If a motor can produce the same mechanical power at a higher system voltage, it should draw less current, reducing resistive losses in the battery, wiring, and controller. For example, a 36 V, 500 W motor might run around 3,250 RPM under load, while increasing the voltage to 48 V could raise its speed by roughly 33%. In an electric-bike mid-drive, however, the motor may be slowed substantially by internal and external gearing, and high bicycle gears can make the motor operate far below its most efficient speed. The question is why manufacturers do not simply use higher-kV, higher-RPM motors and overvolt them, especially when the wiring insulation and many controllers appear capable of handling the voltage.
2 Answers
Higher voltage can reduce current for a given power level, which lowers I²R losses in the battery leads, windings, connectors, and controller. But that does not automatically make an existing motor suitable for the higher voltage. The motor’s insulation, winding temperature, magnets, bearings, controller voltage rating, and mechanical speed limits all matter. Increasing voltage also increases RPM and can push the rotor, reduction gears, and drivetrain beyond their designed limits. A motor designed for the higher voltage or a different gear reduction is usually the safer solution.
The basic voltage-versus-current argument is sound, but the conclusion is too broad. A higher-voltage setup can be more efficient when the motor, controller, battery, insulation, gearing, and cooling are all designed for it. Simply overvolting a low-voltage motor can cause excessive no-load speed, core and iron losses, saturation, commutation problems, or overheating under load. Also, an inefficient gear choice cannot always be fixed by adding voltage; selecting gearing that keeps the motor near its efficient operating range is often the better remedy.
Reducing current in one part of the system does not eliminate every loss. The controller and motor still have their own voltage, current, switching, magnetic, and mechanical limits, so the heat may simply move to another component if the system is not designed for the change.