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60Hz power THD - newbie needs help

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60Hz power THD - newbie needs help

Postby BruceM » Fri Apr 14, 2017 4:24 pm

I'm working on a low frequency stepped sine inverter design. I'm just now starting to use the spectrum-THD measurement and having some trouble.

I find that changes to the frequency range window (199KHz, 100KHz, 50KHz) will change the same signal results rather hugely; using a generator source from 2% to 8% and 15% THD respectively, with no change to the input signal. I also see this sort of relative change in results when using the internal frequency generator set to 60Hz but with chopping the waveform tops via the arbitrary waveform editor.

I don't understand what's going on and how can I assure that I'm getting legitimate values. How can I set up this so that results would be similar to a power line quality/THD analyzer result?

I find the Picoscope an amazing tool. Thanks in advance for any assitance.
Bruce
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Re: 60Hz power THD - newbie needs help

Postby Gerry » Mon Apr 24, 2017 10:48 am

Hi BruceM,

If you change the spectrum rage over which you perform the THD measurement then you are looking at different ranges of harmonics. So, if there are significant high order harmonics you would expect to see a difference in THD. However, without looking at actual data it's not possible to make any kind of quantitative analysis of what is going on (I certainly wouldn't expect to see a huge difference - so there could be some other factor creating an anomoly here). I would suggest posting a psdata file of your results and then an explanation of what you were expecting to see.

Regards,

Gerry
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