Liquid cooling for a motor and controller: when it becomes necessary

A motor and controller are rarely limited by their electrical capacity, but rather by their temperature. As long as the air is sufficient to dissipate heat, liquid cooling only adds weight and complexity. It becomes necessary when the machine needs to sustain high power output for extended periods, in a confined space, or without adequate airflow: for example, in endurance applications, with heavy and slow vehicles, or when the motor is enclosed within a body. This guide explains how liquid cooling works and when it becomes worthwhile.

Why does heat limit power?

A motor dissipates heat in its windings (copper losses, which increase with the square of the current) and in its iron core (losses that increase with speed). A controller heats up in its transistors. Our guide on electric motor overheating details these losses.

What matters is the balance between the heat generated and the heat dissipated. During a short acceleration, the components absorb heat within their mass; you can demand significantly more power than the "continuous" power rating. Over time, the temperature rises until the dissipation balances the production. If this equilibrium exceeds the temperature limits of the winding insulation, the magnets, or the transistors, it's necessary to reduce the power or improve the cooling.

That's why more efficient cooling primarily increases the power that can be sustained for a longer period. Peak power, on the other hand, depends more on the current that the controller and the battery can deliver.

How does liquid cooling work?

The principle is similar to that of a car: a liquid circulates in a closed loop, propelled by a pump. It passes through the component to be cooled, where it heats up, and then flows into a radiator where air cools it before returning to the circuit.

• Regarding the motor: We encase the stator in a jacket, a double-walled housing through which liquid circulates close to the windings. This requires a motor where the stator is fixed to the housing, like a motor with a shaft output; on a motor-wheel, where the outer casing rotates, this is much more difficult.
• Regarding the controller: The transistors are mounted on a cold plate, which is a metal plate with channels through which liquid flows.
• The coolant: Typically, it's a mixture of water with antifreeze and anti-corrosion additives. It must be compatible with the metals used in the system.
• The radiator: Positioned where airflow is present, potentially with a fan for low-speed operation.

The main advantage is the ability to dissipate heat over a greater distance. A motor enclosed in a casing or a controller housed in a waterproof enclosure can thus be cooled by a radiator placed at the front of the machine, in the airflow.

Air, the first resource to be depleted

Before moving on to the liquid, we need to ensure that the air is being used correctly:

• The controller mounted on metal: When attached to an aluminum frame or heat sink using thermal paste or a thermal pad, it maintains its performance much better than when enclosed in plastic.
• A motor that breathes: On our MHS, MHT, XHS, and XHT hub motors, our 6-blade motor fan is mounted on the motor shaft near the cables, forcing air through the motor. A motor with this type of ventilation is no longer waterproof; it's a solution for dry conditions and track use. See our guide on how to ventilate an electric scooter motor.
• Liquid in the air gap: In a motor-wheel, a ferrofluid can help transfer heat from the stator to the bell. See our guide on ferrofluid.
• Reasonable settings: Appropriate current limits, temperature sensors connected, and thresholds configured within the controller.

When coolant is needed

Here are some situations where air alone is no longer sufficient:

• Sustained high power: Endurance, climbing steep inclines with a heavy load, continuous operation for construction or material handling equipment.
• A slow and heavy machine: At low speeds, there is little relative wind for cooling, while the effort required is high.
• Enclosed components: The motor is housed in a waterproof casing, and the electronics are protected in a water and dust-resistant enclosure.
• Weight constraint on the motor: A liquid-cooled motor can deliver the same continuous power with less mass than an air-cooled motor, which partially offsets the weight of the cooling system.

The pitfalls

• The pump is a critical failure point: if it stops, the temperature rises very quickly. It's necessary to have a temperature sensor on the component itself, not just on the liquid, and an automatic power reduction feature.
• Leaks near the high-voltage area: The circuit runs close to the electronics; fittings and clamps must be carefully installed and checked.
• Air in the circuit: An air bubble trapped in the cold plate can create a hot spot. The circuit is purged during the filling process.
• Freezing: A liquid without antifreeze can freeze in the winter and crack a component.

On a machine to be designed

The choice of cooling system is made from the beginning, along with the motor and controller: it determines their size, weight, and placement. When we design a controller specifically for a machine, cooling is part of what we adapt, along with the format, connectors, and sealing. Describe your project to our engineering department: the solution is designed to fit the machine and its intended use.