22 Jul 2024


Single-tooth motors could bite into the EV market

German researchers are developing a new generation of high-efficiency motors for electric vehicles (EVs) with the aim of challenging two perceived limitations of EVs – their limited ranges and high purchase prices.

The researchers – from the German drives specialist FEAAM and the Institute for Electrical Drives & Actuators of the Universität der Bundeswehr (University of the Federal Armed Forces) – are examining the various drive-train components, and basing their work on driving cycles similar to those used to test the efficiencies and emissions of internal combustion engines. They are focusing on efficiencies during partial-load operation, rather than at maximum load conditions.

A promising approach to optimising motor efficiency that has emerged from the work is to use asynchronous motors with concentrated windings. Unlike conventional induction motors with distributed windings, where the coils are wound around multiple lamination teeth, the windings use only a single tooth. Although this technique is not new, it has not previously been applied in practice because of the interfering harmonics that can occur. (The photos above show motors with conventional windings on the left and the concentrated windings on the right.)

In the joint research project, FEAAM and the university team analysed the magnetic fields in the motor using a combination of simulation and practical experiments. As a result, they were able to devise various measures to suppress unwanted harmonics. The closer these harmonics are to the working waveform, the more they can interfere with the motor’s operation, causing electrical losses or acoustic noise.

The researchers are damping the harmonics using a special winding technique in which adjacent tooth coils are wound in opposing directions. By configuring the number of wires in each coil appropriately, they can reduce the harmonics. In the research project, the coil structures were first simulated using mathematical models, and the effects then verified by measurements.

The development process has taken about two years. The researchers have simultaneously been trying to simplify manufacturing, reduce production costs, and achieve high efficiencies at partial loads. This parallel development approach offers potential attractions both in terms of lowering purchase costs and extending the driving range of electric vehicles.

The new winding technology could have attractions in terms of in production. The stator’s stamped sheet metal parts can be wound individually and then plugged together into a motor. This contrasts with the situation for a conventional induction motor, where the plates are assembled first, and the winding is then applied in a more complicated fashion. This high-cost approach means that it would not be ideal for producing the volumes of several million motors per year typically needed by the automotive industry.

The German researchers have built prototype motors and made several patent applications. The prototype three-phase 400V motors have a power rating of about 50kW – similar to that needed for a typical traction motor for an electric car.


“Previously, electric motors in this power range were produced by medium-sized companies in quantities of a few thousand per year,” says Prof Dr-Ing Dieter Gerling from the University. “In the automotive industry, we are looking at quite different numbers, which means that costs become much more significant.

“This industry has a lot of experience in cost management”, he adds. “In principle this, of course, also applies to power electronics and battery technology.”

As well as developing the electric motor, the FEAAM and University teams have also been optimising the vehicles’ power electronic circuits, motor controls and wiring systems. Again, they have ideas for optimising efficiencies at part-loads. In all of these areas there is still a great potential for improvement, says Gerling.

•  As part of the development programme, the German researchers have been measuring currents and voltages using transducers in each phase of their motors. In this way, they can identify any asymmetries. As well as making efficiency measurements, they record torque/speed characteristics, and use this data to optimise their mathematical models. Because rotor resistances – and thus losses – increase with temperature, they are held constant using a cooling liquid and monitored using a thermal imaging camera.

The researchers have been taking measurements for different driving cycles. For power and efficiency measurements, they have been using Yokogawa power analysers. The measured values, including torque and speed and the resulting efficiency, are transferred automatically to an Excel spreadsheet.