Ohms Law for DC currents. Our instructors taught easy as P-I-E. In that Power (P ) expressed in watts is equal to Amps (I ) times Volts ( E ). In longhand Power= Current X Voltage.

So any increase or decrease in either will increase or decrease proportionately. If you double voltage you cut the current in half so if motor is designed to run at a certain ampere rating. It probably will not operate. By decreasing the Voltage by half you double the Current and probably burn the windings in the motor.

Motors that are designed to run at 6/12 or 12/24 voltages have special windings and are designed to handle or operate at either load level.