The Planet Motor: TU Wien’s Innovative Electric Drive

The planet motor developed at the TU Wien

The planet motor developed at the TU Wien

The electric motor is a tried-and-tested concept. This, however, does not mean that there is no more space for revolutionary improvements. Usually, motor and gearbox are two separate functional units. Researchers at the Vienna University of Technology have developed a novel motor concept, which enables the integration of these two components. Instead of a single rotor, four mechanically linked rotors are used. Thereby, a so-called “planet motor” comes into existence, which is characterized by its high power density, energy efficiency, simplicity of use and failure safety.

Convincing Quadrupled Spin
“At first, we were looking for a way how to combine several electric motors into a single component,”, said Prof. Manfred Schrödl from the 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.

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

Two rotors together power a large, inner-toothed gear, whereas the two counter-revolving rotors power a somewhat smaller, externally-toothed gear. In this way, an image similar to a single-stage planetary gear comes into existence, name giver for this invention: electric planet motor.

“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,” said Schrödl.

Sensorless Control
Just because there are several rotors at work does not mean that motor control becomes more complicated: from the outside, the planet motor is regulated and powered in the same way as a conventional synchronous motor. Another advantage of the new motor has already been integrated; it does not contain any fault-prone sensors. Already years ago, Manfred Schrödl has attracted attention with a synchronous motor, which operates sensorless, in which case the rotor’s actual position does not have to be measured with specific sensors anymore, but can be calculated from the actual current and voltage values in the power cable. This sensorless technology has been used successfully in many thousands of instances in practice.

A planet motor prototype was produced and its functionality comprehensively verified at the TU Wien.

“The characteristics of this new machine type are extraordinarily good,” said Schrödl. “Not only is it extremely compact, the performance can be even doubled compared to conventional machines with the same peripheral rotor velocity.”

The application of the new planet motor is suitable in numerous fields; it is extremely useful not only in electric vehicles, but also in the area of machine tools as well as lifting and production technology. Finally, the electric planet motor developed at the TU Wien is particularly suitable for safety-critical and high performance operations.

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