be quadrate axis component of the current under the actual loading condition. So the role of quadrate axis inductance will be highlighted. Contrast Test and Analysis of Prototype According to analysis above, it is known that the characteristics of flux weakening depend on both magnetic circuit structure and current control strategy. This paper adopts the same flux weakening control method to test two sets of prototype with the same rated parameters for the purposes of comparison. The difference between the two sets of prototype is reflected in the back EMF and salient pole rate. As is shown in Tab. 2. The test platform is shown in Fig. 8. TAB. 2 THE MAIN PARAMETERS CONTRAST OF THE PROTOTYPE

Parameters Rated power (kW) Rated speed (r/min) Rated current (A) d-axis inductance (mH) q-axis inductance (mH) Salient pole rate E0/UN

1# prototype 25 3800 107 0.1114 0.2239 2.010 0.97

2# prototype 25 3800 135 0.1439 0.4064 2.824 0.70

FIG. 8 PROTOTYPES TEST

FIG. 9 TORQUE VS. SPEED

B.Change the adjacent pole spacing direct axis adjacent pole inductance Ld spacing (mH) (per-unit value) 0.029 0.057 0.229 0.286

0.111 0.162 0.173 0.178

per-unit value of back EMF/(E0/UN)

ratio of flux weakening (nmax/nN)

0.988 0.943 0.865 0.835

1.322 1.652 1.976 2.157

The ratio of the rated speed to the highest speed is smaller, that is, the ratio of flux weakening is higher in (11). And the back EMF should be lower. Take a prototype of 25 kW for example. Adjust the structure 22

(A)1 # PROTOTYPE

(B) 2 # PROTOTYPE

FIG. 10 EFFICIENCY MAP DIAGRAM


International Journal of Energy Science (IJES) Volume 4 Issue 1, February 2014

The control strategy is MTPA under the rated speed, and flux-weakening control is adopted upon the rated speed. The characteristic curve of speed-torque in all the speed range can be acquired in Fig. 9. The current of the first prototype is much smaller than the second one. The output torque decline obviously over the rated speed in constant power speed regulation. The efficient MAP of the two set of prototype is separately shown in Fig. 10. According to the experimental data, by contrast, the rated efficient of the first one is higher than the second one. According to the above experimental data, the characteristics of flux weakening often contradict performance indicators at the rated state, such as efficiency, torque output, etc. If the character within the rated speed of constant torque is pursued, the back EMF should be close to the rated voltage. If the character of flux weakening is pursued, the magnetic field should be weakened. Conclusion In the field of PMSM used for electric vehicle, the characteristic on certain working point should not be over emphasized. The scope with high performance should be broadened. If the working condition has low demand for the characteristic of flux weakening, the magnetic chain of rotor can be increased appropriately. On the one hand, it is helpful to improve index in the area of constant torque control. On the other hand, the rated current will be decreased. If the working condition has high demand for the characteristic of flux weakening, the permanent magnet should be reduced. In this way, the back EMF is low enough to realize a wide speed range. At the same time, in order to increase the torque output, the salient pole rate should be improved. Make full use of reluctance torque. Therefore, the adjacent pole spacing should be increased and the implant depth of the pole center

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should be reduced appropriately in the built-in "V" type magnetic circuit structure. REFERENCES

Ai-meng Wang, Wei-fu Lu, "Performance Analysis and Comparison of Five PMSM Topologies". Small & Special Electrical Machines, 2010,30(4) ,20-23 Chen Lixiang; Zhang Zhaoyu; Tang Renyuan; A New Structure of Permanent Magnet Traction Motor for Improving Flux-Weakening Level [J]. Transactions of China Electrotechnical Society, 2012,27(3): 100-104 Shu-kang Cheng, Ping Zheng, Shu-mei Cui, Li -Wei Song, and Li-yi Li. "Fundamental Research on Hybrid Magnetic Circuit Multi Couple Electric Machine". Proceedings of the Csee. 2000, 20(4), 50-54. Bingyi Zhang, Born in 1954, professor, PhD supervisor, Mainly engaged in the special motor design and control of low speed high torque motor, low-voltage, high-power motors, etc. zby541108@vip. sina.com

Qingxu Li, Born in 1985, PhD, Electrical and electronics, Shenyang university of technology, Shenyang, China, Mainly engaged in the motor design of PMSM and control strategy research zhenliqingxu@163.com.

Hao Liu, Born in 1990, Postgraduate, Electrical and electronics, Shenyang university of technology, Shenyang, China, Mainly engaged in the motor design and analysis of PMSM. 416520056@qq.com Guihong Feng, Born in 1963, professor, Mainly engaged in special motor design. fenggh@sut.edu.cn

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Study on PMSMs with wide Flux-weakening Speed Range for New Energy Electric Vehicles  

http://www.ijesci.org/paperInfo.aspx?ID=14278 This paper analyzes the permanent magnet synchronous motor used for electric vehicle, and th...