Why do electric motors overheat, and how hot can they get?
The power of an electric motor is initially limited by its temperature. A motor can handle a large current for a few seconds, but then it must decrease the current to avoid damage. Understanding where the heat comes from and what it threatens is crucial for tuning a machine, choosing a motor, and interpreting a loss of power while riding.
The essentials
- The primary source of heat comes from the current flowing through the windings; this heat increases with the square of the current.
- In addition to the losses at high speed, there are also losses in the iron core, which increase with frequency.
- Two components are particularly susceptible to heat damage: the wire insulation and the magnets.
- A temperature sensor in the motor allows the controller to reduce power output before any damage occurs.
Copper losses
The windings have a small electrical resistance. The current flowing through them dissipates heat, which is proportional to the square of the current: doubling the current multiplies this heat by four. These are the copper losses, and they become significant when a large amount of torque is required: starting, climbing hills, and accelerating.
The phenomenon is self-perpetuating: the resistance of copper increases by approximately 0.4% per degree. Therefore, a hot motor heats up even faster, at the same current.
That's also why the choice of winding matters: a high-speed motor, which produces less torque per ampere, requires more current for the same effort. See our guide on Kv, Kt, and motor poles.
Iron losses
The stator is made of thin sheets of magnetic steel. With each electrical cycle, the magnetic field reverses, which produces two types of losses:
- Hysteresis: A small amount of energy is lost with each reversal of the magnetic field within the metal;
- Eddy currents: These are induced currents that circulate within the metal sheets and cause them to heat up. The use of laminated sheets, separated by insulation, is intended to limit these currents.
These losses increase with the electrical frequency, and therefore with the speed and the number of poles. They are particularly significant at high speeds, and they increase further with field weakening.
What the heat threatens
The winding insulation
The copper wire is coated with an insulating varnish, designed for a specific maximum temperature based on its insulation class. Beyond that temperature, the varnish deteriorates quickly, then cracks: the coils come into contact, the motor loses efficiency, overheats even more, and eventually burns out.
Magnets
Neodymium magnets lose some of their strength when they heat up: the motor produces less torque per ampere, which can be reversed by cooling. However, beyond a temperature that depends on their grade, a portion of the magnetization is permanently lost: this is called demagnetization. The motor then becomes permanently weaker. The grades of magnets are indicated by letters (e.g., H, SH, UH), which indicate their temperature resistance.
The risk is highest when a high temperature combines with a strong current flowing in the opposite direction to the magnets, such as during aggressive regenerative braking.
How does a hub motor dissipate heat?
A scooter hub motor is a sealed unit. The heat from the windings, located in the center, must pass through the internal air before reaching the rim, and then the external air; some of it also travels through the axle to the scooter's frame. This path is long, which explains why a hub motor quickly heats up during repeated periods of high power output.
Several solutions can improve this process:
- allowing for openings and airflow, such as our motor ventilation system;
- a special liquid (ferrofluid) placed in the air gap, which conducts heat more effectively than air;
- A larger motor that produces the same torque with less current and offers a greater surface area for heat dissipation.
Monitor the temperature
Many electric scooter motors incorporate a temperature sensor within the windings. Connected to the controller, this sensor allows the controller to gradually reduce the current before it reaches a dangerous level. This is what causes the feeling of power loss after a long uphill climb: it's a protective measure, not a malfunction.
Workshop tip
If your motor has a temperature sensor, connect it and set the thresholds in the controller. This is the best way to prevent the motor windings from overheating or the magnets from weakening, especially when riding on a track.
Frequently Asked Questions
At what temperature does a scooter motor become dangerous?
This depends on the wire's insulation class and the magnet grade, which are specific to each motor. The reliable value is provided by the manufacturer; if not available, it's best to set conservative thresholds in the controller and monitor the temperature during the initial rides.
My motor has been less powerful since it overheated. Why is this happening?
The magnets may have partially demagnetized, or the insulation of the windings may have been damaged. A new motor detection on an adjustable controller will show if the magnetic flux has decreased.
Why does the motor get hotter when going uphill compared to maintaining a constant speed?
When climbing hills, the motor provides a lot of torque, which requires a significant amount of current. Consequently, copper losses increase with the square of the current. At a constant speed on a flat surface, the current is much lower.