Creating a three phase current system with e.g. 70Hz instead of 50Hz

Question

I am doing magnetic field measurements on a 3-phase cable dummy, in order to develope a distance measurement system for power cables. This is a uni research project.

For a test setup I want to create a three phase current system that does not oscilate with 50Hz but something distinctly different, to be able to distinguish the magnetic fields of my own system from the grid frequency, and avoid confusion as to what caused my measurements. In my current 50Hz setup I have peak currents of about 5A. I would like similar currents (if not greater) in this new test setup. I dont need 230V, current is what I want. Something harmless like 5V or even less is fine.

It would be nice (but optional) to have pure sine waves, meaning without harmonics or noise on them. I will use a FFT of the resulting magnetic fields, and I expect a little noise and 50Hz interferance from lab equipment anyways. I would drive a simple ohm load.

One solution would be to get a syncronous machine and spin it at 70Hz with an other machine. The drawback is that that will give me more magnetic fields at 70Hz in my vicinity, and I dont have the space to get away from those nore the means to fasten them to the foundations.

Would it be easy to modify a high frequency dc-to-ac module to output 70Hz instead of 50Hz, and combine three of those into a three current system?

High end audio amplifiers can output roughly 7A, if I understand the specs correctly, and their output could be free of harmoinics. But it would also be quite expensive.

Are there other, simple, cheap and robust ways?

In any case I would need to syncronize the currents and maintain 120° phase shift between them. How could I do that?

Answer 1

The simplest way to go is to use a VFD. Then you would need a sine filter. If you need a low voltage, relatively cheap solution, you can use some evaluation board for permanent magnet synchronous motor.

Typical switching frequency of the industrial VFD is 4kHz IGBT (most of them support also 2kHz and 8Khz), of expensive servo drivers is 16kHz - MOSFET. Industrial sine filters (look Schaffner sine filter) are rather expensive, bulky, made for larger powers and mostly all of them are designed for 4kHz switching frequency.

Now, at low voltage/high frequency a MOSFET is a better choice, over IGBT. Still, there is big derating factor when switching frequency is high. But these eval boards run at 20kHz+ by default, so the output filter can be a very very simple. The challenge in a sine filter design is that switching frequency 4kHz is relatively close to the fundamental frequency 50Hz. At high switching freq. a simple lowpass LC filter does the job and you get a pure sine wave.