Electric vehicle (EV) traction motors must withstand sustained high power and high-speed operation. Keeping them properly cooled is essential to EV powertrain design and implementation. Several options exist for cooling EV traction motors depending on the overall design goals for a specific EV.
This article begins with a short review of the distribution of thermal losses in a typical EV traction motor and typical temperature gradients between the internal motor structures and the ambient air when passive air cooling is used. It then compares water and oil cooling techniques and closes by presenting an advanced two-phase cooling system proposed for EV traction motors.
EV traction motors are around 90% efficient. That’s good, but it also means that a 70-kW motor dissipates about 7 kW of heat. The distribution of heat dissipation in an EV traction is highly variable and dependent on the motor design and packaging.
The stator usually accounts for 75 to 85% of total thermal dissipation. The distribution of the heat in the stator varies with 40% to 65% of the dissipation in the windings and 25 to 35% in the iron. Heat losses in the stator are a primary factor driving cooling system design.
The second factor relates to the thermal gradients and transfer of heat from the inside of the stator to the external environment. For example, in an air-cooled design, there can be a temperature drop between 80° to 140° C with several critical thermal interfaces and temperature gradients (Figure 1).
Figure 1. The typical thermal interfaces and thermal gradients in an air-cooled EV traction motor. (Image: MDPI energies)
- A is the hottest spot in the center of the stator windings.
- AB is an area of high heat transfer from the center of the stator windings to the outer windings.
- B is an area of low heat transfer due to insulation between the stator and core laminations.
- BC is a second area of high heat transfer through the stator’s laminations (light blue).
- C is a second low- heat transfer area between the stator core and frame.
- CD is the drop in temperature due to conduction through the frame’s thickness.
- D is the temperature of the surface of the housing (dark blue on right).
- DF is the temperature drop due to convective heat transfer to the ambient air.
- F is the ambient temperature.
Water or oil?
While air-cooling is an option for smaller traction motors, water jackets, and oil spraying are the main ways larger EV traction motors are cooled. The traction motor cooling system must be optimized to meet the specific needs of the overall EV design.
An effective motor cooling system can simplify the cooling of the battery pack and power electronics, effectively supporting the heating ventilation and air conditioning (HVAC) system.
Water jacket cooling is the simplest to implement. It consists of an integrated water-filled casing around the stator. The water flows through a radiator to dissipate excess heat and maintain the proper motor temperature.
Oil cooling is more complex but can have advantages in high-performance systems. Oil can be sprayed under pressure or dripped onto the stator. A sump collects the oil, which is passed through a radiator for cooling. In some advanced designs, oil flows through the rotor for additional cooling (Figure 2).
Figure 2. A Honda racing EV motor using combined stator and rotor oil cooling. (Image: Racecar Engineering)
Oil cooling has the advantage of improved heat transfer due to the direct contact between the oil and the components that require cooling. The same fluid can be circulated to the transmission for cooling that system.
In addition to being used with conventional traction motors, oil spray cooling has been applied to in-wheel motors in EVs.
Two-phase cooling
A two-phase evaporative cooling system has been demonstrated, which includes co-packaging of the traction motor and high-efficiency wide bandgap (WBG) motor drive electronics (Figure 3).
Figure 3. A closed-loop, wick-assisted evaporative system for cooling stator windings (left). The motor cross-section shows the windings and a possible configuration of the heat exchanger (right). (Image: Georgia Tech)
The system uses wicks with micropillar patterns on a high-performance silicone polymer substrate that enables evaporative cooling of the stator. The system combines capillary action through the wicks with active mechanical pumping. The combined pumping and wicking action ensures the system achieves high heat transfer efficiency. After removing heat from the system, the coolant is condensed within the heat exchanger and then pumped back into the cooling structure.
Air cooling is the simplest way to implement traction motor thermal management, but it’s also the least capable technology. Water cooling is better, and various forms of oil cooling are used for today’s high-performance EV traction motors. Even higher thermal performance may be possible using a proposed wick-assisted evaporative two-phase cooling technology.
References
- Honda’s secrete F1 hybrid, Racecar Engineering
- Keeping it Cool: Thermal Management Solutions for Electric Traction Motors, Ansys
- Recent Developments in Cooling Systems and Cooling Management for Electric Motors, MDPI energies
- Two-Phase Thermal Management System for Integrated Motor Cooling, Georgia Tech
Images
- Figure 1, MDPI energies, Page 5, Figure 2
- Figure 2, Racecar Engineering, two-thirds down the page
- Figure 3, Georgia Tech, bottom of page
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Filed Under: Electric Motor, FAQs