Why the speed of a permanent magnet motor increases at constant torque as its temperature increases?
I am not sure about this. I have never seen it mentioned in motor specs. Here are some possibilities… I doubt it is due to armature resistance increasing. The extra armature resistance leads to greater voltage drop, so reduced speed. An increase in supply voltage would be necessary to maintain the torque. I expect this would just bring the speed back to where it started. The motor gets hotter still. The carbon brushes (if used) may have a negative resistance temperature coefficient, so reduce the resistance, so improving the efficiency. Probably not enough to matter. It would depend on what the ratio of armature resistance and brush resistance were, and the temperature coefficients of each. Some other effect to do with commutation, like damping of the armature coils. The magnetic flux density of the permanent magnet could reduce with increasing temperature, so the motor goes faster to get back to the self EMF compensating the supply voltage and the voltage drop in the series resistan
