Dr. Dan M. Ionel - Professor of Electrical Engineering and L. Stanley Pigman Chair in Power at the University of Kentucky - used his seminar at the recent CWIEME Berlin to highlight record-breaking developments in the design and materials for brushless PM machines.

In recent times brushless permanent magnet (PM) machines, especially of the AC synchronous type, have become a preferred technology when both high efficiency and high torque density are required. But having undergone long-term development, is there still scope to extend the limits of performance and competiveness?

"Yes, and a recent record ultra-high torque density motor for Formula-E racing cars is an excellent demonstration in this respect," says Dr. Ionel. "Originally developed by our UK-based colleagues and collaborators: Motor Design and Equipmake, the concept is now applied for a remarkable 15Nm/kg design rated at 120kW. This has approximately two to three times the torque density of electric motors from commercially-available EV/HEVs. Our computational techniques, based on differential evolution algorithms and parallel processing, enabled the study of thousands of candidate designs in search for the optimum solution."

Extremely high magnetic loading is enabled by a special flux-focusing spoke rotor design. Record high electric loading in the concentrated stator winding is supported by an innovative cooling system. An 18-slot 16-pole configuration maximizes average torque, and minimizes ripple and unbalanced magnetic pull.

"Key to the design process is the ability to model with high fidelity the coupled effect of electromagnetic field, thermal and mechanical stress. Combine this with the detailed knowledge of the duty/traction cycle and engineering innovation and here it is - a new technological frontier," continues Dr. Ionel.

The work builds upon the academic research of the groups from the University of Kentucky and Marquette University on the multi-physics analysis and optimization of electrical machines, which has been covered in journal and conference publications, and won an IEEE best paper award.

New carbon nano-tubes wires and yarns

Yet another question relates to improvements achieved through the use of new materials. Traditionally, the focus has been on soft and hard magnetic materials: laminated steel, SMC, ferrite and rare-earth magnets. Today, developments in electrically-conductive nano-carbon tubes (NCT) wires and yarns are offering opportunities for a new generation of winding technology.

"Together with our colleagues and collaborators from the University of Kentucky Center for Applied Energy Research and ANSYS, we are developing not only new NCT materials but also new machine design concepts that benefit from their different characteristics, such as electric conductivity and ampacity, mass density and thermal conductivity," continues Dr. Ionel. "This is a joint, multi-disciplinary effort. One of the Universitys current projects considers a novel concept for an ultra-light coreless multi-disc high speed machine."

While industrial interest is initially anticipated in premium applications, such as aerospace motor drives, the US Department of Energy (DOE) has recently announced support for NCT demonstrations for the stator windings of single-phase induction motors. On-going research challenges include practical aspects of wire insulation, varnishing and soldering mechanisms.