Electronics > Beginners

Oscilloscope for AC work ?

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SND:

--- Quote from: plurn on May 04, 2019, 05:29:22 pm ---
--- Quote from: SND on May 04, 2019, 11:41:24 am ---Thanks for all the replies.
I'm currently considering the AEMC 407 clamp meter, with their free bluetooth software it might get me close enough to what I'm trying to find, at least it has some THD functions. I'm not 100% sure it'll get to the higher frequencies that might be in there but for a bit over $500 could be worth a try.

--- End quote ---

AEMC 407 clamp meter does "Individual Harmonics (to 25th)", so I would think that means 60Hz x 25 = 1500Hz only? So probably won't show 5kHz.
https://www.aemc.com/userfiles/files/resources/datasheets/Clamp-On-Meter/2139-51.pdf

--- End quote ---


I'm not sure, Specs say it displays to 25th, but the android app for the 407 and 607 models has a picture showing higher harmonics on a graph so maybe it senses them but just can't display higher than 25th? It does say the hz setting goes to 20khz but I'm not 100% clear on if it'll show me data up to that in the app, can't seem to be able to open the app I downloaded without having the unit in hand.   I like that it appears capable to check THD on current and voltage.


plurn:

--- Quote from: SND on May 05, 2019, 12:05:50 am ---I'm not sure, Specs say it displays to 25th, but the android app for the 407 and 607 models has a picture showing higher harmonics on a graph so maybe it senses them but just can't display higher than 25th? It does say the hz setting goes to 20khz but I'm not 100% clear on if it'll show me data up to that in the app, can't seem to be able to open the app I downloaded without having the unit in hand.   I like that it appears capable to check THD on current and voltage.

--- End quote ---

It certainly does look like a useful device for THD and as a general multimeter/clampmeter. I like it too from what little I know about it and I would not mind owning one.

From the manual it says "The frequency of the fundamental is determined by digital filtering and FFT for the network frequencies of 50, 60, 400, and 800Hz".
800Hz * 25 = 20kHz. So maybe you need the fundamental to be 800Hz to see that bandwidth? Or maybe you can see it for everything including 60Hz fundamental? I don't know. You could ask their customer support.

Also from the manual it says:

"Frequency Analysis Band - 0 to 25 times the fundamental frequency, from among the network frequencies 50, 60, and 400Hz
- 0 to 12 times the fundamental frequency of an 800Hz network". So that might be a limit of up to 10kHz?

I think maybe the 5kHz you are talking about is not really "harmonic" distortion related to the fundamental frequency of 60Hz. So something that just analyses harmonic distortion might not take it into account and probably should not be expected to. It could be considered some other sort of distortion (there are different types), or noise, or a seperate fundamental frequency. You have not really said why you expect a 5kHz noise to be present and a significant contributor (unless I missed it).

So if it is just "harmonic" distortion you are interested in, something like this device sounds great and is relatively affordable. If you really need to measure/view other sorts of distortion and noise, it may or may not do it - not sure. Hope it works out for you.

SND:
I expect most of the noise to be from big IGBT's that run at 5khz. Its in the audible range anyway. There probably is some other regular harmonics since its tied to other vfd's and 3ph motors.

BravoV:
My 2 cents, think of your probing mistakes in the past at low voltage circuits, and count as you can memorize, how many ? 1 ? 10 ? or even more ?  :P

Now, when it comes to high lethal voltage, all you need is just "one" mistake, is enough to zap your expensive measuring equipment, your circuits or even worst ... your limb or your life.

Again, think about it, is it worth the saving ?

As many had pointed out, the easiest & possible the most bang for buck is using HV differential probe. As probing mains related devices will be very comfortable, an example scope that was probing the mains line that powered the scope it self.


Also many pointed out, battery powered scope is not always a suitable one for probing lethal voltage.

Here, a design illustration, how a "proper" battery powered handheld scope that was designed for probing mains. Watch carefully how they separate & isolate each functions in the scope.


Also a proper scope and its accesories have NO exposed metal, including the BNC and also the whole probe's body.

An example of Tektronix THS7xx handheld scope for mains probing, watch the BNC connectors at the scope, no exposed metal.


Also the suitable probe, watch again carefully, all covered by non conductive material and its not easy for  human body parts can touch the metal parts.


The bad news is, handheld scope that is qualified for probing mains line, even used, especially top brand ones, still very expensive, and most of the times still more expensive than a brand new "decent" HV diff. probe.

DDunfield:

--- Quote from: BravoV on May 05, 2019, 09:11:27 am ---As many had pointed out, the easiest & possible the most bang for buck is using HV differential probe. As probing mains related devices will be very comfortable, an example scope that was probing the mains line that powered the scope it self.

Also many pointed out, battery powered scope is not always a suitable one for probing lethal voltage.

--- End quote ---

An HV rated portable scope is as suited to probing high voltages as an HV rated differential probe. In most industrial cases the battery scope is the better solution.

In addition to being great at demonstrating probing mains sockets, a bench scope with an HV-diff  probe is ideal when looking at non-ground referenced HV signals on an appliance on your repair bench.

But things like distribution panels, VFDs, large main connected inverters etc. aren't usually conveniently on your bench when operating under a load. These things are also not (usually) in the realm of things a typical do-it-yourself'er works with, but if you have particularly exotic hobby needs, you might "make due" with a bench scope and HV diff. probe.

This setup doesn't work on a job site.

Below is a small collage of images (shamelessly snarfed from web) which shows places where you might see someone comfortably taking mains related scope measurements on a job site.

I suppose you *could* use bungee cords to hang your bench scope on the panel.  Or perhaps drag in a table and extra long leads, making do with the scope where you can't quite see it (also fun when you need to move between several pieces of equipment).  But... where would you plug it in?  Better bring some extension cords too!

I suppose you *could* juggle a 1054Z while standing in front of a panel/equipment wearing heavy protective gear... but IMO much more likely to make a mistake, and again where to plug in.

I suppose you *could* balance your bench scope on the motor ... Not as stable as the flat rubberized scopemeter, but hopefully it won't fall off... But what if it does. How badly will it be damaged?

Making measurements of this kind, you avoid extra cables (the bench/diff-probe adds 2),  you avoid things that need to be plugged in, you avoid things that are hard to hold, you avoid things that are breakable ... bench scope fails on all of these counts. The scopemeter designed for this purpose is the right tool for the job.

And yes, sometimes the right tool for the job is expensive. If you're doing it once (or very infrequently) you might make due like MacGyver. If it's part of your job, you get the right tool, it will pay for itself (otherwise you may encounter a different meaning of  "bang for the buck").

Dave

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