Motor overheats

1. Symptoms

• The motor case is too hot to hold, or the driver reports over-temperature or thermal limiting.
• Performance degrades after some minutes at load.
• The motor or driver shuts down after sustained operation.

2. Possible causes

• Undersized motor or ratio: continuous torque required exceeds the rated torque.
• Mechanical drag: bearings, belts, misalignment, binding gearbox, brakes dragging.
• Higher load than assumed: payload, slope, floor friction.
• Driver or controller settings: poorly tuned loop causing oscillation, high current limit holding stall current, wrong commutation or phase timing.
• Duty cycle: many accelerations and stops, or holding a position against load.
• Ambient and cooling: enclosed, hot environment.

3. Measurements to collect

• Motor current (phase or DC bus) under the real duty cycle, logged.
• Case temperature over time (thermocouple or infrared thermometer), and ambient.
• The torque the load needs (see the motor sizing guide) and the torque implied by the current and the torque constant.
• Free-running (unloaded) current of the motor and of the motor with gearbox.
• Whether the controller oscillates (current ripple at the loop frequency).

4. Diagnostic commands

Commands are not executed by the site’s automated tests (they need a ROS 2 Jazzy installation or the robot).

No ROS commands are needed; measure with a clamp meter or the driver’s own telemetry, plus a thermometer. If the driver publishes motor current over ROS, plot it with the encoder velocity and the commanded velocity.

5. Step-by-step investigation

1. Compute the expected continuous current. From the load (motor sizing guide) get the shaft torque, divide by the motor’s torque constant from its datasheet.
2. Measure the real current over the actual duty cycle and compare. Much higher than expected points to drag or an underestimated load.
3. Measure free-running current with the load disconnected, then with the gearbox, then with the wheel on the ground. A large step at one stage locates the drag.
4. Check the controller: current ripple at the loop frequency or audible hum means oscillation; tune the loop.
5. Check the duty cycle: holding torque against gravity or a wall draws full current with no motion.
6. Check the thermal path: mounting to a heat sink, airflow, enclosure.

6. Corrective actions

• Re-size the motor or change the gear ratio so continuous torque is below the rated torque with margin.
• Remove mechanical drag: realign, lubricate, replace bearings or belts.
• Lower acceleration or avoid holding against load; add a brake or a self-locking drive if holding is required.
• Tune the control loop and set sensible current limits.
• Improve cooling or reduce ambient temperature.

7. Validation after the fix

• After a full worst-case duty cycle the case temperature stabilises below the datasheet limit with margin.
• Measured steady current matches the prediction within about 20 %.
• No thermal limiting or shutdown over a test at least as long as the longest mission.
• The result is repeatable on a second unit.

References

• Datasheet fields: rated torque, torque constant, thermal resistance, maximum winding temperature and maximum continuous current of your motor.

• Tune a PID Controller Labs · Lab