Testing AC Line Quality with an Oscilloscope - Generator power quality

[Nigel Goodwin]

Ok thanks for that Nigel.
But tell us, what exactly does it mean if it works just as well for the House AC as it does for the generators (since it did, no problem)?
why did I not have the problem you mention?

Presumably you don't have a GFCI/RCD?.

I understand you are saying that the house AC should have Earth bound to Neutral? (And yes, the Oscilloscoipe has a three prong, grounded plug)
Perhaps my 1982 home does not due to older electrical code?

Or worse, could there be a problem with my homes wiring?

Well 1982 is fairly modern, I would expect that to have GFCI/RCD, at least of some kind.

Earth and Neutral, at least in the UK, are connected together at the sub-station - and depending on the circumstances the earth at the house may be via an earth spike, or via PME (where essentially the earth is connected to the incoming neutral.

If you have a PME type earth, then the GFCI/RCD 'may' not be triggered by connecting the scope to neutral, but in the case of an earth spike it almost certainly should be (due to the small voltage drop across the neutral cable).

 

[The Electrician]

The reason I would choose not to use a transformer is in my own opinion the generator under test should also be tested under a load and I also feel adding a transformer may not afford a true look at what the actual generator output is. That being just my thinking. Now if someone would like to explain in detail how to measure a generator's power quality please do explain it.

Ron

Edit: I made a typo in the paragraph below. Originally it said "The primary of the transformer is connected to the output of the loaded transformer". It has been corrected.

The generator can be tested under load when using a filament transformer. The primary of the transformer is connected to the output of the loaded generator; the load is not passed through the filament transformer. The transformer therefore indeed sees what the actual generator output is.

Here is what the grid waveform looks like where I am. The first image shows the waveform as seen by a 100 MHz isolated differential probe (yellow), and through a filament transformer (green). The two waveforms are offset so it's apparent that there are actually two waveforms:

In this second image, the two waveforms are superimposed so the fidelity of the filament transformer provided waveform can be easily compared to the differential probe waveform:

Apothecary, if you use a filament transformer you will avoid any possibility of damaging your scope due to differences in safety ground and neutral voltage, and you will get an accurate picture of waveforms. It wouldn't take all that long to hook it up, and you will be pleased with the result.

 

[Reloadron]

OK, then the thread starter can use a transformer. My best guess is that he doesn't have a transformer at hand. Then too I really don't see the use of an oscilloscope to measure quality as a real good choice, especially an older analog scope lacking FFT as a feature. However, if a scope is all there is then I guess it's the only viable option.

Ron

 

[The Electrician]

OK, then the thread starter can use a transformer. My best guess is that he doesn't have a transformer at hand. Then too I really don't see the use of an oscilloscope to measure quality as a real good choice, especially an older analog scope lacking FFT as a feature. However, if a scope is all there is then I guess it's the only viable option.

Ron

From post #23, Apothecary said: "I ordered a 117v to 25v filament transformer and it arrived yesterday. But since I still have to make connections to live 120AC to the transformer, using the probes still seems safer......but again, you are the expert. Thanks. "

He should at least give it a try; it would be a lot safer than his metallic connections to the grid.

It seemed to me that all he wanted to do was have a look at the waveshape and see just how close to sinusoidal it was.

Some time ago I measured some grid waveforms in the Puget Sound area. Here, just to show the variability, are some of my captures:

 

[Reloadron]

Well then if he has a transformer he can use it. Here are a few examples of using a transformer:

This transformer has a CT (Center Tapped) secondary:

Looking at both secondaries with respect to the CT is a good representation of how power is delivered to a US residence. The two wave forms are 180 degrees apart. While a reduced voltage if we were to algebraically add both channels the sum would be about zero so we can invert channel 2. Doing so will yield the following:

Now we let the scope add both vertical channels to present a single trace.

This would be the equivalent of what residential mains would look like. About the only way this can be done is using a transformer or an expensive differential scope probe rated for AC mains.

The end result of any of this is merely going to show what the wave forms look like. There is no way to actually calculate any distortion but yes, the wave shapes look this way.

Moving along we can look at the wave form out of a MSW (Modified Sine Wave) inverter.

Ron

 

[Reloadron]

Earlier in this thread or it may have been the thread in another forum I mentioned the use of an inexpensive verse better quality DMM. That is also a way to compare an AC signal. When looking at AC mains an inexpensive AC voltmeter will be average responding RMS indicating. The inexpensive meter will work fine as long as the AC signal is a nice clean sine wave.

This image reflects house mains as passed through a UPS (Uninteruptable Power Supply.

Here is that same signal as metered by an inexpensive average responding RMS indicating DMM and a better quality true RMS responding RMS indicating DMM.

Both meters are reading the above sine wave. They are reading relatively close or within 1% anyway. The actual voltage is 122.8 so my money is on the better meter.

AC mains power is removed and the UPS kicks in resulting in a 117 VAC MSW.

Not visible on the scope is jitter in that the MSW inverter is not a stable exact 60 Hz, but obviously the signal is a long way from a nice sine wave. This is what a MSW looks like.

Now lets look at the same two meters.

So the inverter voltage while being 117 VAC RMS only reads correctly on the better quality meter. What both meters tell us when compared is the signal being measured while 117 VAC about 60 Hz it is not a very good simne wave.

Really about all the scope images show us with AC mains power is what the signal looks like. That is as good as any of it gets. There is no way to derive a quantitative result as in a number. There is no way we can say for example that the THD (Total Harmonic Distortion) is for example 3.5%.

What really matters with the quality of the AC signal coming out of a generator would be is it good enough for its intended application? The MSW pictured above runs my PC, my wife's PC and several other electronic devices like the CATV set top boxes and cordless phones on a mains power loss just fine until the automatic backup generator kicks on which is about 30 seconds but would power those same devices till the UPS batteries went dead.

That is about all I can think of to say on the subject.

Ron

 

[unclejed613]

What about this? My Oscilloscope is a 4 channel unit......

differential measurement is the safest way to measure line voltages (it's also good for looking at SMPS waveforms on the line side)

 

[ronsimpson]

it's also good for looking at SMPS waveforms on the line side

The scope needs to substract out 220Vac while looking at 10V of gate drive. It works but …. it can really trick you.

 

[Apothecary]

Ok,
Beginning to attempt some photos of the waveform with my new camera.
I'm still learning to use the Manual settings and Macro focus but I'm getting there.
ReloadRon has superb photos....that's my goal.
Not perfect but MUCH better......

House AC

Once I get all the settings right I'll do the NON Inverter generator. It still has a sinusoidal wave form, but nowhere near
as nice as the House AC or the Inverter Generators. Most of all, I'm just having fun with it all.

 

[Reloadron]

There you go. Years ago, more years than I care to remember I was pretty much into photography. Then came the DSLR (Digital Single Lens Reflex) as I drifted into other hobbies the (AUTO) setting became a new best friend.

The sine wave looks good. I have yet to drag my butt out to the garage and start my old 4.0 KW unit and see what it looks like. I really should run that sucker to make sure if I need it it still runs. Fuel tank is intentionally left empty.

Anyway, that sine looks pretty good.

Ron

 

[Apothecary]

Thanks Ron,
Your photos are the Gold Standard I'm shooting for.

If you haven't started in it more than several months and didn't put Stabil fuel stabilizer in it it might be a problem.
But all it would probably need is a good carburetor cleaning.
I use the Blue, Stabil Marine 360 in all my generators (which are stored in an air conditioned garage) and even in my lawn mower which sits outside in the shed in sweltering humidity.
I also have a 7 year old pressure washer out there that sits for years between uses in that 90% humidity and still starts and runs like new thanks to that Blue stuff.

Engines are my strong point. The only bad thing about leaving a tank empty (if it's metal) is that every evening when it gets cool, then the next morning as it warms, that cool metal attracts moisture out of the air like a sponge and will condensate on the inside top of that metal fuel tank and rust it. It's better to fill it with Stabil Blue 360 because it has an additive that coats the metal and prevents rust. The stuff is absolutely worth it's weight in gold. Maybe you already use it?

Hope it starts !

 

[Reloadron]

Thanks Ron,
Your photos are the Gold Standard I'm shooting for.

If you haven't started in it more than several months and didn't put Stabil fuel stabilizer in it it might be a problem.
But all it would probably need is a good carburetor cleaning.
I use the Blue, Stabil Marine 360 in all my generators (which are stored in an air conditioned garage) and even in my lawn mower which sits outside in the shed in sweltering humidity.
I also have a 7 year old pressure washer out there that sits for years between uses in that 90% humidity and still starts and runs like new thanks to that Blue stuff.

Engines are my strong point. The only bad thing about leaving a tank empty (if it's metal) is that every evening when it gets cool, then the next morning as it warms, that cool metal attracts moisture out of the air like a sponge and will condensate on the inside top of that metal fuel tank and rust it. It's better to fill it with Stabil Blue 360 because it has an additive that coats the metal and prevents rust. The stuff is absolutely worth it's weight in gold. Maybe you already use it?

Hope it starts !

Naw, I always run the tank totally empty when I know it won't be used. I do use Stabil in my bike. Then too if we get a nice day I will take the bike out. During the winter months I always put Stabil in the gas tank and leave the battery of the bike on a battery maintainer which is really just a float charger. I keep saying I want to insulate the garage and add heat but that hasn't happened yet. I'll have to look for the Stabil Blue 360 you mentioned as I have just been using plain Stabil.

I can't find an example image but if you can find something you can stand up, I used rifle cartridges, space things like maybe 1/2 to 1.0 inch apart over for example 12 inches slightly angled. Then using a fixed shutter speed and changing aperture size take pictures focused on the center objects. Pretty cool watching the depth of field change the area in focus. Maybe drill holes in a board with 1" centers and put pencils in the holes?

Ron

 

[unclejed613]

It works but …. it can really trick you

once you get the two channels nulled, it's not so bad... it's easier in the US because it's only 120V. of course the better the scope's input stages, the better. i might try it on 220V if it was a Tektronix, but not with a B&K or a Sencore.

 

[Apothecary]

So here's a few (not so good, had a hard time with these) photos of the output from my only NON inverter generator.
Really ugly waveform as you can see.
Not sure if this is what would be called a "modified sine wave" or just a noisy sine wave? I've seen Inverters that make 90 degree block modified sine waveforms. So I think this is slightly better than that?

This is all I was really looking for as was quite expected.
The waveforms from ALL the Inverter generators are PERFECT sinusoidal waveforms. Better than the house AC
Only caveat, I haven't tried them under load yet....but I will.

Waveform from Coleman PowerMate 6250 NON Inverter Generator

An uneducated WAG on this waveform would be that the rotor and stator have less magnets and coils than better generators might have?

 

[Apothecary]

I may have solved the mystery.
Apparently my particular generator does not have an AVR.

Instead, it uses capacitors to help smooth out the power.
That said, I wonder if changing to different, larger, or just replacing the capacitors might help?

After all they are 14 years old (with about 120 hours use on them).

 

[JimB]

Interesting about the capacitors.
Unless they are electrolytic types, they are probably OK.

I have been examining the waveform displayed above, my best guess is that there are vot many low order harmonics (ie 120, 180, 240 Hz), but the wrinkles are due to frequency components at about 720 Hz.

JimB

 

[Apothecary]

Very interesting Jim.
I wonder if there's a way to add a filter of sorts to suppress those 720 Hz spikes?

Would larger capacitors help?