Testing AC Line Quality with an Oscilloscope - Generator power quality

Thread Starter

Lumenosity

Joined Mar 1, 2017
461
I have an older analog Hitachi Oscilloscope V-1050F.

I want to see the sinusoidal waveform for several power generators I have.
I've watched probably a dozen YouTube videos on this but am still not sure.

If I use an isolation step transformer, will that change the waveform or just the voltage?
If I must use a transformer, can you suggest one appropriate for this?

If my probes have a 10X feature, why can't I just set the probes to 10x and check the voltage directly at the AC outlets?

This is directly from the User Manual for this Oscilloscope.....
Do not apply an excessive voltage. The input withstand voltage of each in put connector and probe input is as follows. Never apply a voltage higher than specified.
INPUT direct 250V(DC + AC peak at 1 kHz)
When probe is used 500V(DC + AC peak at 1 kHz)
EXT TRIG INPUT 250V(DC + AC peak)
EXT BLANKING 20V(DC + AC peak)



( If I do not connect anything to the ground terminal, should it be safe to just use the Hot and Neutral ? )
 
Last edited:

Danko

Joined Nov 22, 2017
798
Yes, it is easy and safe.Only double check both probes are in X10 state.
And ground clips should be removed or not connected to anything.
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
461
Ok, after checking the generator(s) in question......(VOM, generator NOT running)

They ALL have continuity between the chassis and the GROUND PIN of the outlets only.
None of them have any continuity between the chassis and Neutral or Hot

These are all portable generators not connected to any other equipment.
 

Danko

Joined Nov 22, 2017
798
For your measurements it does not matter, connected ground pin of outlet to Neutral, or not.
It is better to use mentioned method for safety.
EDIT:
In previous post I talked about GROUND clips of probes.
 
Last edited:

DickCappels

Joined Aug 21, 2008
5,895
You can get a good idea by just looking at the Line voltage, but you might get a better picture by connecting channel A to Line and channel B to Neutral, both using X10 probes and connect the ground clips to earth. Set the vertical mode of channels A - channel B and there you have it.

Transformers add distortion, though if you are lucky the distortion will be too small to bother about, particularly if the transformer is not loaded.
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
461
You can get a good idea by just looking at the Line voltage, but you might get a better picture by connecting channel A to Line and channel B to Neutral, both using X10 probes and connect the ground clips to earth. Set the vertical mode of channels A - channel B and there you have it.

Transformers add distortion, though if you are lucky the distortion will be too small to bother about, particularly if the transformer is not loaded.
Connect the Ground Clips to Earth ???
 

Danko

Joined Nov 22, 2017
798
Doesn't this go directly opposite of the advice given in post #4 ?
1. Only case of oscilloscope should be connected to ground.
2. Ground clips are destined for delivering ground potential from oscilloscope case to circuit under test.
3. For floating technique (A-B channel) ground clips not used. Even oscilloscope case may be not connected to ground. It absolutely does not matter with differential measurements.

See examples in post #18. In both cases ground clips are floating.
 
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Reloadron

Joined Jan 15, 2015
4,517
Below is a very basic diagram of what most low power (Below 10 KW) AC generators look like.
Gen Basic.png

Most I have seen or worked with have their frame ground and neutral bonded including 120 and 240 VAC units. Apparently the units you have do not.

Ok, after checking the generator(s) in question......(VOM, generator NOT running)

They ALL have continuity between the chassis and the GROUND PIN of the outlets only.
None of them have any continuity between the chassis and Neutral or Hot

These are all portable generators not connected to any other equipment.
You are only looking at the generator output correct? The oscilloscope is not powered at all by the generator(s) so with respect to the scope the generator is totally floating correct?

Something you may wish to consider when looking at generator power AC line quality is using a 4.0 KW generator as just an example. The power quality with respect to distortion (harmonics, hum and noise) will not be the same for an unloaded verse a loaded generator. In other words a nice clean looking sine wave on a 4.0 KW generator as in the above schematic unloaded may not be the same nice clean sine wave with a for example 3.0 KW load on that same generator. The best test results as to power quality should be done with the generator under a good load. Electric heaters work well enough.

Anyway, just things to consider.

Ron
 

Thread Starter

Lumenosity

Joined Mar 1, 2017
461
Here is the result.
A few notes.
1). I attempted to check the house AC first.
2). This connection was to a multi outlet (cheap) adapter (Not directly to a home outlet)
3). The probes did not have to be unsheathed....this signal was generated when the probe was placed within 1 mm of the hot or neutral.
4). The probes were set to 10X

sinewave1.jpg
 

Reloadron

Joined Jan 15, 2015
4,517
Placing the probe in close proximity to the AC source is not good enough. The probe needs to be connected directly to the source. You can use either method suggested as to using a single or both channels. You can get the same results by just placing a finger tip on the probe tip and increasing the vertical gain of the scope. You don't mention the scope's time base settings or the vertical gain setting but what you see pretty much looks like general 60 Hz noise.

Ron
 

crutschow

Joined Mar 14, 2008
23,370
Picking up the signal without a direct connection will also pick up a lot of noise, as your display shows.
Direct connection will likely show a cleaner waveform.
 

Reloadron

Joined Jan 15, 2015
4,517
Here, I just connected my scope to my AC mains. Using a 10X probe with probe tip connected to Hot and ground connected to Neutral here is what I get:
Scope AC1.png

Vertical Gain set at 50 Volts Div using 10X probe. The actual AC value is 123 VAC 60 Hz RMS. So 123 X 1.414 = 173.922 Volts Peak and twice that equal 347.8 VAC PK to PK. Horizontal time base is set for 5.0 m/Sec Div. Pretty much what we should expect to see.

Now here is the same AC line signal with my probe tip in close proximity to the AC line Hot.

Scope AC2.png

The vertical gain is down around 2V/Div. The signal is far from a nice clean sine wave and the probe tip is merely picking up 60 Hz garbage. The probe needs direct connection to the source. Even then the sine wave reveals little. Tomorrow I can try and fire up the old 4 KW unit I have in the garage and see what it looks like but I can pretty much guarantee it will look like the above and there is no way to really discern any useful power quality numbers using a scope.

That or I can do a whole house generator test on a much larger generator and automatic transfer and see what thet power looks like on the scope.

So when you say power quality how would you define that? What exactly are you looking to see or find?

Ron
 

crutschow

Joined Mar 14, 2008
23,370
If it looks like a decent sinewave with no noticeable distortion, then it should be okay for mains power usage.

You are looking for significant distortion, such as the peaks appearing flattened, or looking more like a square-wave than a sinewave.
 

DickCappels

Joined Aug 21, 2008
5,895
The reason the distortion shows up when capacitively coupling to the line is that you are forming a high pass filter + a little bit of capacitive voltage divider, making the harmonics stand out.

By the way, I believe most electrical codes do not permit the Neutral to be connected to ground at more than one place, which is why connecting it to your scope's ground is a poor idea. Add to that the possibility that some current might try to return to earth though your probe's ground lead and you create a safety risk for your scope and you,
 

Reloadron

Joined Jan 15, 2015
4,517
The reason the distortion shows up when capacitively coupling to the line is that you are forming a high pass filter + a little bit of capacitive voltage divider, making the harmonics stand out.

By the way, I believe most electrical codes do not permit the Neutral to be connected to ground at more than one place, which is why connecting it to your scope's ground is a poor idea. Add to that the possibility that some current might try to return to earth though your probe's ground lead and you create a safety risk for your scope and you,
That brings up something interesting Dick. Generators aside. Ground bonding to neutral in my residence is of course done at my service entry. OK, with that in mind, anytime I use my older Tektronix 2235 / AN/USM-488 and clip my probe ground clip onto a chassis using an old tube type radio or any other grounded chassis appliance aren't I effectively creating another bond to earth ground? The ground connections are tied to earth ground in the scope. The same is true of a few other scopes I have laying around. I really never gave it any thought till I read your post. The same is true of some other test, measurement and diagnostic equipment I have laying around here.

The NEC does mention in 250.24:
Service equipment supplied from a grounded system must have the neutral conductor grounded.

Again, I just never gave it any thought till you mentioned it.

Ron
 
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