Motor Thermal Derating / Overheat Protection

A motor heats up with use, so the driver limits current by temperature and shuts down if it gets too hot.

As a motor runs, current flowing through its windings generates heat (copper loss, proportional to the square of the current), and the core loses some energy as heat too, so temperature keeps climbing. Too much heat can burn the winding insulation or demagnetize the permanent magnets, so a driver typically uses a thermistor to monitor winding and power-transistor temperature: as it nears the limit, the driver derates itself, actively lowering the maximum current and torque it allows; past a threshold, it faults and shuts down. This is also why motors have separate peak and rated torque figures — peak torque can only be held for a few seconds, while sustained operation is limited to the rated value. For humanoid and quadruped robots, standing for a long time, holding a half-squat, or carrying a heavy load puts sustained high torque on the knee and hip joints, which is the most common trigger for overheating; common fixes include liquid-cooled joints, added heat sinks, and penalizing joint torque in reinforcement-learning reward functions.

ExampleWhen a humanoid robot holds a half-squat while carrying a box for a long time, its knee motors heat up, the driver automatically lowers the current limit, and the robot's legs weaken and its motion slows — in severe cases it shuts down into protection mode.

Also called

Thermal Derating, Overtemperature Protection

Related

Peak Torque · Rated Torque · Liquid-Cooled Joint Actuators (Active Thermal Management) · Torque Limiting (Saturation) · Servo Drive (Motor Driver) · Torque-Speed Curve

Sources

moteus r4.11 - mjbots（连续电流与散热条件对照） (Chinese)