# What governs the output voltage of a three-phase generator?

If you have a three-phase generator (or motor run as a generator), what part of it governs the output voltage? For example the speed at which you rotate the shaft governs the output frequency, but what governs the output voltage?

The reason I ask is that I'm looking for cheap ways to produce three-phase power in an off-grid scenario, and electric motors from damaged EVs are relatively cheap. Many of these seem to be three-phase permanent magnet motors, but finding out what voltages they produce when driven as generators is very hard to come by.

I am wondering whether it's a function of some part of the motor design (e.g. the strength of the permanent magnets) or whether it's something you can control as part of driving the generator, as you do with frequency by varying the shaft speed as required. For example does the output voltage go quite high if there is no load, and enough of a load must be placed on the generator output to drag the voltage down to the desired level? I'm not sure if that's right because I thought the load was related to the frequency (and how hard it is to turn the shaft at the correct speed in order to maintain the desired frequency).

In other words, if you are designing a three-phase generator from scratch, what (if anything) do you change in the design to adjust the output voltage it produces?

• If you are talking about synchronous generators, it's the excitation current.
– Bart
Jul 6, 2020 at 12:31
• Same relation that governs their speed when driven from a voltage ... Kv the speed constant. (But losses subtract from the voltage generated instead of adding to the drive voltage required.)
– user16324
Jul 6, 2020 at 12:32
• @Bart: Please excuse my ignorance - I am not sure what role an excitation current plays in a permanent magnet motor? I thought it was only applicable to induction motors. Jul 7, 2020 at 13:24