If the same power is delivered, a higher-voltage system generally draws less current, which can reduce resistive losses in the battery, wiring, and motor windings. For example, a 48 V system can deliver the same power with less current than a 36 V system. I'm wondering why small motors aren't routinely overvolted instead of replacing them with a higher-voltage model.
As an example, consider a 36 V, 500 W motor rated around 3,250 RPM, with an internal reduction and additional gearing for an electric-bike mid-drive. Depending on the selected rear gear, the motor may be operating well below its rated speed. Increasing the supply to 48 V would raise its unloaded speed by roughly 33%, potentially moving it closer to a more efficient operating range and reducing heat from high current.
The motor's plastic housing may tolerate much higher voltage, and many controllers support both 36 V and 48 V. So what are the practical limitations of simply overvolting a small motor? Is the main concern winding insulation, overheating, controller limits, mechanical speed, or something else?
2 Answers
Lower current at higher voltage can reduce losses in the battery cables and some winding resistance, but it doesn’t make all losses disappear. A battery pack, controller, motor, and reduction system still have their own losses, and changing the voltage can simply move where the heat is produced. The relationship also isn’t automatically a universal square-law improvement once duty cycle, motor speed, controller behavior, and load are included. Running batteries in parallel or choosing a properly matched motor and controller can achieve lower current without overdriving the original motor.
The motor is designed around a particular voltage, winding, speed, and thermal load. Applying more voltage increases its back-EMF constant relationship, so it spins faster under no load and can draw substantially more current when loaded. That can overheat the windings, damage the insulation, exceed the controller’s limits, or push the gears and bearings beyond their intended speed. The plastic outer housing is not the important voltage rating; the insulation between the turns of the winding and the electronics are much more relevant. If the application needs a different voltage or speed, using a motor designed for it is usually safer.
So the possible benefit is mainly lower current and cooler wiring or battery operation, not a guarantee that the motor itself will run cooler. The motor still has to be checked for its winding temperature, maximum RPM, controller voltage, and mechanical limits.