The Voltec 4ET50 Electric Drive System

General Motors' Chevrolet Volt is an Extended Range Electric Vehicle (EREV). This car has aggressive targets for all electric range with engine off and fuel economy with the engine on. The Voltec 4ET50 transaxle has gears, clutches, and shafts and controls that execute two kinematic modes for engine off operation or Electric Vehicle (EV) operation, and two additional kinematic modes for extended range (ER) operation. The Voltec electric transaxle also has two electric motors, two inverters, and specialized motor controls to motivate to execute each of those four driving modes. Collectively these are known as the Voltec Electric drive. This paper will present the design and performance details of the Chevrolet Voltec electric drive. Both the machines of the Voltec electric drive system are permanent magnet AC synchronous machines with the magnets buried inside the rotor. The motor has distributed windings. However, as opposed to a conventional stranded winding the Chevrolet Volt motor has bar-wound construction to improve the motor performance, especially in the low to medium speed range. At higher speed the skin effect and proximity effects in the stator bars lead to increased stator winding losses but are addressed in the design. The bar-wound construction also has excellent thermal performance in both the steady-state and transient conditions necessary for full EV driving. The generator uses concentrated windings. The concentrated winding construction has good slot fill and extremely short end-turn length. These features resulted in good performance in the intended operational region and were an enabler for machine packaging inside the transmission. Both the machines exhibit excellent efficiency and exceptionally smooth and quiet operation. Machine design and construction details as well as the measured thermal, electromagnetic and acoustic noise performance are presented in the paper. The Traction Power Inverter Module (TPIM), which is designed to commutate these motors and uses state-of-the-art power electronic components, adopts flat and compact structure to achieve high current density and exhibits improved manufacturability., Hardware and software design tradeoffs were considered to meet TPIM reliability, efficiency, performance and overall security requirements. These features will be discussed in this paper. To address the aggressive targets for fuel economy and EV range of Chevrolet Volt, engineers were forced to closely examine all loss mechanisms involved in the electric drive system and optimize motor controls to provide the best drive system efficiency. While optimizing the drive system efficiency, other constraining requirements such as vehicle Noise Vibration and Harshness (NVH), high voltage bus ripple and resonance, component thermal performance and controllability, etc. were addressed. This paper will further discuss the issues encountered and solutions adopted to optimize the drive system efficiency to help the Chevrolet Volt meet the fuel economy and EV range targets.