Measuring harmonic distortion 1

[eeprom]

Hello,
I'm looking for instruments for measuring harmonic distortion. I have looked at the Fluke 43 Power Quality Analyzer, but it is pretty pricey. I have also considered buying a good digital scope with storage. If I could import the waveform data into Matlab, I could do an FFT and get the same results as the Fluke, and I'd have an oscilloscope. Does anybody have any other methods or meters that they use for measuring harmonic distortion?

thanks,
Andy

 

[Skogsgurra]

It depends a lot what you are going to use it for. If you are doing this professionally, there is "regognition factor" to consider. Use an instrument that is recognized in the power engineering society if you want your reports to be accepted by professionals.

If you do it to learn or for research, then most techniques are good. Even using the microphone input on a PC works. Or lots of low cost USB data takers. You need to make sure that the microphone input of the PC does not have a lower frequency that blocks power frequencies.

Not too long time ago, people even used a camera to take a screen shot from an oscilloscope, enlarged it and took data points manually. Often also did a manual fourier transform. Took some time, but worked. At least for higher level harmonics.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

If you think the Fluke is pricey, check out Voltech. Different class of instrument - very accurate, very reliable, pricey. Yokogawa have a similar range aimed at the same market; of the two I prefer the Voltechs but that might be down to familiarity and experience.

Another instrument I've found useful is a spectrum analyser with an extended low frequency response. HP's 3561A and 3562A dynamic signal analysers are good value although they certainly aren't cheap even on the surplus market.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Unless the Fluke 43B has inproved, it is a very nice meter but it's storage/recording capabilities are very limited Basically, you get a capture of what you see on the screen with no storage of the data points.

Another option to consider is a Tek TPS2000 scope with the "power bundle".

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13304&lc=EN

 

[eeprom]

Thanks for the info. I definitely lean towards the o-scope because it is not a specialty item. A power analyzer, or a spectrum analyzer has pretty much one purpose. A scope has lots of uses. I was hoping to keep this adventure below $2,000, but I don't see that happening.

Thanks again for your inputs.

 

[ScottyUK]

The TDS2000 series, which Tek's TPS line is derived from, is a full generation behind Agilent's current equivalent range. Big differences in capability, especially on transients. Late last year I asked Tek if they are intending to release an upgraded TPS but they gave a non-commital answer. Just before changing jobs last year I was intending to buy an Agilent DSO5034A scope with four high bandwidth isolation probes rather than the TPS2014: the sacrifice in convenience is more than balanced by the better performance and capabilities. Sure, the isolated inputs are nice and battery power is convenient but the isolation ain't as good as that from a decent isolating probe (and I'd rather wipe out a probe than the scope), and most places have mains power given a bit of effort.


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If we learn from our mistakes I'm getting a great education!

 

[eeprom]

I've never used an Agilent scope. One of my intended uses for this scope is monitoring harmonic distortion on VFDs. So I will need probes to read 480V. Would the DSO5034A scope be a good tool for this application?

 

[ScottyUK]

I had the luxury of a Voltech PM3000 and an HP3561A at my last place so I rarely bothered with the scope (early 90's LeCroy) for harmonic measurement. From what I recall six months on the Agilent's FFT capabilities were reasonable when we had the demo one in (we had a 100MHz one on loan, but basically identical except bandwidth). I was more interested in it's ability to capture fast non-recurring events.

Your best bet is to get demo instruments from a Tek and Agilent distributor and see if you actually like the user interface and so on as well as whether it will make the measurements you need. You're looking at a fair amount of money so you have every right to ask for a trial period.

Personally I would not directly connect the TPS scope to a 480V circuit - in my opinion the inputs are not protected heavily enough for direct connection to an industrial power circuit. You might be pushing the limits of the isolation too, I haven't got the spec in front of me.

From experience I know I was pretty unhappy when the steam turbine's excitation system blew my isolation probe to bits; I would likely have been crying (or fired!) if it took out the scope. The two other lessons learned that day were to replace the crappy leads on the isolation probe with a set of HRC fused multimeter leads, and the value of having a piece of earthed sheet steel between you and it.


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If we learn from our mistakes I'm getting a great education!

 

[zappedagain]

When you install a firmware upgrade (3.41 or later) in a Tek TDS3000 scope it now includes FFT capability for free; you no longer have to buy the FFT cartridge.

Thus you may be able to buy an older used TDS scope and update the firmware to get FFT for a reasonable price.

John

 

[ScottyUK]

That is a good idea - worth exploring for sure.

Have you looked at the auctions? With so many companies going bust there are some real bargains to be had. Worth thinking about.


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If we learn from our mistakes I'm getting a great education!

 

[sibeen]

For many years I ran around in the field with the Tek THS720P. I really liked this scope. It really was hand-held, unlike the later scopes from Tek. I could never understand why Tek never followed up with this line.

Sure, it wasn't the greatest scope in the world, the software was fairly clunky, but for harmonic and power measurements it was excellent.

It has ben discontinued now for quite a few years but you may be able to pick one up around somewhwere.

 

[damiens]

I think TCI (Trans-Coil) manufacture some harmonic distortion mitigation equipment. They might have a cheaper analyser.