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Custom In-Wheel Motor Powers City Car

30 April 2015

Printed Motor Works is a UK manufacturer of compact electric motors and motor gearboxes providing complete solutions for motion control applications. The company is focused on the industrial, aerospace & defence, medical, marine and other specialist markets. Here, they describe their latest challenge in more detail:

Objective:

To design and build a lightweight in-wheel motor for a small city car, with integral brake disc mounting.

Summary:

Designing and manufacturing in-wheel motors is nothing new to Printed Motor Works. Constructing a motor that mounts on a stub axle and in turn onto which the wheel mounts is also straight forward. What made this project challenging was the requirement to build it with an integral mount for the brake disc while still maintaining the external rotor motor layout of the in-wheel XR series.

The in-wheel XR series is a permanent magnet brushless motor with an external rotor. Permanent magnet brushless motors are generally the most efficient and highest torque density motors available today. The XR series gains from two distinct additional advantages:

1. The magnets are positioned on an externally running rotor which gives the motor the fundamental geometric advantage that torque is being delivered on the outer-most point of the motor.

2. The torque density of the XR series is maximised by a high degree of slot fill. The high slot fill is achieved by winding each of the stator teeth individually before assembling them together into a stator

The solution was to design and build a motor with a fully stressed external rotor, with integral brake disc mounting on the rear face. The result was a strong, powerful, versatile motor that can be used for a variety of markets, not just the small town car for which it was designed.

We foresee future variants of the XR series motor being used in self-powered and traction-assisting trailers, specialist off-road vehicles and autonomous guided vehicles (AGVs).

The Challenge

Using the motors for regenerative braking can be very effective whilst the vehicle is moving. Once it is stationary, the motors are unable to stop a vehicle from rolling down a moderate slope without being powered and therefore wasting precious energy. To mitigate against the risk of electrical failure in braking, mechanical safety brakes must be fitted to electric vehicles as a matter of legislation country by country.

Whether used as part of an Hybrid Electric Vehicle (HEV), or as a full Electric Vehicle (EV), there are various configurations for fitting electric motors to vehicles, generally:

1. A centrally mounted motor driving wheels via a gearbox and/or a differential

2. In-board mounted motors driving wheels via a driveshaft.

3. In-wheel motors driving the wheels directly.

If motors are fitted in-board, conventional axle hubs, containing wheel bearings, brakes and wheel mounts, can be employed. This is a tried and tested technology that requires little innovation to implement a satisfactory design.

The design intent of the intended vehicle and the construction of the suspension components demanded that brakes needed to be integral to the motor design. Further constraints included the use of conventional, externally mounted disc calipers, a low target weight and the whole package was required to fit within a 15” wheel rim.

Possibilities

A variety of possible layouts were considered in relation to the arrangement of brakes and bearings.

The constraints of using conventional, externally mounted calipers and the proximity of the suspension components to the wheel mounting excluded all but one solution.

The location of the brake influenced the mechanical arrangement for the rotor and this in turn impacted the bearing location and type to be used.

Consultation with our customer narrowed the options to one strategy, with a choice of two of bearing configurations. The challenge then lay in selecting the best combination of bearing arrangement coupled with standard bearings, to minimise weight whilst maintaining a normal lifespan for vehicular wheel bearings.

With the support of bearing manufacturer SKF’s expertise and bearing life simulation facilities, and Finite Element Analysis (FEA) of the structural components of the motor, a predicted life of over 1,000,000 km has been achieved.

The Solution

The only satisfactory location for the brake disc was inboard of the motor, but still attached directly to the rotor. There was no scope for indirect connection. This demanded that the rotor wrap around the stator, from the outboard face where the wheel is mounted, to the inboard face where the brake disc is mounted.

A lightweight racing bearing offered the best balance of weight and size to enable the stub axle mounting and electrical outlets to pass through its static centre. The outer bearing is of a more conventional size for this type of application.

A short run of motors have been built and validated on dynamometers to confirm they meet the drive cycle requirements. They are now being fitted onto prototype ‘Mule’ vehicles, prior to road going trials. News of testing and results to follow.

Conclusion

The result is the XR32-11, a brushless DC motor, tailored specifically for it’s application as an in-wheel motor in a small city car, but configurable for other markets. Details of the motor can be found on our website here: www.printedmotorworks.com/in-wheel-motors/

This custom motor is an example of the fast development that Printed Motor Works is able to offer its customers. The XR32-11 complements Printed Motor Works' existing range of specialist high-power density external rotor motors. The specialist service that we offer our clients is appreciated for its rigorous engineering and specialist knowledge. Plus at every stage it eliminates client commercial risk, as the motor design is already proven before prototyping and then full scale manufacturing.

For more information, please contact:

Printed Motor Works Ltd
Newman Lane
Alton
Hampshire
GU34 2QW
United Kingdom
Tel:  +44 1420 594 140
Email: sales@printedmotorworks.com
Web:  www.printedmotorworks.com
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