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A new concept in electric motors

A new concept in electric motors

It’s not every day that something radically different comes along in the world of electric motors. Appropriately named, therefore, the Stepchange motor is designed around a radically different concept. The brainchild of serial inventor John Hammerbeck, it is already attracting interest from some of the world’s leading motor manufacturers, as well as robot makers who see it as a possible solution to some key motion challenges.

A background of reading history at Oxford doesn’t immediately suggest the credentials for a breakthrough in motor design, but Hammerbeck argues that a historian can offer real engineering insight. “To the historian, there is never the perfect answer or the ultimate solution. Things are continually changing.”

It was that sort of thinking that led Hammerbeck to his solution for addressing some of the challenges of electric motor design – most notably how to achieve higher power density but combine this with inherently low output speed, all in a compact, simple and low weight product. “Improving the power density of electric motors can be achieved by increasing the switching rate of the coils,” he says. “Unfortunately this results in higher rotor speeds, more precise bearing requirements, more winding losses in the rotor gap, more heat transfer problems and higher rotational inertia.

“It’s a more costly and complex motor to manufacture, and if you want useful output levels below, say, 1000 rpm, you need more expensive and heavier reducers.”

In contrast, the Stepchange motor increases the power density of electric motors without these drawbacks. It has no rotor, no bearings, no air gap, no precision parts and no reduction gears. It inherently reduces from high switching speeds at ratios in excess of 500:1.

The motor is light, quiet and has a large open centre. It works by rotating a gap in the highly compressed interface between a compliant material layer containing permanent magnets to which the output is attached, and a circular stator in which electromagnetic coils are mounted. As groups of coils are activated in sequence round the output, so permanent magnets in the compliant material are repulsed by the magnetic fields created by the coils. This creates a small gap between the stator and the compliant layer. The gap is then moved round the output, by the successive switching on and off of coils. And the smaller the gap the greater the reduction ratio.

The underlying principle isn’t easy to get your head around, but Hammerbeck says a simple way of demonstrating the principle is to put a pencil into a glass and push an appropriately sized orange in beside it, then roll the pencil. “You will find that the orange rotates in the opposite direction from the rolling pencil at a medium reduction ratio,” says. “In the motor there is just an empty gap rotating round the orange and producing the same effect.”

In the Stepchange motor, then, the rotor is directly connected to the load and moves at the same speed; there is no gearbox. No clutch is required because the compliant material stores the output until there is sufficient force to move the load. The electric drive controls both the switching speed of the coils, how many are switched on, and where, thus controlling the size of the gap. The gap can also be controlled by varying the current to produce a very small gap. The size of the gap is proportional to the reduction ratio. The friction of the compliant material also means that it acts as a brake and prevents rotation when the power is turned off.

“The low weight, hold on power-off and large central opening make this motor compelling for applications such as robotic arms, in-hub motors and car seat motors,” says Hammerbeck. “There has already been interest from robot manufacturers – on the one hand because it addresses limitations of current motor/gearbox combinations in traditional robot joints, but also because it offers the potential to get much closer to a human-style shoulder joint, which is a real challenge with conventional motors.”

The inherent simplicity and low cost of manufacture lends the Stepchange motor to electric seat drives in automotive applications, but it could also be scaled up just as easily for something like a drive to rotate a restaurant or viewing platform at the top of a building. “Going back to our pencil and orange illustration of the concept, imagine the orange is a large pneumatic tyre and the glass is a partial torus round the tyre,” says Hammerbeck. “If we inserted a small rack locomotive running on a rack and rails in the torus and picking up current from the rails, this would act like the pencil. The locomotive supports the tyre with rollers. Such an embodiment could act as a slew ring at diameters large enough to rotate viewing platforms.

For a vehicle in-hub drive using this technology, the friction available in the controlled environment within the motor is much greater than that between the tyre tread and the road, particularly if the compliant material and the stator have complementary ribs like a ribbed belt on a pulley. So a motor in hub could be built that has no clutch, no gears and no bearings and with integrated parking brake. The life of the compliant layer would be similar to that of a tyre and the whole corner unit could be replaced and reconditioned. Such a motor in hub would greatly decrease the unsprung weight of the vehicle.

Another possible application would be tank turrets, as Hammerbeck explains: “It would be low weight, offer cushioning, and be easily removable for engine change by deflating the tyre and any securing lugs. It would also offer inherent sealing against ingress of gaseous chemical weapons.”

While the primary application of the Stepchange motor is rotary power, Hammerbeck says that the principle could also be adapted as a linear motor. It could also be built using smart materials such as piezos, electro- and magneto-strictive polymers, carbon nanotube muscles, etc. Hammerbeck is currently looking for development partners and to licence the technology for specific applications. Find out more at stepchangemotor.com.

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