Designing Thermally Secured Electric Motors with Simulation

Component-Level Design & Analysis for Motor Thermal Security

Most of today’s traction motors in battery electric vehicles (BEVs) are permanent magnet synchronous machines (PMSMs) that use interior permanent magnet (IPM) rotors with rare-earth magnets embedded in the rotor (automotive companies are starting to explore or even build other technologies, but that is a topic for another blog).  These magnets generate heat and tend to demagnetize if they reach critical temperatures; moreover, because they are embedded in the rotor, cooling these magnets can be challenging. 

In the design process of a traction motor, a variety of stator and rotor cooling options should be studied, and simulation gives motor designers the ability to study trade-offs of different cooling strategies without having to build and test physical prototypes (saving both time and money). Traditionally, steady-state, component-level simulations that couple finite-element approaches for both electromagnetics and thermal conduction and convection are used to ensure the thermal security of the motor.   

For an example of this, see the model results below that utilize both GT-FEMAG and GT-SUITE. These simulation solutions from Gamma Technologies were used to couple the electromagnetic and thermal finite element solutions to study different stator cooling topologies for a traction motor. The results below include the trade-offs of structure temperature (windings and magnets), coolant temperature rise, and coolant pressure drop. 

Coupling GT-FEMAG electromagnetic and GT-SUITE thermal finite element solutions for motor cooling design analysis

System-Level BEV Design & Analysis

System-level engineering of BEVs, on the other hand, requires an understanding of the global energy management puzzle and temperature distribution of its components (such as batteries, motors, inverters, and the occupants) to predict detrimental hot spots and occupant comfort in either hot or cold ambient temperatures during transient events. For more on this topic, see a blog written by my colleague, Brad Holcomb. 

In the case of automotive applications, the most common transient analyses performed are drive cycle tests that can be 30 minutes, or longer. For these long, transient simulations, the traditional finite element model of the motor (introduced earlier) would be too slow to be integrated into system-level simulation for hot spot prediction. 

The challenge is how can system-level engineers have an accurate, fast-running representation of a traction motor capable of hot spot prediction that can be integrated into a system-level model? In other words, how can we blur the lines between component-level and system-level simulation to engineer better electric vehicles? 

Blurring the Lines Between Component-Level & System-Level Simulation with GT-SUITE and GT-FEMAG 

With GT-FEMAG and GT-SUITE, Gamma Technologies offers an innovative way to have physics-based models of these coupled electromagnetic-thermal models to be used in system-level simulation.

First, the electromagnetic solver of GT-FEMAG is integrated into GT-SUITE as a seamless pre-processor that can automatically generate either map-based versions of motors with detailed component losses (for example, winding, iron, or magnet losses) or equivalent circuit models of motors (commonly referred to as “Ld, Lq” models) for the transient solver of GT to use in system-level simulations. 

GT-FEMAG as a pre-processor to GT-SUITE System-Level Simulation

Second, the thermal solver of GT-SUITE has a generalized, physics-based, and one-click switch that automatically converts 3D finite element models into 1D lumped thermal network models. 

Automatic model order reduction in GT-SUITE to reduce thermal finite element models to thermal network models

These two capabilities enable users to quickly traverse different modeling fidelities between fully detailed electromagnetic and thermal finite element and fast-running and accurate 1D models.

Electric Motor Simulation Demonstration & Results

In a demonstration model we created, we combined these technologies to be able to run back-to-back Worldwide Harmonized Light-Duty Vehicle Test Cycles (WLTC) on an electro-thermal motor model by imposing speed and torque on the motor for one hour of simulation. To give the model more transient warm-up behavior, we connected the motor to a simplified cooling system that includes a thermostat, a pump, and a heat exchanger. We also modeled three different ambient and initial soak temperatures of the system, modeling the warmup of the motor at –10 °C, 0 °C, and 10 °C ambient conditions. 

The simulations each only took 70 seconds to complete (over 50 times faster than real-time), but captured the transient warmup of the various components in the motor, including the windings and magnets in the rotor: 

Transient warmup of various components in an electric motor

Below is an animation showing the transient coolant temperature through the stator for the -10 °C ambient case over the course of the simulation (please note contour scale is non-linear to help visualize results).

Transient stator coolant temperature through 2 WLTCs at -10 °C ambient conditions 

Model Integration 

Because the standalone electro-thermal motor model was processed over 50 times faster than real-time, we can bring it directly into a system-level model for integrated simulations. These simulations allowed us to have a deeper understanding of the energy management and trade-offs between different cooling strategies for the entire system, including the motor, inverter, battery, and cabin.

A complete multi-physics BEV model with GT-SUITE and GT-FEMAG

Learn More About our Electric Motor Simulation Solutions

If you’d like to learn more or are interested in trying GT-FEMAG or GT-SUITE for component-level or system-level simulation of electric vehicles, contact us!