Air-Barrier Width Prediction of Interior Permanent Magnet Motor for Electric Vehicle Considering Fatigue Failure by Centrifugal Force

Abstract

Recently, the interior permanent magnet (IPM) motors for electric vehicle (EV) traction motor are being extensively researched because of its high energy density and high efficiency. The traction motor for EV requires high power and high efficiency at the wide driving region. Therefore, it is essential to fully consider the characteristics of the motor from low speed to high-speed driving regions. Especially, when the motor is driven at high speed, a significant centrifugal force is applied to the rotor. Thus, the rotor must be stably structured and be fully endured at the critical speed. In this paper, aims to examine the characteristics of the IPM motor by adjusting the width of air-barrier according to the permanent magnet position which is critical in designing an IPM motor for EV traction motors and to conduct a centrifugal force analysis for grasping mechanical safety.

1. Introduction

Due to the recent exhaustion of fossil fuels and the strict environmental regulation, new methods of developing vehicles for the future have been widely studied to replace conventional engine driving methods. Therefore, the hybrid electric vehicle (HEV), the electric vehicle (EV), and the plug-in hybrid electric vehicle (PHEV) have been developed in order to replace the conventional dieselpowered or gasoline-powered vehicles that are the major cause of air pollution. The HEVs are already marketed for sale. The performance requirements of the EV traction motors include high torque, high speed, high power, high efficiency and miniaturization [1, 2]. In the permanent magnet (PM) motor for EVs traction, the magnetic flux of field does not depend on external power, but is supplied by means of the PM of a rotor. Therefore, its efficiency and output density is high in comparison with other motors, and it can implement high efficiency and miniaturization. It is increasingly used in each field of the industry [3, 4]. As development of power electronics advances control technology, control motors by vector control are applied to more fields to contribute to popularity thereof for precise control, for example, small AC servo motors.

For this reason, the control system had become simplified along with the development of control technology and the price of rare earth magnets with high energy density like Nd-Fe-B, which was expensive, had dropped and thus become competitive. Furthermore, the problem of impossible self-starting, which had been the biggest weakness of synchronous motor, was resolved by magnetic pole position detection and improved control performance, to the extent that even micro speed control became possible. On the other hand, because the rotor of a motor is connected directly to the mechanical load rotating at high speed, stable stress distribution is essential and it should be designed to fully tolerate critical speed [5-9]. Therefore, an important technology is the structural design of the motor. When designing the structure of a rotor, the width of air-barrier depending on the magnet position affects the operation characteristics of the motor. The wider width of air-barrier makes the centerpost narrower to let the magnetic flux by means of the magnet of rotor not stay in the rotor and send it in the air gap direction to improve torque. However, because the wider width of air-barrier contributes to narrowing the centerpost, the centerpost may be broken by centrifugal force in driving the motor at high speed [10-12]. In this paper, aims to examine the relation by adjusting the width of air-barrier which affects the operation characteristics of the IPM motor and the resulting characteristics of the motor by using the finite element method (FEM) based on 2-D numerical analysis. Moreover, the centrifugal force analysis is conducted to identify an sufficient safety factor of the centerpost is ensured and the structural safety of the IPM motor is also ensured if the width of air-barrier is widened to make a narrow centerpost.

 

2. Shape and Specifications of IPM Motor

The specification of the IPM motor for the 2-D numerical analysis is shown in Table 1. The IPM motor for EV traction motors requires high starting torque as well as driving performance of wide speed region. As the IPM motor for EV traction motors implements higher driving speed regions, motors with a driving speed faster than 18000 rpm have been developed. In this study, a motor with a maximum speed of 14500 rpm is selected to establish the maximum speed as a speed based on loading for the centrifugal force analysis in order to draw the width of air-barrier to ensure structural safety. The cross-sectional diagram of the IPM motor for the 2-D numerical analysis is shown in Fig. 1.

Table 1.Specifications of IPM motor

Fig. 1.The cross-sectional diagram of IPM motor

As shown in Fig. 2, the air-barrier in the IPM motor is a hole for cooling which also functions as a flux barrier that interrupts the flow of the magnetic flux. Also, the design parameters for an air-barrier of the IPM motor for driving EV traction motors are shown in Fig. 2. The design parameter X1 and X3 represent the width of air-barrier, and X2 is the width of the centerpost.

Fig. 2.Design parameters of air-barrier

The IPM motor must be designed so that sufficient magnetic saturation can be achieved for the centerpost or the bridge part to maximize the reluctance torque. This is intended to reduce the magnetic flux through the centerpost or the bridge when the magnetic saturation is done so that the magnetic flux generated by the magnet of rotor can be sent to the stator [13]. For sufficient magnetic saturation of the centerpost, the width of air-barrier must be wide. However, when the width of air-barrier is wide, the centerpost is narrowed to result in the risk of damaging the centerpost in high speed driving. Thus, it is necessary to grasp the width of air-barrier in consideration of the fatigue strength of the material of rotor core. Table 2 shows the design parameters for the basic model and the model for adjusting the width of air-barrier. As shown in Table 2, the electromagnetic field analysis and the centrifugal force analysis was conducted by adjusting the air-barrier width of basic model from 9.9 mm up to 10.3 mm at the intervals of 0.1 mm to grasp the structural limit width of air-barrier.

Table 2.Design parameters

 

3. Electromagnetic Field Analysis of IPM Motor by Adjusting Width of Air-Barrier

Fig. 3 shows the magnetic flux density distribution. Fig. 3(a) shows the magnetic flux density distribution of the basic model with the air-barrier width of 9.9 mm, while Fig. 3(b) shows the magnetic flux density distribution of the adjustment model IV with the air-barrier width of 10.3 mm.

Fig. 3.Magnetic flux density distribution: (a) Basic model; (b) Adjustment model IV.

As shown in Fig. 3, the wider width of air-barrier contributes to the severer saturation of magnetic flux at the centerpost. The basic model has the maximum magnetic flux density of 2.10 T but the adjustment model IV has 2.31 T.

3.1 Cogging torque

Fig. 4 shows the cogging torque changes according to the width of air-barrier. The combination of both the number of poles and the number of slots contributes to repeating waveforms at each mechanical angle 5 deg.

Fig. 4.Cogging torque changes according to the width of air-barrier: (a) Cogging torque waveforms; (b) pk-pk and the average torque against of cogging torque.

The cogging torque waveforms according to the width of air-barrier are shown in Fig. 4(a), and Fig. 4(b) shows the cogging torque pk-pk and the average torque against. As shown in Fig. 4, the maximum of cogging torque of the basic model is 0.94 Nm, and the maximum of the adjustment model IV is 0.94 Nm. Hereby, it was confirmed that adjusting the width of air-barrier does not change the cogging torque.

3.2 Torque

Fig. 5 shows torque changes according to the width of air-barrier at the maximum speed of 14500 rpm. Fig. 5(a) shows the torque waveforms according to the width of air-barrier. Fig. 5(b) shows both the average and maximum torques according to the width of air-barrier, and Fig. 5(c) shows the torque ripple according to the width of air-barrier. As shown in Fig. 5, the wider the width of air-barrier, average torques and maximum torques are increased.

Fig. 5.Torque changes according to the width of air-barrier at maximum speed of 14500 rpm: (a) Torque waveforms; (b) Average and maximum torques; (c) Torque ripple.

As a result of Adjusting the width of air-barrier from 9.9 mm to 10.3 mm, the maximum torque and the average torque increased by about 1.54 Nm, 1.47 Nm, respectively. In the basic model, the torque ripple was generated by 10.32 %. However, in the adjustment model IV with the adjusted width of air-barrier of 10.3 mm, it is 10.29 % reduced by 0.03 %. From the result above, it is observed that the wider width of air-barrier results in torque increases but torque ripple decreases.

 

4. Centrifugal Force Analysis of IPM Motor by Adjusting Width of Air-barrier

In order to examine the structural safety of the IPM motor at maximum speed driving, the centrifugal force analysis was conducted by using each model of which the width of air-barrier where the maximum torque occurs was adjusted. The centrifugal force analysis was conducted in the static load condition while driving the motor constantly at the maximum speed of 14500 rpm in consideration of the material properties values.

In this analysis, Von Mises stress is applied. The centrifugal force is proportional to square of rotating speed as shown in (1),

where, F is centrifugal force, m is mass, v is rotating speed and r is radius of rotor. Centrifugal force is proportional to square of speed [14]. Therefore, it is necessary to calculate the mechanical stress caused by centrifugal force at high speed. Fig. 6 shows the stress distribution of the IPM motor obtained through the centrifugal force analysis. Fig. 6(a) shows the stress distribution of the basic model with the air-barrier width of 9.9 mm. The stress distribution of the adjustment model IV with the air-barrier width of 10.3 mm is shown in Fig. 6(b).

Fig. 6.Stress distribution of IPM motor at the maximum speed of 14500 rpm: (a) Basic model; (b) Adjustment model IV.

As shown in Fig. 6, the stress is concentrated on the centerpost.

The maximum stress according to the width of air-barrier is shown in Fig. 7. As shown in Fig. 7, the maximum stress value of adjustment model IV with the width of air-barrier of 10.3 mm is about 448.580 Mpa. This value which is greater than the yield point 393 Mpa of silicon steel (35PN230) for the rotor, and the rotor may be broken during high speed driving in this case. The safety factor according to the width of air-barrier is shown in Fig. 8. The safety factor is 0.87 even smaller than the safety factor of stable general electrical machinery. Therefore, it is ideal to select the adjustment model IV of which the width of air-barrier is 10.3 mm in consideration of just the electromagnetic characteristics. However, if the structural characteristics are considered as well, it is ideal to select the width of air-barrier under than 10.2 mm to implement an ideal safety factor between 1.3 and 1.5. Although it is not described in this paper, an analysis was conducted on the dynamic load condition according to the operating and static testing for the motor durability test.

Fig. 7.Maximum stress of each model.

Fig. 8.Safety factor of each model.

 

5. Conclusion

This paper has presented the electromagnetic field analysis and the centrifugal force analysis according to the width of air-barrier in the high speed driving region of the IPM motor for EV traction motors by using the FEM based on 2-D numerical analysis. As the electromagnetic field analysis result, it was confirmed that more improved operation characteristics of the IPM motor are achieved with the wider width of air-barrier. The cogging torque does not have a significant effect on the width of air-barrier. However, both the average and maximum torques increased by approximately 2 % by adjusting the width of air-barrier. Moreover, the torque ripple which adversely affects the driving characteristics of the IPM motor decreases as the width of air-barrier is wider. Also, the structural limit width of the air-barrier is identified through the centrifugal force analysis. By integrating the result of this study with the optimum design, a more improved and advanced design technology for the IPM motors will be established to ensure the resulting stability as well.