Hub Motors for All-Electric Vehicles Still Have Some Technological Challenges to Overcome

Aug. 10, 2010
A look at the advantages of hub motors, as well as the disadvantages and how engineers plan to overcome them.

Authored by:
Stephen J. Mraz
Senior Editor
[email protected]

Fraunhofer Institute (USA),
Siemens VDO,

As engineers strive to devise and refine alternative powerplants for cars and trucks, ones that won’t burn fossil fuels or make each vehicle a point source of pollution, some are looking to the past for inspiration. One such approach, the electric hub motor, seems to show promise. The idea is to build an electric motor right into the wheel. Stationary windings, usually concentric with the wheel, generate electromagnetic fields which force the outer windings mounted on the wheel to follow them and, thus, rotate. The idea has been around almost as long as the automobile, In fact, Ferdinand Porsche, founder of the famed German auto company, mounted a pair of hub motors that used electricity from a gas-powered generator on a car back in 1900. The car, the Lohner-Porsche, could hit speeds of up to 35 mph, letting it set several world speed records.

But technological limitations of the hub motor and a more-appealing alternative, the gas-powered internal-combustion engine (ICE), kept hub motors out of cars and the vast majority of trucks, except for a few concept vehicles. So as gasoline loses favor due to price or its pollution, researchers are trying to develop an affordable, durable hub motor that consumers will like.

Hub motors have several advantages.

• From a designer’s standpoint, hub motors offer flexibility. They can be used to power rear or front-wheel drive vehicles, as well as all-wheel-drive versions. They can even be used to augment other powerplants, including ICEs.

• Their relatively compact size means more room for other components, which could be a battery pack, fuel cell, or generator. Designers also have the option to mount these components to improve the weight distribution between front and rear wheels or lower the center of gravity for better stability and handling. And if there is no need for these other components, auto engineers could add more cargo room, arrange crush zones around the passenger compartment for a safer car, or simply design a smaller, lighter, more-energy-efficient car.

• By eliminating the main powerplant, hub motors negate the need for a heavy transmission, driveline, differential, and axles. This cuts mechanical losses, inherent in every component standing between the engine and wheel, and makes the car or truck run quieter. It also cuts weight, which makes for more-efficient travel.

• Like most electric motors, hub motors generate high torque at low rpms. By comparison, ICEs need to be revving beyond 1,000 rpm to create enough torque to get a car or truck moving. So most hub-motor designs do away with transmissions, letting the electric motor’s flat torque curve do the work. But there are some designs that use simple, relatively lightweight planetary-gear transmissions to let them run more efficiently at higher rotational speeds. Either way, doing away with the transmissions found in most cars means less weight, less complexity, and more efficiency.

• Hub motors can be used as brakes by acting as a generator rather than a motor. The spinning wheels slow down as they are forced to work against the electromagnetic fields to create electricity. This regenerative braking also lets the vehicle create electricity that can be stored and reused later.

• With all the advances in electronic-motor control, engineers can now design controllers able to fine-tune each hub-motor’s torque, rpm, and even direction of spin. This means features such as antilock brakes, traction control, and even cruise control could be handled by a single master controller. There’s even the possibility of adding new, previously impossible, or at least impractical, features. For example, having the right wheels turn one way and the left wheels turn the opposite direction could give a car a near-zero turning radius for getting out of tight parking spots. It might also turn out that new features in development such as lane following, collision avoidance, and active cruise control would be easier to implement with separate wheel motors and a master controller.

Disadvantages and potential fixes
Hub motors have been around for more than a century, and the fact they haven’t caught on speaks for itself. At least that’s the feeling of one researcher at an organization dedicated to electric vehicles. “If they haven’t found success yet, it’s unlikely they ever will due to the problems they have.”

Of course, this attitude doesn’t take into account human ingenuity, technological advances, and that over the last 100 years, there have been few incentives to wean drivers from gas-powered cars. But to make hub motors viable, there are some hurdles that need to be cleared.

• Hub motors don’t provide energy, just a way to use it. Car engineers still need to supply the electricity needed to run these motors. The easiest approach is to put rechargeable batteries onboard the car or truck. But this adds weight and puts some potentially dangerous materials into the waste stream. It also puts an additional load on the nation’s energy grid and increases the need for new power plants.

• The major challenge facing hub motors is the issue of unsprung weight. Unsprung weight is the mass of all components not supported by a car’s suspension. Conversely, sprung weight is the mass supported by the suspension, including the frame, motor, passengers, and body. Unsprung weight includes wheels, tires, and brakes, and it travels up and down over any bumps, potholes, and debris as it tries to follow the contours of the road. The sprung mass, however, is shielded from most of these movements, especially the smaller ones, by the suspension. And the sprung weight and suspension act to press down on the wheels so that they are in contact with the road.

As a rule, designers try to minimize unsprung weight to improve handling and steering. That’s why hot-rodders and auto enthusiasts invest money in alloy wheels — to reduce the unsprung weight and make steering more precise. Lighter wheels and tires also mean it takes less energy to spin them or bring them to a stop. So lighter wheels mean more responsive acceleration and braking. (It’s estimated that every pound of additional rotating mass is the same as adding 10 lb to the overall vehicle weight.) So adding hub motors could vastly increase the unsprung weight and hurt performance.

The obvious solution is to cut the weight of the hub motors, and engineers have been trying to do this for the last 50 years. Some use lightweight, yet powerful, rare-earth magnets. In one program, General Motors engineers designed a hub motor with stator windings fixed in epoxy rather than wound around a heavy iron core. Others use Litz wire in the windings to cut eddy losses, which also reduces motor weight for a given output. Another approach is to remove the cast-iron friction-brake assembly, replace it with a hub motor that weights about the same as the assembly, and let the car rely on regenerative braking. Current designs can supply about 1 g of braking.

But one of the side effects of shaving the weight off of engineered components is reduced durability. And with hub motors being part of the vehicles unsprung weight, they will feel the impact of every pothole, bump, and high-speed turn. They will also be exposed to road dirt and mud, dust, water, and road salt. So engineers must balance durability and weight, and it has yet to be seen whether any current hub motor can last 100,00 miles of everyday driving. It’s also unclear what two or four durable hub motors would cost.

There are still more than a few technological and economic issues to be settled before hub motors are widely accepted. Two minor issues involve stopping vandals from stealing hub motors, and drivers’ willingness to invest in an expensive spare tire with a ready-to-go hub motor mounted in it.

The Frecc0
Researchers at Germany’s Fraunhofer Institute are working on a demonstration vehicle, the Frecc0 (Fraunhofer E-concept car Type 0), that will be used to devise and evaluate all the components needed in an all-electric car. (These component comprise technologies in the newly named area called electromobility.) One key component is the Fraunhofer hub motor.

According to a Fraunhofer manager, the two motors used on the car’s rear wheels are relatively light and compact, so they add little to unsprung weight. The German designers plan on compensating for the loss of handling performance by altering the chassis. They claim they can do this by reconfiguring the muffler setup, for example. Right now they are confident they will be able to devise a chassis and suspension that compensates for the additional unsprung weight. They are also sure that simply adjusting the spring and damper settings on the Frecc0 will let them proceed with the demonstrator project. They are basing their suspension adjustments on data gathered from an instrumented “measurement wheel.” That same data will be used to eventually come up with the new chassis/suspension design.

The motor itself is a six-phase permanent-magnet synchronous version with relatively high power and torque, according to the Institute. The motor is actually split into two independent three-phase subsystems, each of which can power the wheel in case of a failure in the other. And in case of an inverter fault, the induced voltage in the motor would be high at high speeds, which would lead to currents in the batteries and destroy the power electronics. To avoid this, the hub motor is built to short circuit in case of an inverter failure.

All of the motor’s power electronics and control unit are in the hub. But it will likely have a master controller coordinating the output of all the vehicle’s hub motors. Individual components, as well as the assembled hub motor, have been tested to ensure they can withstand the shocks, vibrations, and temperatures involved in daily driving. The hub motor is also sealed to an IP65 level, enough to keep out dirt and water. The rotor, for example, is protected by a shaft seal. The design team is also using components that lend themselves to mass production. The stator, for example is made using lost-foam casting, which can reproduce the complex internal cooling channels. And the rotors are well suited to high-pressure die casting.

The Fraunhofer design will not use any transmission or gearing. The motor provides enough torque at low speeds and puts out nearly constant power (55 kW and 700 Nm) at high speeds, says the Institute. Though they are not sure what batteries will be used yet, they estimate the Frecc0 will need a battery pack with 30 kW to give it a 90-mile range and the speed and acceleration of a sports car.

The Active Wheel
The Active Wheel, a hub motor from Michelin, Greenville, S. C., that has been in development for at least a dozen years, has appeared on several concept cars but no actual production vehicles. It is an advanced hub motor in that it uses two electric motors: one to turn or stop the wheel and the other to power an active suspension. It also contains a brake and uses regenerative braking. The total weight is 95 lb per motor.

The drive motor, a 30-kW, water-cooled version, mounts off-center where its shaft turns a spur gear which drives a ring gear on the hub and, thus, turns the vehicle’s wheel. The suspension motor operates the active suspension via a gear rack and pinion that replaces a hydraulic shock absorber. It controls ride height, pitch under braking, and roll during cornering, and reacts within 0.0003 sec. A coil spring in the wheel supports the static load of the car. The Active Wheel mounts to the vehicle chassis using a single lower-control-arm suspension.

Two Active wheels deliver about 40 hp, which can peak at up to 80 hp. That’s enough to take a 2,000 lb car from 0 to 60 mph in about 11 sec, and give it a top speed of 95 mph.

© 2010 Penton Media, Inc.

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