Thermal Management Solutions for Electric Vehicle Motors

FEV Europe GmbH has made a document introducing the thermal integration issues of high-speed traction motors in electric vehicles and proposed some future trends and solutions.

● Motor thermal management

In recent years, electric vehicle technology has made great progress, but the issue of motor thermal management has always been one of the challenges in the field of electric vehicles. The motors in pure electric vehicles have shown high efficiency, but still cause energy loss in the transmission system. Future energy consumption targets require further improvement of motor efficiency to reduce energy consumption and increase driving range. Thermal management of motors is considered to be one of the key factors to achieve this goal.

● The impact of temperature on motor efficiency

Research has found that the temperature of a motor has a significant impact on its efficiency. At a specific operating point, higher temperatures can improve the efficiency of the motor, so in daily driving, "hotter is better!" By actively controlling the temperature of the motor, efficiency can be effectively improved.

The efficiency of electric vehicles shows a clear difference when the motor temperature rises from 60°C to 140°C, for a compact electric vehicle driven by a single electric axle with a peak powertrain of 150 kW. The study used a state-of-the-art permanent magnet synchronous motor (PMSM) for simulations, with a motor design using a delta-pole shape and splint windings. When the motor temperature reaches 140°C, positive numbers (green) indicate a clear improvement in the efficiency of the motor, especially in the low torque region. In this region, the motor requires less overexcitation, reducing the need for field-weakening currents, but also requires thermodynamic behavior.

But in the high-load region, the efficiency of the motor drops, mainly due to increased copper losses, so a better cooling system is needed to maintain performance.

For everyday driving, "hotter is better!". Under normal driving conditions, higher motor temperatures increase efficiency. By actively controlling the temperature of the motor, active thermal field mitigation can be achieved, which further increases efficiency. This technology is particularly suitable for high-speed and lightweight motors, as they have higher power density, higher loss density and shorter heat-up time. In WLTC tests, higher performance and efficiency can be achieved by optimizing the motor temperature.

● Increase motor power density

Increasing the power density of a motor is one way to improve its performance, which can be achieved by increasing the speed at which the motor can run. This also requires a trade-off between power density and speed to ensure the reliability and performance of the motor.

High-speed motor design is an effective way to increase power density, which usually includes improved air gap radius, higher material density and efficient cooling methods, which can achieve higher power density, less material consumption and higher efficiency.

● Cooling concept of the motor

Different motor cooling concepts, including direct stator cooling, direct rotor cooling and stator stack cooling.

1) The motor stator housing cooling solution has the following characteristics:

Low additional workload

No contact between coolant, windings and insulation

Will not affect the magnetic properties of the motor

There is a large thermal resistance between the heat source (winding) and the cooling medium

Additional radial space required

Sealing and manufacturing are more complicated

2) The motor stator laminate cooling solution has the following characteristics:

Low thermal resistance

No contact between windings, insulation and cooling medium

May affect the magnetic properties of the motor

There may be leakage between the steel sheets

Possible loss of torque support surface (with press fit)

3) The direct stator winding cooling solution includes the following features:

It can dissipate heat directly to the heat source, cooling the winding very effectively, providing the highest cooling potential for the winding

Contact between windings, insulation and coolant

Possible impact on insulation system

There may be leakage between the steel sheets

These different stator cooling options have their own advantages and disadvantages, and choosing the right one is a trade-off based on the specific application and performance requirements.

4) The rotor shaft cooling solution includes the following features:

When a hollow shaft is used, the additional workload is low and the design and implementation are easy.

There is a long heat transfer path between the heat source (magnets, rotor bars or field windings) and the cooling medium.

5) The rotor laminate cooling scheme has the following characteristics:

Closer to the magnet excitation source.

No contact with magnets, use the rotor hole.

There may be leakage between the steel sheets.

Cooling channels are provided in the magnetic flux area.

The supply of coolant in the different channels may not be uniform and may lead to imbalances (e.g. due to pressure differences).

6) The direct rotor excitation cooling solution includes the following features:

It can quickly dissipate heat near the heat source and has a high thermal dynamic response capability.

Challenging mechanical design.

Possible loss of magnetically active material.

The routing of the coolant is more complicated.

Each of these different rotor cooling options has its own advantages and limitations, and selecting the right option for a specific application requires comprehensive consideration.

● Motor thermal modeling method

To address the thermal management of electric motors, researchers used a variety of thermal modeling methods, including electromagnetic finite element analysis and computational fluid dynamics simulation. The combination of these methods can more accurately assess the thermal characteristics of electric motors and help optimize cooling solutions.

● The impact of cooling on motor performance

Different cooling methods have different effects on motor performance, including temperature rise rate and power density. A well-designed cooling method can significantly improve the continuous torque of the motor, thereby increasing the performance of electric vehicles under high load conditions.

● Conclusion and Outlook

Motor thermal management is one of the key factors in the development of electric vehicle technology, with new challenges in manufacturing the next generation of electric drive units, the selection and development of coolants, and the optimization of the entire thermal system through model predictive control to improve the efficiency and performance of electric vehicles.

Motor thermal management issues will continue to drive the direction of electric vehicle technology development. Through more efficient motor design and innovative thermal management methods, we can expect to see more competitive electric vehicles in the future that will offer longer driving range and superior performance.