The wheel motor
(also called wheel hub motor
, hub motor
or in-wheel motor
) is an electric motor
that is incorporated into a wheel and drives it directly.
- Wheel motors are applied in industry, driving e.g. wheels that are part of assembly lines.
- Wheel motors are standard for most modern powered railroad vehicles of all sorts, including locomotives and multiple units.
- They have been little applied in cars, yet that is what they were invented for. See the "History" section.
Application to cars: Comparison with conventional EV design
Compared with the conventional electric car
(EV) design with one motor situated centrally driving two (sometimes four) wheels by axles, the wheel motor arrangement has certain advantages:
- 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 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 1000 rpm. An electric motor has 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. A fundamental fact about gear power transmission is that each gear in a mechanical transmission introduces power loss. With several gears between an engine and wheel the losses add up to approx 30% in a two wheel drive vehicle and 40% in a four wheel drive vehicle. By mounting an electric motor directly inside a wheel and deleting the mechanical transmission then there are exactly zero transmission losses between motor and wheel.
- Regenerative braking is another advantage of wheel motors. A 3 phase AC motor will easily turn 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% 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. Additionally if an EV has the motor driving the rear wheels only, then regenerative effort is limited to 20% of the total available energy due to weight distribution changes that occur during braking. Since 70% of brake effort is generally applied to the front wheels then to capture 70%+ 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. Wheel motors provide a good solution because they are able to provide a balanced vehicle in both acceleration and deceleration.
- Each wheel motors can be precisely electronically controlled and therefore, modern control systems such as ABS, traction control and stability control can become software functions and can be included in any wheel motor equipped vehicle at very little extra cost. In fact with electronic control of brakes (and acceleration) opens a whole world of new automated vehicle dynamics options including:
- Intelligent cruise control where a vehicle can actively keep a set distance to a vehicle ahead,
- Collision avoidance braking where a vehicle can automatically brake to avoid an object within a collision path,
- Emergency brake assist where the vehicle can adjust braking effort in response to the proximity of an object within a collision path.
Some of these features have already appeared on top of the line vehicles but are mechanical systems and most often require a specialized sub system be added to the vehicle as some expense. With wheel motors all these features become software that once developed is free to reproduce.
- Eliminating mechanical transmission inc. gearboxes, differentials, drive shafts and axles provides a significant weight and manufacturing cost saving.
- 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 and amazingly 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.
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 a 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.
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.
- 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 the Hydro-Québec power company in Canada develops and produces wheel motors for automobiles. They have 20 years' experience.
- Freepistonpower.com conceived of an engine with valves in the crown of the piston, whose reciprocal movement generated electricity when two opposed pistons were attached to permanent magnets moving through coils wrapped around the shared cylinder. A working model (2004) portends greater efficiencies for HEV designs.
- 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.