What determines voltage usage for electric AC motors?
Do you mean what determines what voltage you apply to an AC motor? You apply the voltage and frequency and number of phases the motor was designed to use. The designer-engineer designs a motor for a specific voltage, frequency, and number of phases, and power out, as well as size, RPM etc. And to operate it correctly, you apply that voltage, etc to the motor. .
For the mechanical characteristics you want (torque, RPM) you get a motor to suit the power supply you already have. You could change the voltage with a transformer (but not the frequency) to suit an existing motor designed for another voltage, but this is probably not the best option.
The design. It will be designed for a particular voltage usage.
