thats because the voltage of the motor is fairly irrelevant for a bipolar motor. Its the current that matters and assuming you have a constant current driver (e.g. either a chopper or PWM) that is the key value. The other key parameter is inductance of the windings as this tells you how the motor torque will roll off at speed and therefore the ideal driver voltage, which is 32 x sqrt(inductance in mH). so a 3mH motor ideally would like a 32 * 1.732 = 55v driver. Of course a lower voltage will work but wont get the same torque at the higher speeds.

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