Do I need to fit a heatsink to the motor driver?
No heatsink requirement is stated in the product data for the ST42, KS250 and PT500. Lowering the current and ventilating the mounting usually solves heating.
No heatsink requirement is stated in the product data for the ST42, KS250 and PT500, so it cannot be said that these modules do or do not need a cooler. The general principle holds: because the loss grows with the square of the current, running the module at 70-80% of its continuous current rating and leaving air circulation at the mounting location keeps the heating under control in most applications.
The logic of the thermal balance
If the heat produced exceeds the heat that can be removed, the temperature rises. The heat produced grows as P = I² x R; the heat removed depends on surface area, air circulation and ambient temperature. That is why the same module at the same current reaches completely different temperatures in a closed box and in an open panel. A heatsink improves only the second half of the equation; the first half is still in your hands. The kind of mounting surface sits inside that equation too: there is a serious difference between screwing the module to a plastic cover and seating it on a metal section with air around it, because metal spreads the heat and works like a second surface.
These come before a cooler, in order
- Reduce the current. This is the most effective step, because the loss varies with the square of the current. Run the module at 70-80% of its continuous rating.
- Fix the mounting location. Squeezing the module into an airless cavity, or resting it against the motor body or a sheet-metal surface in the sun, is the most common cause of heating.
- Raise the voltage. For the same power, using 24V halves the current and cuts the loss to a quarter.
- Thin out the duty cycle. Leave cooling time between back-to-back starts.
Is the heating really a problem, and how do you tell
"Too hot to touch" is not a measurement; metal surfaces feel hot to the hand even when they are only warm. The right method is to measure the hottest point on the module with an infrared thermometer or a thermal camera during the real duty cycle, and to note the ambient temperature at the same moment. What means something is not the absolute value but how many degrees the rise above ambient reaches, and whether it settles during the cycle. If the temperature keeps climbing, no equilibrium is being established. If the ambient temperature is high the internal temperature goes higher at the same loss, so a summer measurement does not stand in for a winter one.
Where these steps are not enough
The steps above usually solve the heating. If the module still runs too hot in your application, that shows the selection is marginal; moving from the 15 A KS250 to the 30 A PT500 is the more correct solution. Where convection cannot be established at all inside a closed box, forced air may be needed instead of a passive cooler. If you are considering any mechanical modification or fitting an external cooler, consult the support team first so that the ingress protection (KS250 IP45, PT500 IP67) is not compromised; drilling the enclosure does not let you restore that sealing, however good the thermal interface material is.
What to do
First measure the current under load and work out its ratio to the module's continuous rating; if the ratio is high, consider going up a model rather than hunting for a cooler, because that ratio is what really sets the loss. Then change the mounting location so that air can reach it, repeat the same cycle and measure the temperature again. Leave only one variable between the two measurements; if you move it and lower the current at the same time, you will not know which one worked.
The full topic: DC Motor Driver Selection Guide