Temperature Limits of E-Motor Cooling Fluids
Why oil-cooled and immersion-cooled e-motors age their fluid at high temperatures – and how oxidation, varnish and polymerisation can be controlled.
Bulk temperature is not the whole story
In an immersion-cooled or oil-cooled motor, the bulk fluid in the sump or heat exchanger may be well below the temperatures that the fluid sees at the windings, in the slot area or at the bearings. These hot spots, and thin fluid films on hot surfaces, decide how fast the fluid ages. A fluid can look fine in the sump while deposits are already forming on the hottest components.
The flash point is a fire-safety property, not an operating limit. A fluid with a flash point above 250 °C is not automatically suitable for continuous operation at 200 °C.
How fluids age: oxidation, varnish and polymerisation
At high temperature, and especially in contact with air, the fluid reacts with oxygen. Radicals and hydroperoxides form, followed by acids and higher-molecular products. These products can polymerise into sludge and varnish – sticky or hard deposits on windings, bearings and heat exchangers. As a rule of thumb, the reaction rate roughly doubles for every 10 K increase in temperature.
Esters bring a second mechanism: in the presence of water, they can hydrolyse into acids and alcohols. Low water content and closed systems keep this under control.
What slows ageing down
- Base fluid: saturated polyol esters are thermally very stable
- Antioxidant additives: they interrupt the radical chain and delay oxidation and polymerisation
- Limited air contact: sealed systems or nitrogen blanketing reduce the oxygen supply
- Circulation: renewed films on hot surfaces age far less than stagnant films
- Dry fluid: low water content limits hydrolysis
How oxidation stability is tested
| Test | What it shows |
|---|---|
| PDSC oxidation induction time (ASTM D6186) | Resistance to oxidation at high temperature under oxygen pressure |
| RPVOT (ASTM D2272) | Oxidation reserve of the fluid and its antioxidants |
| Thin-film and deposit tests | Tendency to form varnish on hot surfaces |
| Acid value and viscosity trend | Ageing in service or in long-term tests |
Monitoring in service
Acid value, water content, breakdown voltage and viscosity are the key indicators. For IsoTec EC 68 we recommend checking them every 1,000 operating hours or at least once a year.
IsoTec fluids for e-motors
| IsoTec EC 68 | IsoTec EC 68 HT | |
|---|---|---|
| Continuous bulk, sealed system | Up to 120 °C | Higher thermal loads – values on request |
| Local hot spots (circulated) | Up to 150 °C | Values on request |
| Short-term peaks | Up to 180 °C | Values on request |
| Additives | No aggressive EP/AW additives | Additive package against oxidation and polymerisation |
IsoTec EC 68 · IsoTec EC 68 HT · Immersion cooling fluid selection guide
Frequently asked questions
Is the flash point the maximum operating temperature?
No. The flash point describes fire safety. The permissible operating temperature depends on oxidation and thermal stability, air contact and circulation.
What is varnish?
Varnish is a thin, sticky or hard deposit formed when oxidation products polymerise. It impairs heat transfer and can affect bearings and insulation.
How can I extend fluid life at high temperatures?
Use a sealed or nitrogen-blanketed system, keep the fluid circulating over hot surfaces, keep water content low and choose a fluid with suitable antioxidant protection.
Pushing your drive to higher temperatures?
We help select and test the fluid for your thermal profile. IsoTec products are supplied to customers outside Germany.