Study finds vertical force of in-wheel switched reluctance motors deteriorates vehicle stability and comfort
In a study investigating switched reluctance motors (SRMs) for in-wheel motor applications, researchers at Chongqing University in China have found that the vertical component of the residual unbalanced radial force of the motor deteriorates the lateral and anti-rollover stabilities of the vehicle in addition to having a considerable impact on vehicle comfort. (The unbalanced radial force is the radial force difference between a pair of opposite stator poles.)
In their paper, published in the Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, they suggest that a control method addressing these issues will be needed if SRMs are to see use in in-wheel applications. In an earlier paper, members of the team had proposed the use of an FxLMS (filtered-X least mean square) controller based on active suspension system to generate controllable force to suppress the vibration caused by SRM vertical force. In that paper, they found that utilizing active suspensions could reduce the effect of SRM vertical force on suspension performance.
In a switched reluctance motor (SRM), torque is produced by the magnetic attraction of a steel rotor to stator electromagnets; there are no permanent magnets, and the rotor carries no windings. A controller energizes each stator winding only when it can produce useful torque. With suitable timing of the stator excitation, the machine can operate as a motor or generator. Switched reluctance motors are simple, inexpensive, robust and can offer very good efficiency over a wide load range.
SRMs also offer a high torque density, high operating efficiency, and excellent power-speed characteristics. Accordingly, there is some interest in exploring their use as vehicle traction motors. As one example, Cobham Technical Services is collaborating with Jaguar Land-Rover (JLR) and engineering consultancy Ricardo UK to develop a switched reluctance traction motor. (Earlier post.)
However, one of known challenges with SRM devices is delivering a torque-dense motor that is quiet enough for vehicle use. While SRMs can have very high power density at low cost, they have had issues with high torque ripple when operated at low speed, and the acoustic noise caused by torque ripple and vibrations.
Unbalanced radial force caused by rotor eccentricity may degrade the performance of SRM, increasing vibration and acoustic noise. In practice, some degree of rotor eccentricity is always present due to the tolerances introduced during the manufacturing process, wear of bearings, and static friction especially when the rotor is sitting idle, as well as other reasons. The air gap of the SRM is generally between 0.2 and 1 mm which is much smaller than any other type of motor and is more sensitive to rotor eccentricity. A relative eccentricity between the stator and rotor of 10% is common. On the other hand, SRM unbalanced radial force will be magnified by the vehicle continuously idling, road excitation, and unbalanced load. This phenomenon is particularly serious to IWM-EV [in-wheel motor electric vehicles], because the vertical component of SRM unbalanced radial force, namely, SRM vertical force, applies directly on vehicle wheels and will change the tire load. Although SRM unbalanced radial force is inevitable and serious, the contributions of SRM unbalanced radial force to IWM-EV stability and comfort have not been studied thoroughly yet.—Wang et al. (2014)
In the new study, Wang et al. specifically examined the role of the vertical force of the switched reluctance motor in the stability and comfort analysis for in-wheel-motor-driven electric vehicles.
The results in this paper indicate that the vertical force of the switched reluctance motor has a great effect on the lateral and anti-rollover stabilities of the vehicle. The direct cause of this phenomenon is that the vertical force of the switched reluctance motor is directly applied on the wheels, which will result in a significant variation in the tire load, and the tire can easily jump off the ground.
Furthermore, the frequency of the vertical force of the switched reluctance motor covers a wide bandwidth which involves the resonance frequencies of the vehicle body’s vibrations and the wheel bounce. As a result, the comfort of the vehicle is greatly harmed. Therefore, the effect of the vertical force of the switched reluctance motor on the the comfort of the vehicle is also considerable in some resonance situations. The conclusion is that the vertical force of the switched reluctance motor not only causes the stability of the vehicle to deteriorate but also has a considerable effect on the the comfort of the vehicle.—Wang et al.
Yan-yang Wang, Yi-nong Li, Wei Sun, Ling Zheng (2015) “Effect of the unbalanced vertical force of a switched reluctance motor on the stability and the comfort of an in-wheel motor electric vehicle” Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering doi: 10.1177/0954407014566438
Yan-yang Wang, Yi-nong Li, Wei Sun, Chao Yang, and Guang-hui Xu (2014) “FxLMS Method for Suppressing In-Wheel Switched Reluctance Motor Vertical Force Based on Vehicle Active Suspension System” Journal of Control Science and Engineering doi: 10.1155/2014/486140