When variable speed motors are operated at lower RPMs they have a tendency to overheat. This in part is caused by two factors. Due to a phase shift, the motor generates more heat at lower speeds. Also, the cooling fan is turning slower and moves less air, making it less capable of removing the added heat.
My customer who builds custom machines, called in a panic mode. His customer was burning out motors and refused payment until he got the problem resolved. Having done this application before, I suggested feeding cold air into the fan intake with a Model 3708 Mini Cooler. Worked flawlessly!
The end user was happy, my customer was happy, and I was happy to have been of assistance.