Ferrofluid in an electric scooter motor: Cooling via the air gap, advantages and limitations

Ferrofluid is a magnetic liquid that is applied to the air gap of a motor, the narrow space between the magnets and the stator. Held in place by the magnets, it replaces the air in that space and allows heat from the windings to transfer to the motor casing much more effectively. This is a way to cool a motor without drilling holes in it. However, it has its limitations and is not suitable for all machines. Here's how it works and when to consider using it.

The problem: a layer of air trapped between two hot components

In a hub motor, heat is primarily generated in the stator windings, located in the center. To escape, it must pass through the air gap, then the bell and the rim, before reaching the outside air. However, air is a poor conductor of heat: this thin layer, even if very thin, acts as an insulator. As a result, heat accumulates in the stator, causing the motor's temperature to rise during repeated periods of high load.

The origins of this heat, which comes from losses in the copper and iron, are detailed in our guide explaining why an electric motor gets hot. Here, we'll focus on the path this heat takes as it exits.

What is a ferrofluid?

A ferrofluid is a liquid, typically an oil, in which extremely fine magnetic particles, on the nanometer scale, are suspended. These particles are coated to prevent them from clumping together and remain in suspension. At rest, the liquid appears black and oily; when brought near a magnet, it is attracted and concentrates where the magnetic field is strongest.

These fluids were initially used in industry, for example, to create airtight seals around rotating shafts or to cool certain speakers. The bicycle and electric scooter world has adopted them for use in wheel motors.

How does it cool the motor?

In a hub motor, the area with the strongest magnetic field is precisely the air gap, located between the magnets and the stator teeth. A small amount of ferrofluid placed inside the motor will naturally distribute itself to this area and remain there, even when the wheel is rotating.

The liquid then creates a bridge between the stator and the magnets glued to the bell. The heat from the windings passes through this bridge much more easily than the original layer of air, and then goes into the bell, which is in contact with the outside air and the rim. The motor doesn't produce less heat, but it dissipates it more effectively: its temperature rises less quickly during exertion, and peak temperatures are reduced.

Important point: the ferrofluid only transfers heat to the bell. If the bell itself is already hot, for example, after a long ride with no wind or inside a closed fairing, the benefit is reduced.

Limitations and drawbacks

A slight friction at high speed

The liquid is located between a stationary part and a rotating part. It therefore creates a viscous friction, similar to a thick oil. This friction increases with the rotational speed: it is negligible at low speeds and becomes more significant at high speeds, where it consumes a small portion of the power.

A fluid that degrades over time

With heat and time, the carrier fluid can evaporate or degrade, causing the fluid to lose its properties. In such cases, it's necessary to add more fluid, or to clean and refill the system. The lifespan depends on the product and its usage; only the manufacturer can provide this information.

Cleanliness

Opening a motor that contains ferrofluid is messy: the black liquid spreads over the magnets, the stator, and your hands. Any metal particles or dust that enter the motor become trapped in the fluid. This is something to consider if you frequently work on your motors.

The right amount

Insufficient fluid can create air pockets within the air gap, while excessive fluid increases friction and can eventually leak out of the air gap. The correct amount depends on the motor and the specific product; it's essential to follow the fluid manufacturer's recommendations for the motor size in question. Furthermore, the motor must be clean and dry before filling.

Ferrofluid or ventilation?

Both solutions aim for the same goal through different approaches. Ventilation, like our PETG six-blade motor ventilation system, opens the motor to allow air to flow directly over the windings. Ferrofluid keeps the motor sealed and improves heat conduction to the bell.

We do not sell ferrofluid. For our MHS, MHT, XHS, and XHT motors, the solution we offer is ventilation, for use in dry conditions and on tracks. For everyday use in all weather conditions, a larger motor, which generates less heat at the same power output, is often a simpler solution: see our guide on motor sizes and the one on motor ventilation. All of our motors are presented in the electric scooter motors category.