Designing High-Performance Drive Systems for Electric and Hybrid Electric Vehicles

With the US economy's increased dependency on foreign oil, and with more stringent emission standards, electric vehicles (EVs) and hybrid-electric vehicles (HEVs) are receiving renewed attention from the US government and automotive manufacturers.

In these vehicles, electric traction systems supplement or replace conventional internal combustion engines. According to the National Renewable Energy Laboratory (NREL), the use of these alternative systems could increase the average gas mileage to 80 mpg. The design of these systems involves a wide variety of applications and requires sophisticated simulation tools.

For instance, the design of electrical traction systems involves electromagnetic and electromechanical aspects, power electronics, and sophisticated controls. Designers at General Motor's Advanced Technology Vehicle Center (GMATV) in Torrance, California, are now analyzing a brushless DC (BLDC) motor-drive concept. The drive contains a complex power-electronic circuit and innovative control concepts. GMATV is using Ansoft's electrical and electromechanical design solutions. Ansoft provides a complete multidomain design environment that includes Finite Element Analysis (FEA)-based component design software, as well as circuit and system-level analysis (see Figures 1 and 2).

"At GMATV, we're using Ansoft's finite-element software for the accurate design of electrical motors for traction systems in electrical and hybrid-electric vehicles," said Costin Stancu, a Design Engineer at GMATV. "The specifics of electric traction require high-performance electrical machines combined with innovative power electronics and controls."

To evaluate the overall performance of the drive, engineers require a complete system model. Given the specifics of traction systems, it is necessary to consider the motor, even on the system level, with nonlinear effects, such as saturation. To obtain an accurate representation of the motor behavior, Maxwell® 2D can generate a lumped parameter model. This is a state-space-type model where parameters are swept through the entire operating range; thus, space harmonics and saturation effects are included. The model generation is completely automatic. Furthermore, the model can be imported directly into Simplorer®, Ansoft's versatile system-level simulation tool. Here, the nonlinear motor model is embedded into the system environment. Due to the unique combination of circuits, block diagrams, and state machines, design engineers can quickly model the entire power-electronic and control system.

"With the addition of Simplorer," said Stancu, "we can now design the whole traction system in a single design environment under consideration of all aspects and interdependencies between the system components. The analysis of sophisticated controller concepts, such as field weakening, requires the combination of accurate, nonlinear Finite Element Analysis-based motor models with versatile electrical, mechanical, and system-level modeling capabilities. By adding Simplorer to our existing Maxwell infrastructure, we now have all of these capabilities. And by implementing complete system simulation tools in our design cycle, we expect faster and more accurate performance prediction of our traction systems and, as a result, a shortened design cycle."