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In modern EVs the typical design is to use a single-ratio transmission, so the motor must work over a broad speed range. To achieve this, the motor is designed to reach high speed by using high voltage, with the same power available over most of the speed range. This also keeps the motor out of the high-load high-current low-speed regime most of the time, where efficiency appears to be low.

As major explained, the same speed range could be covered with lower motor speed by using less gear reduction, but it would need to put out more torque for the same low-speed acceleration, making the motor larger and requiring higher current from the controller.

My guess is that two factors which may have led to increasing voltage are that the high series cell count required might have been unworkable with earlier battery technologies, and that high-voltage power electronics are getting more reasonable in price. Of course before the switch to AC motors, the commutator and brushes of traditional brushed DC motors limited practical rotational speeds.
 
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