Professors J. Rhett Mayor, School of Mechanical Engineering, and Ron Harley, School of Electrical and Computer Engineering, Georgia Institute of Technology

This research considered applying advanced heat transfer techniques to the thermal- mechanical design of high-power density, variable loading electrical machines in order to increase thermal capacitance. The machine thermal capacitance provides a passive ability to absorb transient overload conditions, albeit
typically for short durations. The maximum duration of overload condition is proportional to the machine frame mass degree of overload (on per-rated current basis). A proprietary approach to the thermal design of electric machines, active thermal capacitance, has been investigated. The proposed active thermal
capacitance design approach increases thermal capacitance without adding significant (thermal) mass to the frame, thereby avoiding the corresponding penalty to the power/torque machine density. The technique is based on
controlling fluid flow rates in micro-channel enhanced heat exchangers integral to the
machine frame in order to provide the unique ability to increase the machine frame apparent
thermal capacitance.

The active thermal capacitance technique was applied to a surface mount, concentrated
winding PM machine. An advanced parametric, self-segmenting half-slot model finitedifference
code was adapted to investigate the spatio-temporal temperature distribution in the
stator and winding of the PM machine under various load conditions. Peak power conditions
of up to 3.5x per unit stator current were considered. The peak condition duration was
allowed to extend to thermal steady state to characterize the maximum duration of overload
conditions achievable through modifying the frame thermal capacitance. Two ambient
thermal rejection scenarios were considered: (a) rejecting to an external radiator at 20°C,
and b) using an engine-coolant base flow system at 40°C (avg), available in a typical hybrid
vehicle application. A characteristic result from the study illustrating the thermal machine
response to a 3.5x stator current overload condition, rejecting to ambient, is presented in
Figure 8.

Figure 8: The tempeture distribution in the stator of the SMPM machine decreases even under 3.5x per unit stator current overload conditions due to the massive increase in heat transfer achieved by the novel micro-channel heat exchangers.

Figure 8: The tempeture distribution in the stator of the SMPM machine decreases even under 3.5x per unit stator current overload conditions due to the massive increase in heat transfer achieved by the novel micro-channel heat exchangers.

The temperature distribution in the SMPM machine stator decreases from 160°C to 110°C even under 3.5x per unit stator current overload conditions. This counter-intuitive response is due to the massive increase in
heat transfer from the winding to the coolant stream with increased flow rates through the
novel micro-channel heat exchangers. The final temperature distribution in the stator (right
view in Figure 8) clearly demonstrates the direction of heat transfer through the winding
to the heat exchanger. Compare it to the more conventional stator-based heat rejection path,
dominant prior to engaging the active thermal capacitance technique, which results in the
heat concentration in the windings (left view in Figure 8).

The results from the study have demonstrated the efficacy of the active thermal capacitance technique incorporating the advanced micro-channel heat exchanger concepts. Active thermal capacitance is a feasible
approach to developing a new generation of electrical machines that are capable of achieving
both high efficiency operation at normal loading and very high power/torque density operation for sustained periods of peak loading. Such machines would have direct application in advanced electric vehicle power trains,
including off-highway construction equipment and “green” inner-city delivery vans. Ongoing
work is aimed at extending the investigation to consider alternate machine topologies, in
particular, field-wound induction machines.

This research was supported by the Grainger Center for Electric Machinery and Electromechanics.