Drives and Controls Magazine
Planetary motor-gearbox could raise power density by 50%
Published:  27 June, 2017

Researchers at the Vienna University of Technology (TU Wien) have developed a novel motor with an integrated gearbox. Instead of a single rotor, four mechanically linked rotors are used, creating what the developers call a “planetary motor”. They say that it offers high efficiency, a power density up to 50% higher than that of a conventional motor of a similar size, ease of use, and failsafe operation, at a relatively low cost. The motor made its global debut at the Hannover Fair earlier this year.

“At first, we were looking for a way to combine several electric motors into a single component”, explains Professor Manfred Schrödl from TU Wien. “Every electric motor has a rotor inside, and the electromagnets are outside. If various motors are arranged next to each other in a geometrically skilful way, some parts of the motionless motor parts – the so-called stators – become redundant. Thereby, the entire structure can be simplified. This saves space and reduces power losses.

“For example, we use four electric machines with a three-phase winding, which means that, in total, there are twelve magnetic coils”, he continues. “Due to the machine’s clever design, however, one can manage with only six coils.”

The motor works in a similar way to a single-stage planetary gear. Two rotors together power a large, inner-toothed gear (the ring gear), while two counter-revolving rotors power a smaller, externally-toothed gear (the sun gear). The ring and sun gears rotate in the same direction and (with appropriate dimensioning) with the same angular velocity, allowing them to be integrated permanently. This results in a powerful output, while reducing the strain on the individual rotor outputs.

“Our drive, which integrates the gear directly with the motor and its output, is extremely simple – which means that we can enhance its efficiency, while reducing its production cost,” says Schrödl.

Although there are several rotors at work, controlling them is relatively easy. The motor is regulated and powered in a similar way to a conventional synchronous motor.

A prototype of TU Wien's space-saving integrated motor with a transparent disc that makes the planetary gears visible

Another advantage of the new machine is that it does not need any fault-prone sensors. Schrödl earlier developed a sensorless synchronous motor, where the rotor’s position does not need to be measured using sensors, but is calculated from the current and voltage values in the power cable. This sensorless technology, which saves space and reduces errors during assembly and maintenance, has been applied successfully in tens of thousand motors.

The TU Wien researchers have built a prototype of their planetary motor with a 1:10 gear reduction, and verified its operation. The electrical behaviour of the motor is said to be similar to that of a conventional inverter-controlled, three-phase motor. “The characteristics of this new machine type are extraordinarily good”, Schrödl reports. “Not only is it extremely compact, but the performance can be doubled compared to conventional machines with the same peripheral rotor velocity.”

He foresees potential applications for the new motor in numerous fields, including electric vehicles, machine tools, robots and lifting and production technologies. The design is particularly suitable for external rotor drives, such as drum motors, wheel-hub motors, or tubular motors. The motors could be assembled using automated lines and their power electronics could be integrated on the gear-driven end-face, resulting in extremely flat designs.

The Austrian researchers are now building a 50kW version of their machine.