The motor driver is overheating, what are the possible causes?
The first cause of heating is current; the loss grows with its square. The second group is the mounting: an airless box, a loose terminal, a thin cross-section.
The first cause of heating is current: because the loss depends on the square of the current through P = I² x RDS(on), doubling the current makes the heat four times larger. The second group of causes comes from the mounting — no air in a closed box, a loose terminal, an inadequate cable cross-section and a high ambient temperature. Follow the order that way: current first, then mounting, then components.
Checklist
- Measure the real current. If you are approaching the driver's continuous current rating, heating is to be expected. The KS250 is defined for 15 A and the PT500 for 30 A continuous current; values above the continuous rating are only for a limited time (45 A peak for 5 s on the KS250, 40 A maximum for 15 s on the PT500).
- Duty cycle: Frequently repeated starts produce a high current surge every time. A ramp (soft start/stop on the KS250, RMP on the PT500) softens the surge.
- Mechanical resistance: An unlubricated bearing, misalignment and a binding guide raise the current needlessly; the heat you see at the driver is really the result of a mechanical problem.
- Mounting: A module left without air in a closed plastic enclosure runs significantly hotter because there is no convection. Mount the module on a metal surface and in a position with air circulation.
- Terminal and cable: A loose connection creates contact resistance, and an insufficient cross-section heats both the cable and the terminal.
- Ambient temperature: As the ambient rises, the component temperature rises at the same loss; since RDS(on) also increases with temperature, the process feeds itself.
From symptom to cause: where to look
| Symptom | Possible cause | Measurement |
|---|---|---|
| Only the terminal area is hot | Loose connection, thin cross-section | Voltage difference across the two ends of the terminal |
| The whole body is evenly hot | High current or inadequate ventilation | Current under load, ambient temperature |
| It only heats up on starts | Frequent cycling, ramp switched off | Number of starts and the ramp setting |
| It grew over time, it was not there at first | Mechanical ageing, grease drying out | Current compared with the value at installation |
| It only heats up in the summer months | Ambient temperature, mounting in the sun | Temperature inside the panel and outside |
Where heating does not count as a fault
A module that goes from warm to hot under load is not on its own a sign of a fault; the power being switched turns into heat and that is expected behaviour. The problem is the temperature not settling over the cycle but continuing to rise, or the module going into thermal shutdown. It is also normal to run hotter at the same current at a high ambient temperature; there the answer is to lower the current or to go up a model. Heating is sometimes itself a symptom rather than a cause: a binding guide or a bearing drying out raises the current and heats the driver. In a case like that, replacing the module postpones the problem by a few months, it does not remove it.
What to do
Note: no heatsink requirement or operating temperature range defined by the manufacturer is stated in the product data for these models; the above are general engineering principles. Measure the current under load first and work out its ratio to the continuous rating, then use the table to confirm the most likely cause. Make one change at a time and repeat the same cycle and measure the temperature again after each one; if you change several settings together, you will not be able to tell which one worked.
The full topic: Locked rotor and thermal protection