Development and Performance Investigation on a 60kW Induction Motor for EV Propulsion

Abstract

This paper introduces the development process and investigation of a 60kW induction motor for electric vehicles. We present performance improvement in the induction motor of electric vehicle using copper die-casting based on a multi-gate process. Copper die-casting motors can reduce the size of the motor, the loss of the rotor, and material costs. We also introduce electromagnetic, thermal, mechanical design and analysis results that satisfy the design and the performance requirements. In order to analyze losses accurately of induction motor, commercial finite element analysis is done considering PWM voltage and thermal characteristics by using lumped-circuit parameters. Experimental tests are also carried out to validate the traction motor design.

1. Introduction

Due to the growing popularity of hybrid and electrical vehicles, traction electric motors are becoming common in the automotive industry [1-4]. The most common form of induction motor utilizes a squirrel-cage rotor, which is composed of a series of longitudinal conductor bars inserted into slots located towards the periphery of a stack of steel laminations. In this study, we designed a 60kW-12,000rpm induction motor for electric vehicles and demonstrate performance improvement of the induction motor using copper die-casting based on a multi-gate process [7, 8]. The multi-gate die-casting process enabled us to obtain a high quality casting product decreasing casting defects from fluid, such as gas traps and mixing of inclusion particles. In addition, copper die-casting motors can reduce the size of motors, the loss of the rotor, and material costs. We include electromagnetic, thermal, mechanical design and analysis that satisfy the design and performance requirements. The electromagnetic dimensions of the motor were determined from the induction motor equivalent circuit and verified from FEM analysis in various operating [5, 6]. In order to accurately analyze the losses of the induction motor, we made a commercial finite element analysis considering the thermal characteristics using a three-dimensional computational flow dynamic analysis. The mechanical designs were made considering structural safety and the vibration response and the thermal conditions from the analysis results. We also made experimental tests on the continuous (30kW) and maximum (60kW) operating conditions to validate the performance of the traction motor design.

 

2. Design of EV Motor

2.1 Performance requirement of motor

The EV motor described in this paper was designed to satisfy the continuous (30kW) and maximum power (60kW) operating conditions as shown in Table 1 and Fig. 1. The motor was designed using an input voltage at the base speed (3,000rpm) and 60% (130V) of the rated voltage. It was increased to the rated voltage (220V) at the maximum speed (12,000rpm) to prevent a decrease in power in the high speed region.

Table 1.Specification of EV motor

Table 2.Design parameters

Fig. 1.Rated performance vs speed

2.2 Design parameters and specifications of the motor

Table 1 shows the design parameters and specifications of the EV induction motor designed for 3,000rpm (base operating speed) using a 130V/100Hz supply voltage; however, it can work up to 12,000rpm.

 

3. Analysis Method of EV Motor

3.1 Electromagnetic analysis

The basic losses, the copper loss and the iron loss, were calculated by 2D electro-magnetic FEM using sinusoidal voltage.

Fig. 2.Flux density distribution

The induction motor was analyzed by time-varying magnetic finite element analysis at the rated conditions (130V/100Hz) using sinusoidal voltage. Fig. 3 shows the flux density distribution. The maximum value of the stator yoke and the teeth flux density were 1.45 [T] and 1.3 [T], respectively.

Fig. 3.Mechanical stress analysis

Table 3 shows the loss analysis by a 2D FEM and equivalent circuit method of the induction motor. Because this machine is driven by a PWM inverter, the efficiency can decrease due to harmonic loss. The stator winding and rotor bar temperature were set at 100℃ and 130℃, respectively. the stray load loss was calculated at about 2.5% of output power and the mechanical loss was calculated using Takeuchi’s formula [11]. In IEC-60034-2-1, the stray load loss was about 1.7% of the output power (1.6% of the input power) at a 60kW rating. In this study, the stray load loss was considered to be higher than the IEC standard because the EV motor is driven by a PWM inverter.

Table 3.Comparison of the loss analysis and test results

3.2 Mechanical stress analysis

To verify the structural safety of the rotor, a stress analysis was performed on the rotor using centrifugal force on the maximum speed as shown in Fig. 3. The maximum stress values of the core, endring and rotor bar were 184, 24, 20 [Mpa], respectively, with safety factors above 2 and the yield strength of core was 380 [Mpa] and that of copper was 55[Mpa]. In the end-ring, the stress of the inner part was higher than that of the outer part because the area decreases toward the inner part with the same centrifugal force. This result shows that the rotor structure was structurally safe enough to withstand the centrifugal force at the maximum speed.

3.3 Modal analysis

A modal analysis of the rotor at maximum speed (l2,000 [rpm]) was done to verify the rotor dynamics characteristics. Fig. 4 shows a modelling of the rotor assembly for FEM analysis, and Table 4 shows the physical properties of the components of the rotor assembly. Non-oriented electrical steel was used for the rotor core and the supported stiffness of the ball bearings was considered. As the 1st natural frequency was 20,400 [cpm] in the analysis results of Fig. 5, the safety factor was 70% compared with the maximum rotating speed, 12,000 [rpm]. The unit cpm is the abbreviation of cycles per minute, which indicates the natural frequency and critical speed in the rotating machine.

Fig. 4.Modelling of the rotor assembly

Table 4.Physical properties of materials for the rotor

Fig. 5.Modal analysis results

3.4 Thermal analysis

Three-dimensional computational flow dynamic analysis has been performed for the induction motor as shown in Fig. 6. Electromagnetic losses from the FEM analysis as in Table 3 were adopted as heat sources for the thermofluidic analysis.

Fig. 6.3D modelling for thermal analysis

Fig. 7 and Table 5 present the temperature distribution and maximum temperature in the specific components of the induction motor. The highest temperature in the motor was predicted to be approximately 129℃ in the rotor and cage with a negligible difference. There was a significant temperature difference between the rotor and stator because of the air gap. It should be also noted that the temperature of the winding part can be compared with thermal saturation test results.

Fig. 7.Thermal analysis results

Table 5.Comparison between the thermal test and the analysis

 

4. Prototype and Test

Fig. 8 shows the process assembling prototype. The rotor bar was made by copper die-casting, and the stator was molded by a synthetic resin to improve cooling capacity.

Fig. 8.Assembling prototype

Fig. 9 shows the test facility for the high speed machine presented in this paper. The test motor was driven by a commercial inverter and cooled by a water jacket. Fig. 10 shows the thermal saturation test on the base operating conditions (30kW at 3,000rpm) which can be compared with the thermal analysis results. The winding temperature was 109℃, which confirms the thermal analysis result (winding temperature: 110.3℃).

Fig. 9.Dynamo system for testing

Fig. 10.Thermal saturation test (30kW_30,000rpm)

Figs. 11 and 12 respectively show the efficiency test results at 30kW and 60kW output power. One was done at temperature saturated conditions to consider continuous operating conditions, and the other was tested instantaneously for the maximum operating condition.

Fig. 11.Efficiency and output test (30kW)

Fig. 12.Efficiency and output test (60kW)

Fig. 13 shows the vibration test results according to the operating conditions. As the vibration characteristics are good under 2.8 [mm/s], the limit of ISO 10816-3, the modal analysis is verified.

Fig. 13.Vibration test

 

5. Conclusion

Recently, the induction motor has drawn more interest for electrical traction because it is less expensive, robust, and inherently safe in case of inverter fault. The induction motor has a rotor cage structure that includes metal (copper or aluminum) for the conducting bar and an end-ring, and the thermal and mechanical analyses are different and more important than in the permanent magnet motor. In this paper, a 60kW-12,000rpm induction motor with a copper die-casted rotor was developed for a traction motor considering mechanical and thermal problems. The electromagnetic dimensions of motor were determined from an induction motor equivalent circuit and verified from FEM analysis in various operating conditions. The design results for the EV traction motor were also verified through experimental tests based on continuous (30kW) and maximum (60kW) operating conditions.