Electric Motor Design of General Motors’ Chevrolet Bolt Electric Vehicle

A permanent magnet synchronous motor (PMSM) motor is used to design the propulsion system of GM’s Chevrolet Bolt battery electric vehicle (BEV). Magnets are buried inside the rotor in two layer ‘V’ arrangement. The Chevrolet Bolt BEV electric machine rotor design optimizes the magnet placement between the adjacent poles asymmetrically to lower torque ripple and radial force. Similar to Chevrolet Spark BEV electric motor, a pair of small slots are stamped in each rotor pole near the rotor outer surface to lower torque ripple and radial force. Rotor design optimizes the placement of these slots at different locations in adjacent poles providing further reduction in torque ripple and radial force. As a result of all these design features, the Chevrolet Bolt BEV electric motor is able to meet the GM stringent noise and vibration requirements without implementing rotor skew, which (rotor skew) lowers motor performance and adds complexity to the rotor manufacturing and hence is undesirable.

A bar-wound stator construction, similar to Chevrolet Spark battery electric vehicle, is implemented in Chevrolet Bolt BEV. Bar-wound construction, which GM has adopted for most of its electric and hybrid vehicle motor construction, is known to provide high slot fill, short end-turn length, improved thermal performance, and improved vehicle efficiency especially at low to medium speed ranges. System design, including gearing takes advantage of these machines to ensure the aggregate majority of the driving energy is consumed in the higher efficiency areas of the motor efficiency envelope. However in order to lower the winding ac effect at higher speeds and expanded the high efficiency portions of the envelope, the Chevrolet Bolt BEV motor implements six conductors per slot design while four conductors per slot design was used in Chevrolet Spark motor design. As a result, individual conductor size is smaller in new design resulting in reduced winding AC effects and improved joule loss at high speed operations. Winding layout design in Chevrolet Bolt BEV motor is optimized to minimize voltage between conductors within the slot. This has allowed to eliminate the slot insulation between conductors, further increasing the slot fill and reducing material and manufacturing costs. Stator design of Chevrolet Bolt BEV adopts a special feature, introduced in Gen2 Chevrolet Volt, the stator slot opening size and placement under each pole are optimized to lower torque ripple and radial force. This feature supplements the torque ripple and radial force reduction features introduced in the rotor design as described above.

The high performing electric machine is coupled with a high performing control algorithm to deliver maximum system efficiency and performance. A six-step mode of inverter control is implemented to maximize the voltage utilization. As the speed is increased control automatically transitions to six-step mode from space vector PWM (SVPWM) seamlessly. Torque response dynamics at six-step control is reduced, as expected, from SVPWM mode of control. However, the torque control dynamics with six-step control is able to meet the torque response requirements of the vehicle. Control is stable and robust even with very fast vehicle acceleration.