DEVELOPMENT OF A SWITCHED RELUCTANCE

MOTOR FOR AUTOMOTIVE TRACTION

APPLICATIONS

Professor: Ming-Shyan WangStudent: Zong-Ren YangID Number:4A02C071

© EVS-25 Shenzhen, China, Nov. 5-9, 2010 The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

Saphir Faid1 , Patrick Debal1 , and Steven Bervoets11Punch Powertrain, R&D Department, Schurhovenveld 4 125, BE3800 Sint-Truiden, Belgium

ABSTRACT+ Electric motor is key part in electric vehicles including hybrid electric

vehicle, fuel cell electric vehicle and battery electric vehicle. Wide torque-speed range and high reliability are needed of the motor applied in electric vehicles. Novel hybrid switched reluctance motor is developed. It combines features of robust as switched reluctance motor and that of high efficiency of permanent magnet motor. Flux strengthening and weakening control give large maximum torque and high speed to the motor drives. They are implemented by only simply controlling the magnitude and direction of the current in an additional coil in the motor. Rotor position is detected by the signal from stationary coils in the motor and rotor speed is calculated according to this signal. To test characteristics of the motor drives, an experimental bench is developed. It is easy to test four quadrants torque-speed and dynamic characteristics of the motor drives. The whole testing system is energy saving.

INTRODUCTION Punch Powertrain develops hybrid and electric drivetrains for passenger

cars. The electric traction motor is a crucial component as it impacts the vehicle’s performance and because the cost of batteries related to range requirements urges for a highly efficient but costeffective drivetrain. For the projects under development, several options for electric traction motors were investigated, namely permanent magnet motors, induction motors and switched reluctance motors (SRM). Induction motors are the most widely used type in industrial applications as well as heavy traction applications (railway, electric buses,…). Permanent magnet synchronous motors offer significantly better efficiency and power density, which has led to increasing popularity in hybrid and electric passenger cars, electric bicycles, scooters,… However, cost and supply concerns regarding the limited reserves of rare earth magnets are a limiting factor for application of permanent magnet motors in a scenario of serious worldwide electrification of mobility . The topic on the most suitable electric machine remains open, and because of some particular advantages, switched reluctance motors may offer an interesting solution for applications requiring a highly performing but cost-effective solution.

MOTOR SPECIFICATION

+ Based on vehicle performance simulations for a hybrid vehicle , the following specifications were targeted for this electric motor design:

Peak Torque: 200 Nm Peak Power: 30 kW Speed Range: 0-10 000 rpm Continuous Power: 15 Kw In order to achieve optimal integration into the hybrid powertrain

package developed by Punch Powertrain , the following dimensional constraints were set:

Diameter: 225 mm Length: 275 mm

SRM PROPERTIES+ A switched reluctance motor produces torque

purely through interaction of the stator field with rotor saliency. The basic operation of a switched reluctance motor is illustrated. The stator consists of laminated iron with stator poles and windings. The rotor is just laminated iron. By exciting a pair of opposing stator windings, the principle of reluctance will cause a torque to align the rotor poles with the stator poles. The simplest design can be a single phase motor, but thiswould not allow total control of the motor. The shown design is an 8/6 configuration with 8 stator poles and 6 rotor poles which is a typical four phase motor. The four phase operation offers the possibility to achieve full torque in each rotor position and allows smoothening of torque ripple at low speeds a demonstrated in this paper.

CHARACTERIZATION+ The manufactured prototypes were validated

on a test rig with full measurement equipment such as a torque measurement device for accurate characterization. The characterization on the test rig allowed more precise determination of the efficiency map and actual torque delivered by the motor. During characterization on the test bench, these calculated control parameters were tested to deliver the requested torque and efficiency. After optimization of the control parameters, the final torque-speed map was determined, the ‘fingerprint’ of the motor from a vehicle point of view. As visible on figure , the motor actually delivers higher peak power at medium speeds and achieves efficiency in excess of 90% (motor +inverter) in a wide speed range.

IMPLEMENTATION & IN-VEHICLEASSESSMENT

+ After characterization the motor was integrated with the other subsystems into a complete parallel hybrid powertrain as illustrated in figure The powertrain was then implemented into test vehicles where performance and drivability could be assessed . During the first assessment, torque ripple was investigated and brought to an acceptable level for comfortable driving, even at low speeds. The motor performance was fulfilling its expectations delivering the requested torque precisely, and demonstrating reliability. Regarding NVH aspects there were no issues with motor vibrations transmitted to the vehicle, and the acoustic noise produced by the motor itself although clearly perceivable at low speeds especially in generating mode, seemed acceptable to most of the users (subjective assessment). It must me noted that controls parameters obviously have a significant impact on acoustic noise as well, and that this is also part of the overall compromise in optimizing the excitation parameters. Another point to note is the fact that the power electronics unit in this case was also a source of acoustic noise and was perceived as more annoying to the users due to the high pitch noise related to the switching frequency of the IGBT modules. However this is something which can be improved by better sealing of the housing of the power electronics unit, positioning of this unit in the vehicle and by shifting the switching frequencies (although the last aspect is related to switching losses and hence to inverter efficiency).

Complete hybrid powertrain and test vehicle

Integration to hybrid powertrain through chain drive

CONCLUSIONS

+ The conducted motor design and optimization has proven successful with regard to delivered torque and efficiency. The issue of torque ripple, which could be a major drawback for traction applications, was successfully addressed by use of a four phase design and application of current profiling. The integration into test vehicles allowed assessment in the actual application, and first subjective assessments by various test users were positive, although these are subjective results, more objective measurements are to be carried out. The operational test vehicles are nevertheless an illustration of the potential of this low-cost technology. The positive feedback, ideas for even better design and control, and availability of options for noise handling offer further room for improvement. Hence, Punch Powertrain will build on this experience and come up with an even more performing solution.

REFERENCES + [1] Oakdene Hollins, Lanthanide Resources and Alternatives,

http://www.oakdenehollins.co.uk, March 2010+ [2] R.S. Colby et.al. Vibrating modes and acoustic noise in a 4 phase switched

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salient variarable-reluctance machine. IEEE Transactions on Industry Applications, 28:1250–1255, 1992.

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+ [7] P. Debal et.al. Development of a Post-Transmission Hybrid Powertrain, EVS24, May 13-16, 2009, Stavanger, Norway

+ [8] Rasmussen et.al. Structural Stator Spacers - The key to silent electrical machines, March-April 2004, IEEE Transactions on Industry Applications, Issue 2, p574 - 581