High-frequency motors: Energy efficiency through intelligent winding design

In order to improve efficiency in the overall system of high-frequency motors, we’ve created a simulation model to specifically address winding losses. Winding losses generally have significant impact on the efficiency of the motor, particularly in high-frequency systems. Winding losses can be categorized into a direct current component (DC) and an alternating current component (AC).

In order to design reliable systems with high efficiency, the accurate prediction of winding losses is more important than ever - from a sustainability perspective as well.

In co-creation with experts from Newcastle University, the CONNACTIVE team has developed a simulation model to correctly classify winding losses in the overall system.

Classifying winding losses correctly

Cause and effect

Winding losses lead to DC losses and AC losses.

There are three sources of loss:

DC-losses occur as a result of direct current resistance
AC-losses occur as a result of skin and proximity effects und particularly through induced fields emanating from the rotor.

Skin Effect

Only the surface of a conductor is available for the transport of charge carriers. In alternating current, eddy currents and electric fields are generated depending on the frequency, and these force the charge carriers into the “skin” of the conductor. The resistance of the conductor increases.

Proximity Effect

When cables are positioned adjacently, the currents flowing in opposite directions repel each other due to the associated magnetic fields. The current flow across the cross-section of each conductor thus becomes non-uniform. This increases the resistance of the conductor.

Modelling Winding Losses

Our Approach:

Comparison of different winding solutions
Development and refinement of a simulation methodology to determine winding losses
Analysis of the results

Implementation

We looked at different winding configurations of profile copper and compared them with Litz-wire-based windings.

Profile copper, wound horizontally

The use of horizontally wound profile copper results in a high fill factor and a greater distance from the rotor. The alternating field is induced in the winding turn opposite the rotor, however, so that heating there is intense and the AC losses very high.

Profile copper, wound vertically

In this concentric winding, the induced eddy currents are evenly distributed across all the turns. This distribution significantly reduces the eddy current loss. Contacting the conductors presents a challenge, however.

Litz-wire — State of the art

The classic approach for high-frequency motors. It’s common knowledge that stranded wire minimizes AC losses.

This is a standard solution – but it has the following drawbacks:

Relatively low fill factor and thus sub-optimal thermal properties
Complex contacting
Higher material and process costs

Evaluation / Conclusion

The DC losses of the Litz-wires are no more than 10 % higher than in the other two options.
AC losses increase at higher frequencies: Operating points and respective dwell times must therefore be taken into account.
Taking into account the application profile, vertically wound profile copper can offer a good compromise between efficiency and cost - if the contacting problem is solved.

The effects of Litz-wire insulation and fill factor are not taken into account here.

The journey is the reward

Overview of the model’s performance:

The simulation model was extensively validated and compared with existing models and measured data.
AC losses were measured for different materials across large duty cycle and frequency ranges, and compared with theoretical models.
Our goal: Increased efficiency in determining losses and selecting suitable components, as well as the optimization of all interfaces within the eDrive system.
Careful selection of the winding as a tradeoff between efficiency and cost – in order to achieve good TCO.
Particularly in the case of high-frequency motors, one should always take into account the application profile and make sure that the design decisions for the windings are arrived at only after careful consideration.

Research Associate at Newcastle University

Dave Winterborne

"Through the AC simulation model developed in co-creation with our CONNACTIVE partners and the University of Newcastle, we have again been able to demonstrate what co-creation can achieve: More than simply an engineering service, it constitutes real development work via system design and simulation – and in record time, too."