# variable frequency three phase AC

I want to generate 1-150Hz+ three phase AC. Output voltage at maximum frequency would be around 240VAC and current around 40A. I have a few 60Hz three phase induction motors, drive shaft coupler, and a 480VAC feed three phase VFD. Yes I can build an excitation supply if needed.

What type of generator would I need to get the above output? What should I know about overspeed and induction motors?

simulate this circuit – Schematic created using CircuitLab

I am expecting a three phase sinusoidal waveform out of the generator to the DUT. About the generator type, can I get an induction motor to make the 40A I need? Do I need a synchronous generator, nonsynchronous generator, AC generator, ect?

• Why would you not use the VFD (or another one) to generate the required 3-phase AC? Apr 8, 2016 at 21:48
• "What type of generator" is a very vague question. I could just say "You need a variable-frequency, 3 phase AC generator" and that would answer the question and - I suspect - not answer your question. Please be more specific, post a schematic or ask about a model number, something that will guide us a bit in helping you. I can't tell from your question if you are trying to specify a VFD or if you are trying to build a VFD yourself for educational purposes. My answers would be very different in each case. Apr 9, 2016 at 0:13
• What kind of waveform are you expecting? A VFD is definitely not going to be anything close to a sine wave. Depending on the technology, it could be anything from a six step pseudo-sine, or a high frequency PWM waveform. Apr 11, 2016 at 14:06
• I would use a Honda generator. Or, a Generac. Apr 12, 2016 at 0:21
• @TimSpriggs: How would a Honda generator or a Generac generate 1 - 150 Hz three-phase power at 240 V AC? Apr 13, 2016 at 18:38

There is a good possibility that the VFD that you have can be adjusted to provide an output of 150 Hz at 240 V with 480 V input. You may be able to get it to operate at 1 Hz, but most motors won't operate very well at 1 hz. A good sensorless vector drive may be able to do that. You can probably not accomplish much trying to use an induction motor as a generator.

When operating induction motors above their rated speed the main limitations will be bearings and rotor balance. At some speed, centrifugal force on the rotor may become an issue, but that speed is likely to be significantly higher than the rating of the bearings and the speed at which rotor balance becomes a problem.

With standard motors, you can probably run a 6-pole motor at twice its rated speed, a 4-pole motor at 1.5X to 2X, and a 2-pole motor at 1.25X to 1.5X. If you don't increase the voltage proportionally for operation above nameplate frequency, the motor will not be able to deliver rated torque. You can probably get constant horsepower operation up to 1.5X rated speed. Above that, the torque must be limited to something below the torque that will provide constant horsepower.

Re Question Edit

There is no way that you are going to get an induction generator to give you the frequency range you are looking for.

The best option would to directly use the output of a VFD if you can get an acceptable waveform. An up-to-date PWM VFD will give you a a pretty good waveform. You can probably find one that will go down to 1 Hz or close to that. You could also consider using a VFD with an output filter. You may have difficulty getting good information about waveform quality. It may also be difficult to put together a filter.

For a really good waveform, the best option will probably be a permanent-magnet synchronous generator (PMSG). That will give you a good sine wave with no filtering. You can use a permanent-magnet synchronous motor as a PMSG. You use a PMSG that has more poles than the driving motor, your speed can be lower. You would need to consider the power factor of the load. I don't know how a PMSG will react to a power factor below 1.0. A VFD will have no problem with a lagging power factor down to 0.7 or so.

A wound-rotor synchronous generator (WRSG) can do as well as a PMSG, but you may have difficulty getting an excitation control unit that will work over your frequency range. Most WRSGs in the size range that you need are sold either with an engine or as "head" units designed to be bolted to an engine.

Do you need to put this together yourself or do you have a budget that would allow you to work with a system integrator?