EV Motor Cooling Systems

Permanent Magnet Synchronous Motor (PMSM) and the Externally Excited Synchronous Motor (EESM). 

The Importance of Thermal Management

Effective thermal management is paramount in electric vehicle (EV) design, especially when it comes to traction motors. These motors convert electrical energy into mechanical motion and are critical for the vehicle's overall performance and efficiency. 

Managing their heat is essential, not only to maintain optimal functionality but also to extend the vehicle’s range and life. One practical scenario where cooling plays a critical role is during high-speed or uphill driving, where motors generate significant heat. Without effective cooling strategies like water jacket and oil spray cooling, the motors could overheat, leading to decreased efficiency or potential failures.

More than a basic heat sink

C/M HOLD PREFERENCE ON NON-OIL COOLING

This aligns with our ability to use less parts & less maintenance improving reliability & affordability in equivlance or better

Non-oil EV motor cooling methods rely on circulating liquids (water-glycol), forced air, or advanced heat transfer materials to dissipate heat from the electric motor and power electronics, avoiding the use of dielectric oil. These methods are designed to maintain motor performance and longevity by keeping internal temperatures within safe operating limits. 

Key Non-Oil Cooling Technologies

• Water-Glycol Jacket Cooling: The most common non-oil method, where a water-ethylene glycol (WEG) mixture circulates through a jacket around the stator. This liquid cooling approach is efficient and can be linked to the same cooling loop as the battery, reducing overall system complexity.

• Forced Air Cooling: Used primarily in smaller motors or older electric vehicles (e.g., first-generation Nissan Leaf, VW e-Golf), this method uses fans or air vents to move air directly over the motor surface or internal components to remove heat.

• Advanced Water-Based Rotor/Stator Cooling (CoolShaft/CoolDisc): This technology enables direct contact with the heat source (rotor and windings) using water-based, sealed systems rather than oil. These systems provide a leak-proof, compact, and high-performance alternative to traditional oil cooling, enhancing continuous power output.

• Nanofluids and Advanced Coolants: Emerging technology involves using high-performance coolants enhanced with nanoparticles (such as graphene or alumina), which significantly increase thermal conductivity over standard coolants.

• Refrigerant Cooling: A specialized method, such as that used in the BMW i3, which uses air conditioning refrigerant for direct, high-capacity cooling of components. 

Pros and Cons of Non-Oil Cooling

• Pros:

• Cost-Efficient: Water-based systems are generally lower cost.

• Integration: Easier to integrate into a single thermal management loop for the entire vehicle (battery + motor + power electronics).

• Efficiency: Modern direct water-cooling methods (like CoolShaft) offer superior performance for high-speed rotors, rivaling oil cooling.

• Cons:

• Less Effective at High Loads: Simple water jackets are sometimes less effective than direct oil spraying at cooling the hottest parts of the rotor in high-load scenarios.

• Air-Cooling Limitations: Air cooling is noisy and less effective in hot climates or high-performance, high-power vehicles. 

Common Applications

• Water-Glycol: Dominant in modern mainstream EVs such as Tesla, Hyundai Ioniq 5/6, Kia EV6, Ford Mustang Mach‑E, and VW ID.4.

• Air-Cooling: Used in some early or lower-cost EVs. 

Sound based accoustics as an additive at C/M

https://notesatcm.blogspot.com/2026/04/thermoacoustic-refrigeration-cooling-is.html

1. Working Principle
PMSM (Permanent Magnet Synchronous Motor)

• Uses permanent magnets (usually rare-earth magnets) on the rotor to produce a constant magnetic field.

• The stator creates a rotating magnetic field (via 3-phase AC), and the rotor locks into synchronisation with it.

• Since the magnets are always “on,” no external excitation is needed.
EESM (Externally Excited Synchronous Motor)

• Instead of magnets, it uses an electromagnet in the rotor, powered by a separate DC excitation current, typically supplied through slip rings or brushless excitation systems.

• This allows dynamic control of rotor magnetic flux, which is not possible with fixed magnets in PMSM.

• But automakers are exploring alternatives to reduce rare-earth reliance.

• Innovations in magnet cooling, hybrid motors (PM + WRSM), and non-rare-earth magnets may keep PMSM viable.
EESM:

• Gaining traction for next-gen EVs, especially in Europe and China.

• More automakers (BMW, Renault) are investing in EESM for cost and sustainability.

• Improved control algorithms and digital excitation systems will drive wider adoption.

Reference. External "Le"

https://thereviewstories.com/eesm-vs-pmsm/

OIL-COOLED SYSTEMS 

Replacing the oil in an oil-cooled electric vehicle (EV) motor is a specialized maintenance procedure, often necessitated by high-performance driving or specific manufacturer recommendations, even though many manufacturers label the fluid as "lifetime". The process involves draining and refilling the motor with a specialized dielectric fluid, such as automatic transmission fluid (ATF), designed to cool the windings and lubricate gears without causing short circuits. 

Why and When to Replace the Oil

• Contamination and Degradation: While not exposed to combustion residues, motor oil still breaks down over time due to heat. It also collects metal particulate matter from bearings and gears that can degrade performance.

• High Performance/Tracking: Frequent or hard driving, especially tracking, can cause the oil to degrade rapidly, requiring changes every 15 track hours according to some manufacturer guidance.

• Warranty and Longevity: Some manufacturers (e.g., Hyundai) may not officially require a change, but reports of dirty fluid ("wheel of misfortune") have led some owners to change it to avoid reduction gear damage.

• Intervals: Some hybrid drives suggest changing the unit fluid every 72,000 km. 

The Replacement Procedure

• Safety First: Ensure the EV is in "Service Mode" and the high-voltage system is shut down to prevent electrocution or damage.

• Raise the Vehicle: Use a lift or jack stands to access the underside of the motor.

• Drain the Fluid: Locate the drain plug on the motor housing. It is often recommended to drain the oil while it is warm to ensure impurities are removed and the oil flows faster.

• Replace Filter (If Applicable): Some systems have an internal filter screen that may not be serviceable without taking the motor apart, while others may have external filters.

• Refill with Correct Fluid: Use only the recommended manufacturer-approved fluid (e.g., synthetic ATF).

• Measure Volume: The most common method is to measure the amount of oil removed and replace it with the exact same amount. 

Common Oil-Cooled EV Motor Systems 

• Stator Oiling: Oil is sprayed directly onto the stator windings and core to remove heat, often reducing temperatures far better than air cooling.

• Rotor Flinging: Oil is pumped through a hollow shaft into the rotor, where it is flung outward by centrifugal force to cool the magnets.

• Drive Unit Integration: Many EVs, such as the Chevrolet Bolt and various Tesla models, use the same oil for both motor cooling and the gearbox/reduction gear lubrication. 

Safety and Technical Tips

• Check for Leaks: Ensure all O-rings and gaskets are in good condition when resealing.

• Do Not Overfill: Overfilling the motor with oil can create excessive "churning" or "splashing," which introduces parasitic drag, reduces efficiency, and can increase internal temperatures.

• Consult the Manual: Always check the specific EV's factory service manual for the correct oil type, capacity, and interval. 

Reference. External 

https://blog.ozeninc.com/resources/advanced-cooling-techniques-for-ev-traction-motors?hs_amp=true

OIL-COOLING 

Just like internal combustion engines, electric motors generate a considerable amount of heat during operation, even if in a much lower quantity. In electric vehicle motors, electromagnetic losses result in the generation and dissipation of energy in the form of heat. These losses are primarily caused by resistance encountered by the flow of electric current through conductive components and variable magnetic fluxes through magnetic components in the motor.

To improve the efficiency, longevity and performance of EV motors, it is essential to reduce these losses. As such, we are examining the optimal solution for oil-cooled motors that enables delivery of the same power output as larger units, but in a smaller, lighter, more-affordable package.

To avoid causing material degradation or demagnetisation, EV motors should be kept at less than 180° Centigrade, allowing the various components to operate at their optimal temperatures. With the highest temperature appearing in the winding copper core shrouded by an insulation layer which leads to poor heat dissipation, innovative cooling directs the fluid exactly where it’s required.

When looking at approaches to direct motor cooling, there are three methods that we’re currently researching and developing, all of which provide enhanced thermal management to improve efficiency and performance.
The first method is manifold dripping cooling, which precisely and uniformly directs cooling fluid on to the motor winding heads. This method requires less cooling fluid – rather than ‘bathing’ the rotor – and therefore removes parasitic losses due to drag, however does require a higher-pressure oil pump.
The second method - shaft centrifugal cooling - offers the same cooling capacity as other solutions, with significantly reduced oil flow rate. Cooling using the rotor-shaft, the centrifugal force sprays oil to the head windings, which has some limitations at lower speeds, but does provide uniform distribution at higher speeds.

And lastly, specific oil channels through rotor and stator, which would allow the fluid to improve the heat dissipation close to the source of the heat.

Using an electrically driven pump to move the oil allows control for maximum efficiency. The pump can be a stand-alone unit or, depending on application, shared with the vehicle’s gearbox.

Reducing losses through advanced thermal management technologies such as these is vital to the rollout and uptake of next generation electric vehicles. Although under continuous development, internally cooled electric motors are already hitting the market, starting in large EVs, before being democratised through mid-sized EVs and, eventually, all EVs in the medium term.

As a development partner to most global OEMs, our ambition is to drive forward these developments in oil-cooled motors, providing reductions in both weight and cost, while offering improvements in efficiency and performance.

The project has been part-funded by Basque country local grants with development taking place at our GKN Driveline Zumaia plant, where GKN Automotive’s eMotor development is concentrated. As we continue to test and refine the direct oil cooling feature, we are able to bring to market the next generation of eMotors which are even more efficient and robust than before.

Reference. External 

https://www.theengineer.co.uk/content/opinion/comment-why-oil-cooled-motors-are-key-to-next-gen-ev-propulsion

EVEN WITH PLANT BASED OILS

Oil-Cooling is not perpetual & acts like Gasoline or Diesel you need to refill like coolant for a combustion air-condition & heat climate control system

C/M looks to perpetual systems & low maintenance in all areas rather than equivalent transitional replacement yet a respectful tiered maintenance schedule focused on zero emissions, zero cycle or close to with balancing exceeding or equal in emissions meeting Net zero

Oil cooling for Electric Vehicle (EV) motors is a, highly efficient thermal management technology designed to handle high continuous power densities, but it is not a perpetual system. While it enables superior, long-term, and consistent performance compared to traditional water-jacket cooling, it operates within the laws of thermodynamics—requiring energy for pumps and a heat exchanger to dissipate heat to the environment, making it a continuous, not "perpetual," solution. 

Key Aspects of Oil-Cooled EV Motors:

• Higher Efficiency and Performance: Direct oil cooling allows the coolant to come into direct contact with heat-generating components like stator windings and the rotor. This method can remove nine times more losses than air-cooling, allowing for a three-fold increase in motor power.

• Continuous Power Management: Oil cooling excels at keeping motor components below critical temperatures, facilitating higher continuous and peak power, which is ideal for high-speed motors and electric trucks.

• Types of Oil Cooling:

• Direct Oil Cooling: Involves spraying oil directly onto the stator windings and rotor, allowing it to penetrate hot spots.

• Indirect Oil Cooling: Circulates oil through channels within the housing to absorb heat without direct contact with windings.

• System Components: The system typically uses automatic oil pumps, specialized heat exchangers, and dielectric oil (such as ATF or Novec oil) that can efficiently transfer heat while providing electrical insulation.

• Performance Benefits: Using oil-cooled systems, such as spray cooling on the end windings, reduces winding temperatures significantly more than water jackets, providing superior heat transfer (20.3% improved performance). 

Why it is not "Perpetual":

The system requires an active, powered, closed-loop system—including pumps for circulation and a heat exchanger (radiator) to transfer the absorbed heat to the outside air. This requires electrical energy from the battery, and the oil itself can degrade over time, requiring maintenance

https://www.emobility-engineering.com/thermal-management-in-electric-vehicles-e-motor-cooling-technology/

26. K.T-CIG