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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)

The eccentric positioning of the SRM creates unbalanced electromechanical radial forces due to asymmetrical magnetic pull. In a 6/4 outside-rotor SRM: Left: The eccentric rotor overlap stators 2 and 5. Right: The eccentric rotor overlap stators 1 and 4. Wang et al. (2014). Click to enlarge.

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



The only wide scale use of in-wheel motors that I am aware of is in the BYD buses.

Any idea what system they use?
I could not track down whether it is inductive or what it is.


This sounds like it is operating in the same fashion as out of balance wheel.
In effect putting a pulsing 'possibly electrostatic stress force thru to the tyre.

In wheel balancing, the larger the diameter and the greater the effect for a given unbalanced weight (required).
As it is resisted by the 'dead weight' of the axle, a heavy say 4wd light truck was less prone than a lighter axled steer only axle.

The lighter the suspension the more sensitive.

The frequency is relevant as the affect occurs at resonant frequency(s) or rotational speed(s). It is also affected by interaction between different wheels especially front and back interactions.

That was usefull to diagnose if front bal or four wheel balance required.

This is saying that the emotor throws or flings the attatched wheel into unbalance.

There are strategies to overcome this that use the same sensing technology as we find in wheel balancing machines to counter the forces by applying the appropriate pulse of energy timed to cancel out the primary unbalanced torque.

It has been shown to work in unbalance washing machines to rebalance the load by sensing then applying appropriate vector forces

Bit like white noise.


I would suggest that BYD buses would be unlikely to be affected very much as the wheels are heavy and rotate slowly for a given road speed.

The area of attachment to the chassis (no axle?) bearing etc is likely also substantial so more resistant to unbalanced input.

However resonant frequency fatigue would be an expected outcome and need to be considered.
However, as the problem frequency combination(s) would be narrow in bandwidth and the total range of frequency experienced in service large, it may not present as problem.


Hub motors are an idea that's been around since the early 1900s and never implemented...No one has solved the main problem yet: That of maintaining the critically close gap between the rotor and stator.

The loads are very high when the motor bearings are also used as hub bearings for the suspension, causing them to wear out prematurely and increasing the gap. A small change in the gap has a marked effect on the torque the motor can produce. Typically the gap is made as small as possible...about nine thousandths of an inch.

Vibration problems could be a contributing factor to maintaining the gap; perhaps these Chinese engineers can produce a hub motor that will work...I wish them well and it would be a welcomed advance in EVs...worth billions.


They evidently work reasonably well in buses.
BYD has thousands on the roads already, and buses clock up some serious mileage.
Whether that translates to lighter vehicles is another question, and Arnold has kindly outlined some of the issues.


@ Davemart
The special architecture (mechanical and electrical design) of the employed reluctance motor is basically an induction motor that avoids the use of rare earths. The particular design employed by BYD is usually mirrored (stator / rotor); s. e.g.:
With BYDs mirrored design, it is possible to reduce the number of coils/windings in the stator and reduce the coulombic losses. I suspect that this measure is part of the problem that BYD is encountering. Just a tip from a daft engineer.

Bob Wallace

How much energy would be lost by frame-mounting the motors and using short connecting shafts to drive the wheels?

Anyone have an idea of how much more efficient in-hub motors would be if the bugs were worked out?


@ Bob Wallace
The difference in efficiency would be negligible to near by wheel mounting but the effort and price increase would be considerable.


@ Bob

I don't think it's about efficiency gains so much as space saving. By putting the motors in the wheels you're taking them out of the cabin space. You're also eliminating the mechanical transmission - including gearboxes, differentials, drive shafts and axles which saves more space. This could provide a significant weight and manufacturing cost saving as well.


I.M.O. this has great potential.

Don't know if anyone else is onto this but if it were on the front axle and had useful battery range, I would see it as current state of the art.

Hub motors are not as mature but it seems the inboard mounting is finding its way to the showroom now.

"The Power Drive Unit, packaged in the center of the vehicle beneath the center console, dictates the power management strategy of the Sport Hybrid SH-AWD system, including motor power and battery recharge."


"The Twin Motor Unit (TMU) is located in between the rear wheels, where a differential on an all-wheel-drive vehicle is typically mounted. Inside its die-cast aluminum housing are two electric motors positioned back-to-back. Each 27 kW motor powers a single rear wheel. A clutch allows each motor to be decoupled from its wheel in certain operating situations to improve efficiency. Each rear motor is independently controlled and can supply advanced torque vectoring—either positive (drive) torque or negative (declaration) torque to its wheel."

So we have the option.
Twined inboard motor pair on the rear axle, but I would like to see something similar on the front axle. This twin motor concept if fitted to the front axle would enable stronger regenerative braking and regenerative and reversible capable traction control without a differential.

While the current version may appear a bit untidy, and space hungry requiring short drive shafts and associated larger drop angles, as well as require some clever dual controller design, it will surely become more refined in future examples.

Meanwhile I doubt there would be any unbalanced forces to contend with. If there were, some sort of rubber coupling harmonic balancer / C.V. sytem would be an option.


Apologies for earlier misleading reference to "bit like 'white noise'"
which has nothing to do with noise canceling or noise canceling technology that I had in mind.

"Sound masking is the addition of natural or artificial sound (such as white noise or pink noise) into an environment to cover up unwanted sound by using auditory .... In noise cancellation the sound is actually eliminated not covered up."

Then the 'electrical equivalent' to 'sound canceling' signal is applied to the unwanted electrical frequency rather than the unwanted 'sound spectrum' f.


Just want to point out that hub motors do see wide, trouble free, use in smaller vehicles: Bicycles, scooters and motorcycles. What's at issue here is the weight of in-wheel motor that a heavier, faster vehicle like a car would need.

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