# What determines the voltage of an electric motor?

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).

• The motor was designed for a certain voltage by choosing the number of turns and the diameter of wire. More turns and smaler diameter for higher voltage, less turns and larger diameter for lower voltage. – Uwe May 15 '18 at 13:09

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.

• Thats not what i was asking, but thank you for this info. I was looking for the knowledge of how manufacturers rate their motors to a specific voltage and how to rate it for lower voltage. – Tom May 15 '18 at 8:24
• Voltage and frequency cannot be set apart. Once these are set, they calculate the cross section of iron needed for the excitation allowed by the material and voltage-time it has to "eat". This figure determines the power the motor may deliver magnetically. The copper comes next, because the copper only determines the power limit from heat losses. – Janka May 15 '18 at 8:28
• This isnt what i was asking. Thanks anyways, great info. – Tom May 15 '18 at 8:31
• I'm sorry, but your question seems very unclear to me then. – Janka May 15 '18 at 8:32
• Ill make it simple mate. Why is the voltage plate on the motor 220v? why not 46000v? What part of the motor needs to be adjusted to give this voltage rating? – Tom May 15 '18 at 8:34

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.