Wheel hub motor

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Raleigh SC30 with an aftermarket electric conversion hub motor kit installed.

The wheel hub motor (also called wheel motor, wheel hub drive, hub motor or in-wheel motor) is an electric motor that is incorporated into a hub of a wheel and drives it directly.

Uses in current and future vehicles

• They are commonly found on motorized bicycles.^{[citation needed]}
• Wheel motors are applied in industry, e.g. driving wheels that are part of assembly lines.^{[citation needed]}
• They have been little applied in cars, yet that is what they were invented for. (See History)
• Hub motors can also be found on buses.^[1]

Concept cars

PML Mini QED electric vehicle

Several concept cars have been developed using in-wheel motors:

• PML Flightlink's Mini QED in 2006, and other cars using its Hi-Pa Drive
• Mitsubishi MIEV concept model in 2005
• Siemens VDO (bought by Continental) eCorner concept in 2006^[2][3]
• Heuliez WILL using the Michelin Active Wheel (which incorporates motorized active suspension as well) in 2008^[4]
• The ZAP-X in 2007 "would use high-tech electric hub motors at all four wheels, delivering 644 horsepower to the ground from a lithium-ion battery pack. The hub motors would eliminate the need for transmission, axles and conventional brakes, opening up space beneath the floor for a giant battery pack."^[5]

Mechanism

Hub motor electromagnetic fields are supplied to the stationary windings of the motor. The outer part of the motor follows, or tries to follow, those fields, turning the wheel to which it is attached. Energy is transferred to a brushed motor through physical contact of brushes to the rotating shaft of the motor. Energy is transferred to a brushless motor electronically, requiring no physical contact between stationary and moving parts. Although brushless motor technology is more expensive, most are more efficient and longer-lasting than brushed motor systems.

A purported advantage of this design is that no additional transmission system is needed, thereby increasing the efficiency of the drive system. However, because electric motors are most efficient at high rates of revolution^{[citation needed]}, direct drive hub motors tend to be inefficient^{[citation needed]}. Integrated planetary gear drivetrains are sometimes included^{[citation needed]}, but re-introduce transmission losses.

Wheel hub motors are increasingly common on electric bikes and electric scooters in some parts of the world, especially Asia.^[6] Most of the wheel hub motors made in China are designed for bikes with Chinese dimensions but there are conversion kits are made to suit other bike dimensions.^[7]

Comparison with conventional EV design in automobiles

Compared with the conventional electric vehicle design with one motor situated centrally driving two (sometimes four) wheels by axles, the wheel motor arrangement has certain advantages and disadvantages:

Energy efficiency

Energy efficiency is one of the biggest advantages of direct drive in-wheel motors. A conventional vehicle uses mechanical means to transmit power from a centrally mounted engine/motor to the wheels. With an ICE (Internal Combustion Engine) vehicle this mechanical transmission must have multiple gear ratios to compensate for an ICE having no usable power at engine speeds lower than about 1000 rpm. An electric motor gives its maximum torque from zero rpm so does not need as many gears as an ICE, but with a single electric motor in a vehicle it still needs mechanical transmission.^{[citation needed]} Each gear in a mechanical transmission necessarily introduces power loss, so with several gears between an engine and wheel the losses can add up to approx 30%^{[citation needed]} in a two wheel drive vehicle and 40%^{[citation needed]} in a four wheel drive vehicle. An electric motor mounted directly inside a wheel without any mechanical transmission will avoid all such losses.

Regenerative braking

Regenerative braking is another advantage of wheel motors.^{[citation needed]} A 3 phase AC motor is easily turned into a generator any time the rotor is traveling faster than the applied drive frequency. In practice this means any time a vehicle is not accelerating, it will naturally regenerate. With a mechanical transmission between the wheels and an electric motor the transmission losses mentioned above apply meaning 30-40%^{[citation needed]} less energy can be collected from regeneration and to compound the problem helical cut gears as used in all differentials and most gearboxes are not designed to transmit power in two directions.^{[citation needed]} Additionally if an EV has the motor driving the rear wheels only, then regenerative effort is limited to 20%^{[citation needed]} of the total available energy due to weight distribution changes that occur during braking. Since 70%^{[citation needed]} of brake effort is generally applied to the front wheels then to capture 70%+^{[citation needed]} of the available energy during regeneration the front wheels must be used to drive the electric motor. Unfortunately a front wheel drive only car design includes a lot of vehicle dynamic compromises that severely limit both forward and lateral traction.^{[citation needed]} Wheel motors provide a good solution because they are able to provide a balanced vehicle in both acceleration and deceleration.^{[citation needed]}

Drive by wire

Each wheel motor can be precisely electronically controlled and therefore, control systems such as ABS, traction control and stability control can conceptually become software functions included in any wheel motor equipped vehicle at very little extra cost.^{[citation needed]} In fact with electronic control of brakes (and acceleration) this opens a whole world of automated vehicle dynamics options such as:

• Active cruise control, where the vehicle can actively keep a set distance to a vehicle ahead
• Collision avoidance, where the vehicle can automatically brake and/or steer to avoid a collision with another object
• Emergency brake assist, where the vehicle can adjust braking effort in response to the proximity of an object within the cars path

These features had however in 2009 already appeared as options for many common vehicles with internal combustion engines. Since most systems in modern cars are electronically controlled in one way or another the benefit from wheel motors is not that large and many of the features do come with a need for specific sensors that add cost even with electronically controlled wheel motors.

Weight savings

Eliminating mechanical transmission inc. gearboxes, differentials, drive shafts and axles provides a significant weight and manufacturing cost saving. However, the independent nature of the motors makes it impossible to send more torque to wheels with more grip during cornering, necessitating heavier, more powerful motors to achieve the same performance and hence offsetting the weight and cost savings.^{[citation needed]}

Unsprung weight

A design consideration with wheel motors is unsprung weight. Most conventional electric motors include ferrous material composed of laminated electrical steel. This ferrous material contributes most of the weight of electric motors. As excessive unsprung weight can lead to less than optimal vehicle handling, several recent wheel motor designs have minimized the electrical steel content of the motor by utilizing a coreless design with Litz wire coil windings to reduce eddy current losses. This significantly reduces wheel motor weight and therefore unsprung weight. Two examples of this are the CSIRO Solar car wheel motor and later versions of the Lucchi R. Elettromeccanica Srl wheel motor developed for General Motors.^{[citation needed]}

Another method used is to replace the cast iron friction brake assembly with a wheel motor assembly of similar weight. This results in no net gain in unsprung weight and a car capable of braking up to 1G.^{[citation needed]} A good example of this is the Michelin Active Wheel motor as fitted to the Heuliez WILL that results in an unsprung weight of 35 kg on the front axle which compares favorably to a small car such as a Renault Clio that has 38 kg of unsprung weight on its front axle.^{[citation needed]}

History

• First wheel motor concept: Wellington Adams of St. Louis first conceived of building an electric motor directly in the vehicle wheel though it was attached via some complicated gearing. The Adams patent is US # 300,827 in 1884.
• First practical wheel motor: Albert Parcelle of Boston, MA developed the first fully incorporated Wheel hub motor in his "Electro-Motor Traction Wheel" and patented it in patent US # 433,180 in 1890.
• High torque low RPM wheel motor invented: The motor was incorporated into the wheel without gearing and addressed torque considerations through the use of a new high torque, low rpm motor invented by Edward Parkhurst of Woburn, MA in patent # 422,149 in 1890 (and mismentioned in Parcelle's patent as #320,699).
• Electric wheel motor advantages revealed in patent: An early Wheel hub electric motor was invented by Frenchman Charles Theryc and patented in 1896 as US patent 572,036 entitled Wheel with Electric Motor hub for Vehicles. In the patent he explained all advantages including no transmission losses because of the absence of classic transmission rods from engines to wheels.
• Diesel wheel motor: Not all wheel hub motors were electric. C F Goddard in 1896 invented a piston hub motor for horseless carriages patented in US # 574,200. He envisioned it powered by expanding gas of some kind. His offcenter flexible bent spoke designs later showed up in the Apollo moon lander rovers wheels in 1960's.
• Using cams, another type of combustion wheel motor:In patent # 593,248 W C Smith in 1897 developed another explosive gas expansion motor inside a wheel hub that utilized cams on a track in the hub to transmit power to the wheel.

The electric wheel hub motor was raced by engineer Ferdinand Porsche in 1897 in Vienna, Austria. Porsche's first engineering training was electrical, not internal combustion based. As a result he developed his first cars as electric cars with electric wheel hub motors that ran on batteries. The Lohner Porsche, fitted with one wheel motor in each of the front wheels, appeared at the World Exhibition in Paris in 1900 and created a sensation in the young automobile world. In the following years, 300 Lohne Porsches were made and sold to wealthy buyers.^[8]

Porsche recognized the efficiencies of power transmission of the electric wheel hub motor over a central engine driving a transmission then the wheels. In an effort to overcome the low battery capacities of the day, he developed an electrical wheel hub motor car that got its electric power from a gasoline driven generator in the car effectively inventing the hybrid auto he called the "System Mixt". He set many speed records of the day in this gasoline powered generator car with two front wheel electric hub motors. He won Austria's Automotive Engineering prize with that car and his career took off.^{[citation needed]} Eventually the growth in power of the gasoline engine overtook the power of the electric wheel hub motors and this made up for any losses through a transmission. As a result autos moved to gas engines with transmissions however they were never as efficient as electric wheel hub motors.

See also

• ActiveWheel
• Locking hubs

References

External links

Wikimedia Commons has media related to: In-wheel motor

• PML Flightlink a UK based company that currently designs and builds the highest power density electric wheel motors in the world. Currently working with Volvo on the ReCharge, a vehicle released at the 2007 Frankfurt Motor Show that uses 4 of PML's in-wheel motors.
• e-traction, a Dutch-American company that has several wheel motors in production. Also the manufacturer of the Whisper, a serial hybrid bus with wheel hub motors.
• Tech-m4, a subsidiary of Hydro-Québec, a Canadian utility company in Canada develops and produces wheel motors for automobiles.
• GM's experimental car, presented in 2003, is driven by a conventional IC engine at the front while fitted with wheel motors at the rear.
• Mitsubishi' experimental car, presented in May 2005, is fitted with wheel motors at the rear.
• L2ES lab staff and topics French staff working on wheel motor
• A new type of motorcycle wheel motor
• An electric motor wheel in a bicycle.
• A Proposal to adapt wheel motors to MagLev Vehicles