What determines the operating voltage of an electric motor? And could an AC motor be run at a lower voltage (i.e. 80v) and how? I'm not sure if its the windings turns or thickness of the wires which determine this but I'm guessing it's something related to this. I'm trying to understand if there is a possibility to build an ac motor to be run on lower voltages with a dc supply into a controller (VFD).
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.
An AC induction motor is designed around a specific ratio of voltage AND frequency because it has to do with the magnetic flux and the resulting strength of the magnetic fields. So if a motor is designed as 230V 60Hz, it is designed around the magnetic flux strength when you apply 3.833 Volts per Hertz (V/Hz). If you give it more volts at that same Hz, the magnetic fields saturate the core and turn to heat rather than useful work. If you give it LESS voltage at the same Hz, the magnetic fields become weaker and the torque that the motor produces drops at the SQUARE of the voltage reduction. So applying 80V at 60Hz to a motor designed for 230V 60Hz, results in a V/Hz reduction by roughly 65% (35% V/Hz), so the motor will produce roughly only 12% of its rated torque and likely stall.
What VFD does is to MAINTAIN the same V/Hz ratio that the motor was designed for, so the torque produced by that motor can remain the same and/or without causing saturation as the speed reduces. So in your example, if you apply 80V to the motor and ALSO change the frequency to 21Hz, the motor still gets 3.833 V/Hz, so you can get full rated torque from it at a slower speed.