Greater electrification requires higher peak torque and power densities, Savagian noted. The extraordinary continuous power density required by an all-electric drive application is a “very different situation” than that required by a hybrid application in which the combustion engine can contribute to the overall tractive effort.

If our intention is to drive completely electrically, the operating point of the electric motor can be buffered a little but, generally it is up to accelerator pedal of the driver. The operating region has to be broader.

—Pete Savagian

In its study, GM used an agressive drive cycle consisting of a hard acceleration to about 20 km/h followed by wide open acceleration to 130 km/h, followed by some cruising and a slowdown. The cycle represents “aggressive behavior, but behavior that cars need to be able to satisfy regularly”, Savagian said.

Continuous motor duty increases with electrification. The drive cycle is at the top. Click to enlarge.

For this drive cycle, GM found, a mild hybrid motor would have an average power requirement of about 3.4 kW. A full hybrid motor—such as Motor B in GM’s two-mode hybrid—would have an average power requirement of 5.2 kW. However, an extended range electric vehicle or a battery electric vehicle motor would have an average power requirement of 24.9 kW—more than four times that of the full hybrid system.

GM set up a thermal analysis to compare bar wound stator technology with conventional wire wound stators for this cycle. Bar winding uses rectangular wires instead of conventional round wire. The technology produces a higher copper fill (85-90% compared to 70%, according to GM).

Wire wound (top) vs. bar wound (bottom). Click to enlarge.

Bar winding lowers winding resistance 30% or more, lowering overall losses when compared to conventional wire wound types. Bar wound motors also have 50% or more greater heat dissipation area when compared to stranded wound types.

For the comparison, GM used the same package envelope, same active length, same rotor and same electromagnetic design (turns/pole) for both bar wound and conventional motors. The bar wound stator used rectangular slots; the wire wound used keystone slots.

GM used a two mode in a mid-sized car simulated for full hybrid driving, and a midsize E-REV simulated with full electric driving. The cooling boundary conditions were common between both cases.

While bar wound motors are somewhat cooler than wire wound for full hybrid applications, they run significantly cooler for Full EV driving, GM found. In a full hybrid application, the temperature delta was around 5 °C; in the EV application, the temperature delta was more than 20 ° C. A 20°C difference could double the life of the motor, Savagian noted, citing data from motor insulation providers.

The different dissipation between the bar wound and strand wound design is the copper loss due to the increased resistance (ie. iron loss held constant). The different heat rejection is due to the different end turn geometries.

GM, which last month announced it plans to manufacture electric motors (earlier post), will manufacture bar wound motors in its Baltimore plant, according to a GM spokesperson.