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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?

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    \$\begingroup\$ For a one-off, I would use a four channel function generator and dito channel audio amplifier. That's what I do to get 60 Hz for type testing in a 50 Hz grid. Are you going to mass produce this? "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?" Depends how it clocks itself internally for frequency reference. Some UPSes and VFDs have service menu with custom output frequency settings. Probably more voltage than current though on them. \$\endgroup\$
    – winny
    Jan 19, 2021 at 17:12
  • \$\begingroup\$ great, we have such a function generator, I hope. \$\endgroup\$ Jan 19, 2021 at 17:17
  • \$\begingroup\$ You'll find commercially available (and pretty cheap) Variable Frequency Drives (VFDs) designed to drive 3 phase motors at your frequency of choice. Any machine tool supplier can point you in the right direction. I don't see any reason you'd need something else. \$\endgroup\$
    – user16324
    Jan 19, 2021 at 17:45
  • \$\begingroup\$ If you have three function generators, you should be able to use a sync pulse and set individual delays for them. Even simpler is a 4+ channel sound card in a PC and generate your 70 Hz "sound" and offset the two last channels. \$\endgroup\$
    – winny
    Jan 19, 2021 at 18:34
  • \$\begingroup\$ If you would have been local to Sweden I could lend you my setup. \$\endgroup\$
    – winny
    Jan 19, 2021 at 18:39

2 Answers 2

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You could use a motor-generator set, but you'll go nuts if you're anywhere near to it - those are true acoustic nuisances (I have some experience). You could use a 3-phase induction motor to drive the generator, and use a VFD to control the speed to get 70Hz on the generator output. And if you move it away sufficiently not to hear it, then you won't have the problem with magnetic field interference from the motor or the generator itself either. If you can do it with components you can scavenge from "junk" or secondhand, it can probably be very cheap, although I do suggest that you rent a hi-pot tester or a megger and test the insulation on both the motor and the generator to make sure it's functional.

But I also think that you don't really need all this complication - it seems that you're doing some EMC measurements, like hum pickup etc. You don't need to use a 70Hz supply for that.

avoid confusion as to what caused my measurements

Such measurements require some care and understanding what's going on, so if you don't know how to set it up to get valid results, then I doubt very much that 70Hz will be of much help.

Are there other, simple, cheap and robust ways?

If your time has any value at all, then the cheapest and simplest way of doing it is to rent a 3-phase power supply that is designed for device testing. Those provide output frequencies up to 1kHz at least, and would be the simplest option. There are many such supplies out there, and you would rent one. That's how it's typically done in development labs that don't need that functionality non-stop.

You should look into test&measurement equipment rental houses in your area.

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    \$\begingroup\$ A cheap option to buy could be old avionics gear, they typically run on 400 Hz. \$\endgroup\$
    – pipe
    Jan 19, 2021 at 17:09
  • \$\begingroup\$ Yes, renting such a power supply would be feasable. Money is less of a problem. Googling for such rental places didnt yield any result so far. I will look further into that. 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. \$\endgroup\$ Jan 19, 2021 at 17:15
  • \$\begingroup\$ @AndreasSchuldei You want to be doing differential measurements, then. Measure with the cable turned off to get a baseline, then turn the cable on, measure again, compute the difference. In practical terms, chop the supply to the cable at a "slow" rate like every 50 line cycles, so that you can synchronously demodulate the output and get very good sensitivity that way. All that's needed for that are zero-switching solid state relays, and a square wave generator synced to line frequency (you can easily make one with an Arduino). The longer you average the more sensitive it'll be. \$\endgroup\$ Jan 19, 2021 at 17:28
  • \$\begingroup\$ @Kubahasn'tforgottenMonica so you would do that at 50Hz grid frequency and switch the grid on or off? can you explain a little further how you mean about the synchronisation of the demodulation? my fields will be 1e5-1e6 time weaker then the earths magnetic field, so any boost i can get is welcome. \$\endgroup\$ Jan 19, 2021 at 18:36
  • \$\begingroup\$ @pipe can you give me more details on that? where can i find that, what is my search term for google? 400Hz is not optimal, as it is a multiple of 50Hz. \$\endgroup\$ Jan 20, 2021 at 9:11
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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.

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  • \$\begingroup\$ I have been thinking about a VFD solution. The issue I see it the generally high voltage that you have to have in inverters intermediate circuit. All VFD devices that I know of have an emergency fault mode for low intermediate circuit voltage. That voltage is really central to all calcuations of the PWM pattern etc. Running this with nominal voltage would be unnecessarily dangerous. If you know of a model that can be run with low voltage please let me know. \$\endgroup\$ Jan 20, 2021 at 9:09
  • \$\begingroup\$ @AndreasSchuldei If you read my answer, you will notice that I suggested you to use a low voltage three phase evaluation board from STM, Freescale/NXP, TI,...and a lowpass filter. \$\endgroup\$ Jan 20, 2021 at 10:51
  • \$\begingroup\$ i used to work with those VFD evaluation boards. They used to come without the power electrocics, and building that part is tricky. Are you suggesting to get an eval board and then designing the power electronics part myself is the simplest way to go? \$\endgroup\$ Jan 20, 2021 at 11:24
  • \$\begingroup\$ @AndreasSchuldei A VFD eval board without power part? It makes no sense, this is not a VFD eval board, then. I am sure that you will find many of these boards at your university \$\endgroup\$ Jan 20, 2021 at 13:23

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