Overcoming the challenges of HEV/EV traction motor design

  • 11-Jun-2011 03:54 EDT
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FEA accurately accounts for nonlinearity and local saturation of a rotor and stator. ANSYS’ Maxwell electromagnetics field simulation software uses the finite element method to solve static, frequency-domain, and time-varying electromagnetic and electric fields.

Hybrid electric vehicles (HEVs) and electric vehicles (EVs) have been rapidly gaining traction in the global automobile market by providing high fuel efficiency and near-zero emissions at increasingly affordable prices. Many OEMs are working on new HEV and EV designs; one of their greatest challenges is designing traction motors, which, in most cases, are interior permanent magnet (IPM) synchronous machines. Minimizing electrical and magnetic losses is critical to deliver maximum range and fuel efficiency to consumers. At the same time, engineers need to consider structural, thermal, and electromagnetic issues that play a crucial role in vehicle performance, reliability, and cost.

OEMs face the challenge of solving these issues quickly to squeeze through tight market windows in the rapidly evolving HEV and EV markets. Integrated multiphysics simulation technology helps to address these challenges by enabling engineers to rapidly evaluate—prior to physical prototyping—functionality, performance, and cost of a wide range of design alternatives. This approach makes it possible to optimize traction motor design performance in a fraction of the time and cost required by traditional design methodology.

In developing the motor/generator, a design team first focuses on the electromagnetics of the electric machine. Initial CAD drawings and related engineering specifications of the assembly provide entry data for electronics design optimization software: defining the main elements of the motor/generator including magnet materials, coil configurations, number of turns, air gaps, and more.

These outputs, in turn, provide data for electromagnetic field simulation software, which computes the torque profile of the machine—that is, how the torque ramps up over time for driving the vehicle in motor mode as well as the electrical resistance in stopping the vehicle in regenerative brake mode. Vehicle weight is brought into the analysis to determine acceleration as well as stopping time for various scenarios. Based on output results, the team modifies the design by changing any basic design parameters (magnet size, for example) to balance machine performance against its size, weight, and cost.

The computed torque output is used further in a structural mechanics solver for computing mechanical stresses, loads, deformations, and vibrations of the physical parts of the powertrain, including the driveshaft and gearing. Vibration analysis is important because traction motors can be a prominent source of noise in EVs. Additionally, a fluid dynamics solver helps in studying thermal management issues, mapping energy losses, and determining heat distributions in the motor/generator assembly.

Putting two or more individually optimized components together does not make an optimized system. This is particularly evident in HEV and EV traction motor design, in which the motor must be designed and optimized as part of a larger system that includes power electronics, controller, and other components. Therefore, a multidomain system simulation program is essential for designing a high-performance product that incorporates electrical, thermal, electromechanical, electromagnetic, controller designs, etc. Such technology must tie the different physical analyses together to arrive at a coherent, optimized electric powertrain.

The traction motor is part of the larger system that includes an insulated-gate bipolar transistor (IGBT) inverter, cable/busbar, and mechanical load, which all must be modeled in a single integrated simulation. Using electronic thermal current tools, engineers specify the geometry of the major heat sources in the powertrain system, such as the IGBTs and current-carrying parts of the motor/generator. Each heat source is applied individually at major points of interest in the system with air circulation and conducted thermal energy taken into consideration. The software then processes this data and generates a thermal model that determines overall temperature profiles of each IGBT. The software also provides temperature-dependent performance variables, such as energy drained from the batteries to ensure that heat levels do not exceed specified limits and adversely affect IGBT performance.

From this temperature profile, engineers can utilize the thermal-structural analysis capabilities of an FEA-based structural solver to determine the resulting thermal stresses. Electronic design analysis tools are applied to calculate electromagnetic forces acting on motor/generator components to determine deformations and mechanical stress distributions on the structure. Engineers can then modify the structure to eliminate stress concentrations and excessive deformation or, conversely, to lighten regions that were overdesigned.

Throughout the electromagnetics and mechanical development processes, a common simulation platform coordinates the actions and exchanges of data between the various physical simulations—all those many computations performed for different load scenarios and in comparing various design alternatives. This multiphysics cosimulation process is efficient only when the software runs together through a single unified environment with a smooth flow of data between programs.

As an example, a common simulation platform is used to simulate a typical multiphysics problem in traction motor design. The end goal of this simulation is to find out the stress/deformation on stator lamination and coils as input for vibration/acoustic noise or fatigue analysis. The geometry is common for both structural and thermal analysis. The magnetic solver computes electromagnetic losses and magnetic force. Losses from the magnetic solver are automatically mapped into the thermal solver as thermal loads on an element-to-element basis to compute the temperature profile. This temperature profile is then automatically mapped into the structural solver to compute thermal-mechanical stress.

At the same time, the magnetic component of the force is mapped from the electromagnetic to the structural solver. The engineer can apply any additional force directly within the structural solver. The final simulation simultaneously takes into account all the loads that would act on the motor under real operating conditions, thereby simulating the motor’s performance with real-life accuracy. Once one such simulation is completed, the common simulation platform allows engineers to change the geometry and update all simulations in different physics in a highly automated way—without having to set up each simulation again.

For each current, output torque peaks at one particular load angle. To optimize the motor and drive design, load angle and current should be used to drive the motor to achieve maximum torque within a given geometry. To derive this type of curve, at least 494 combinations need to be simulated—which doesn’t include potential changes in geometry, motor speeds, and material properties at different operating temperatures. This example shows that hundreds of thousands of designs must be simulated to optimize a typical IPM design.

Zhangjun (Zed) Tang, Ph.D., Lead Engineer, ANSYS Inc., wrote this article for AEI.

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