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GM Study Highlights The Increasing Technical Demands on Motors of Increasing Electrification; Bar Wound Stators as Good Present-Day Solution

While hybrid architectures see a relatively small difference in temperature between the bar wound and wire wound technologies, a full EV application sees a temperature difference of more than 20 °C, according to the GM analysis. Click to enlarge.

Increasing electrification—i.e., moving to an extended range electric vehicle (E-REV) or full battery electric vehicle (BEV) from a mild- or full-hybrid architecture (HEV)—increases the technical demands on motors.

At the recent SAE 2010 Hybrid Vehicle Technologies Symposium, Pete Savagian, GM Engineering Director, Hybrid and Electric Architecture and Electric Motors, presented a study that highlighted the different power and thermal demands on electric motors in a demanding drive cycle for hybrid and all-electric drive vehicles. Specifically, the GM study highlighted the thermal benefits of the bar wound stator technology—similar to Remy’s hairpin technology—compared to a conventional wire wound stator in an EV application.

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.


Stan Peterson

Simple technical advances. That is the way incremental Progress occurs...


What's the downside to bar vs. star? One presumes weight and cost? Is it simply that there's more copper in the bar wound solution?

Or are they harder to manufacture?

The engineer giveth the manufacturer taketh away...


Pros and cons: "Increasing electrification—i.e., moving to an extended range electric vehicle (E-REV) or full battery electric vehicle (BEV) from a mild- or full-hybrid architecture (HEV)—increases the technical demands on motors" but increasing electrification—i.e., moving to an extended range electric vehicle (E-REV) or full battery electric vehicle (BEV) from a mild- or full-hybrid architecture (HEV)—decreases the technical demands on the controls. Witness the current problems of Prius


First, it is good to see GM looking at motors but the challenge is current flow to the motor. The switching semiconductors have to handle much larger current flows. This tends to increase power losses. In contrast, Toyota has headed towards more windings, faster rpms, and using higher voltages with less current. This tends to reduce the volume and weight of the control electronics and transaxle motor sizes.

Kudos for the study but there is no free lunch. Total vehicle performance, not just motors, needs to be part of the study.

Bob Wilson


The Tesla runs up to 13,000 rpm and still needs 2 gears. I guess when you have to have a top speed over 90 mph it takes that. I see no need to go faster, even in a sports car.

If Lotus would make the new Elise a range extended hybrid it would be a winner. More than 60 miles per gallon and 0 to 60 in 6 seconds. Now if they can make if for $30,000 they would have sales.

Oliver Burke

SJC, I believe the initial "2 gear" roadster (if that's what you were referring to) is no longer. I think the roadster is a single forward gear.

From their site: "Transmission: Single speed fixed gear with electrically-actuated parking lock".

And it looks like Lotus has answered your prayers with the Evora :)

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