GM ATV's Generation II Electric Drive - The Technology
December 3, 1998 - John Hepke, director of propulsion systems at GM Advanced Technology Vehicles, acknowledges that competing with internal combustion engines is no small task, "Even though IC engines have well over a century of continuous development on their side, with each design iteration the electric propulsion cost comes down and the performance increases. At the moment, we're taking advantage of the electronics boom that is driving down the cost of a personal computer. Concurrent with each new Pentium processor and the next generation of Windows software, there are similar gains in power and control electronics that help us slash cost, weight and bulk from the EV1. The Generation II electric drive system represents approximately half the cost and one-third fewer parts required in the Generation I design, yet we haven't sacrificed any performance."
GM ATV established very ambitious improvement curves for the EV1. Now that the first-generation product has proven successful, the stage is set to forge ahead with significant changes involving the power electronics bay, drive unit, charging system, battery pack and Heating Ventilation and Cooling (HVAC) system.
Peter Savagian, ATV's chief engineer of traction and thermal systems, uses the technical term "up-integration" to describe improvements under the EV1 hood, "The first generation has three interconnected boxes in the power electronics bay. Up-integration permitted locating everything in one new box that's significantly smaller, lighter, and less expensive. We also switched to field orientation control electronics versus the analog hardware used in Generation I and changed to field-oriented control versus slip control of the three-phase AC motor, allowing for smoother and more efficient driving."
"Electronic circuits previously housed in an accessory power module and a power steering control module have moved into the main power electronics bay. Two significant internal changes saved space, weight, and cost. One is replacing a total of six single IGBTs (Insulated Gate Bipolar Transistors) with three dual IGBTs. These devices invert the DC current from the battery pack to three-phase AC which runs the motor. The second change is the integration of the DC-DC converter function, which is analogous to an alternator. The DC-DC converter now uses modular power supplies instead of discreet power components, saving cost and reducing size. This saved one metal housing and a number of interface wires. The bias control assembly--electronic circuits that translate vehicle information into motor control signals--now takes up half the previous circuit board space. This is because a significant number of discreet components have been combined and replaced by a microprocessor and a digital signal processor that saves bulk, cost, and complexity while offering enhanced programming flexibility. Bus capacitors are also significantly smaller in Generation II. All together we trimmed nearly 30 pounds of weight and were able to position more circuits within a smaller power electronics bay thanks to up-integration aided by the rapid advancement of electronics technology during the past two years."
The major change to the drive unit assembly--the EV1 electric motor, gear reducer, and integrated final-drive assembly--is a switch from pressure to splash lubrication. This permitted elimination of an electric motor-driven gear pump, 2 pounds of weight, and some bulk. Reliability is improved because the Generation II design has fewer moving parts. Lubricant carried within the housing is now circulated by the paddle-wheel-effect of the ring gear. A lower-viscosity lubricant is used for optimum performance over a broader range of climatic conditions. Subtle design improvements have quieted the cruising-speed noise produced by the traction motor's rotor.
A switch to a die-cast aluminum rotor from laminated steel construction was another major source of cost reduction. The new rotor is approximately 25-percent longer because aluminum is magnetically less efficient than steel. The motor's power output--102 kW--is unchanged.
Redesign of the propulsion system's coolant reservoir to a simple molded-plastic component also reduced cost. The heat-pump-based HVAC system is upgraded in two significant areas. A heat exchanger that helped warm the interior on cool days by means of waste heat from the propulsion system has been replaced by an electric-resistance heating element. The system now works better and is less complicated as a result of the elimination of several parts: heat exchanger, control valve, and coolant hoses. The heated windshield power supply regulates operation of the new HVAC heating element.
In preparation for the switch to the optional nickel-metal hydride batteries, the EV1 battery pack now has active cooling. While the batteries are being recharged, a cooling fan at the rear of the vehicle draws cool air supplied by the heat-pump. Baffling within the battery pack tunnel helps balance the air flow distribution.
The Magne-Charge inductive charging system, used in the EV1 and S-10 Electric truck, also has various revisions. Except for a pedestal assembly, wall and floor mount versions are now identical. The cable-retractor system provided in the floor-mount version has been replaced by a simple holder for the inductive paddle. Inside, up-integration has cut the number of circuit boards in half. The new liquid-crystal display panel is more visible in high ambient light conditions. At the front of EV1, the charge port receptacle is smaller and lighter because the rectifier diodes have been relocated to the power electronics bay.
Stressing the significance of an improved second-generation electric drive system, Hepke notes, "Every car manufacturer has advanced concepts but the difference at ATV is we're able to put them in production. The real-world test bed where we prove whether or not these ideas work is very attractive to electronic systems suppliers because it demonstrates the market potential for their products. They're very anxious to become involved on the ground floor of the electric car movement so they bring us new ideas that stimulate our thinking for the next generation."