What determines the voltage of an electric motor?
For an AC asynchronous motor, it's the voltage-time going into magnetization within one half-period. The lower the frequency is, the lower the voltage has to be to avoid over-excitation during the half-cycle and thus, excess currents.

So you can always run a given AC asyncronous motor at a lower voltage than rated, at the expense of a likewise lower torque (and power). Lowering the voltage also offers you the opportunity to reduce the frequency (and power, again). The figures are mostly proportional.