# Verifying Generator Output



## tvl (Nov 14, 2018)

This may not be the best place to ask this question, but here goes anyway:

I have a Honda EU3000is inverter generator. I realize Honda manufactures quality equipment and all is probably well, but I would still like to view the power output quality of my unit with an oscilloscope. Being able to get my hands on a scope, I would like to know the following:

** I really don't want to connect directly to the 120 volt lead at the generator with the scope as I realize this isn't completely safe for me or the scope. Therefore, I would like to utilize a step down transformer which would safely lower the 120 VAC to some lower voltage such as possibly 24 VAC. As a matter of fact, I have a 120VAC to 24 VAC transformer on hand.

My question is as follows: Whether the generator output be a PURE & CLEAN sinewave OR be DISTORTED & DIRTY, will the use of a step down transformer still provide me a true "picture" of output quality OR will the transformer mask or make things appear differently …………… whether it be for the better or for the worse?

Thanks!


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## Osviur (Sep 7, 2015)

Hi TVL,

The best method is to connect directly, via the test probes, better if they are attenuated 10 X, one to channel one and the other to the channel two, using the invert mode for one channel and the add function. Do not connect ground.

You may use a transformer too, in the previous way, but you will have to assume that this transformer is not designed to work in the saturation area, which is not a common condition.

Normally the primary winding create an alternating magnetic field in the laminated iron core that follows the current variations and this, the voltage variations from the input. The active section of this core can be magnetized in a limited way. If exceeded, the core is known as saturated and the magnetic field does not increment as the current in the primary coil. This condition is reflected in the secondary voltage as a flat topped senoid and this is what you will see in the oscilloscope, not the original curve. 

Another inconvenience is that many high speed transients or fast spikes cana not be followed by the magnetic field and don't pass to the secondary. The transformer inductive reactance acts as an undesirable filter. 

A simple resistive circuit can be done wit three resistors connected in series, may be incandescent lams (now, difficult to find) and take the signal in the intermediate lamp terminals.Use the same technique (one channel inverted).

Hope this help you.


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## motormonkey (Jul 10, 2016)

If your scope has a 5 volt per division or larger sensitivity setting, and you have a 10x probe, you're well within the specifications for the equipment and the probe.


If the scope probe won't reach the contacts inside the generator's receptacle, you'll have to improvise some means of getting to the voltage. A hardware type replacement 120v plug with the cover removed will work well.


It should go without saying that nothing should touch the exposed connectors on the plug but the scope probe! I would suggest attaching the probe to the live connector with appropriate clips before starting the generator, and then starting the generator and observing the readings, hands-off.


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## Osviur (Sep 7, 2015)

Thank you motormonkey, very important your safety observations.


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## RedOctobyr (Aug 8, 2018)

Really interesting replies, thank you guys! I figured the best bet would be to measure the output directly, but I had no idea of the subtleties of a transformer, and how they would mess with trying to read it using the transformer's output. 

Unfortunately, I sold my "conventional" (contractor-style) generator before measuring its output with my scope. And I just haven't sat down to try this with my EU2000i, even though it would be an interesting exercise. 

Can I ask why to use two probe channels, along with a math function, rather than to use 1 probe with its ground clip? 

Is that to protect against the risk of scope damage due to accidentally connecting the ground clip to the hot wire, rather than the ground or neutral wires? 

I think I read somewhere about maybe a Honda inverter generator putting out 60V on the hot wire, and 60V on the neutral wire. I have not checked for this on mine (could just use a multimeter, I think, no need for a scope), so forgive me if I'm remembering incorrectly. But if that was the case, presumably that would be another reason to use 2 probe tips, and a math function, and *not* to use the probe's ground clip. 



Osviur said:


> A simple resistive circuit can be done wit three resistors connected in series, may be incandescent lams (now, difficult to find) and take the signal in the intermediate lamp terminals.Use the same technique (one channel inverted).


Just to flesh out this comment a bit more, I believe this is to essentially build a voltage divider, and cut the voltage down to 1/3 the original value? If you connected (3) identical light bulbs in series, then each bulb would see 1/3 of the total voltage drop. So your 120V AC would now be measured as 40V AC, when measuring across the to leads of a given light bulb. The bulb wattages would need to all be identical, but I don't think the actual wattage values would matter (25W, 60W, whatever was available). 

In terms of oscilloscope input-voltage limits, I think 120V AC has a peak voltage of around 170V. So if your scope could take a max input voltage of 50V, hypothetically, you might want to use 4 light bulbs in series, not 3, to ensure you don't exceed 50V peak.


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## Osviur (Sep 7, 2015)

Hi RedOctobyr,

Your questions are really interesting, but I really didn't want to extend too much my Quick Reply.

For safety reasons, evident, the scope must be grounded permanently, the same for the generating equipment. This condition creates a potentially problematic condition: two grounding points connected via the probe grounding shield which is connected to the scope ground and then to the power outlet. Any difference in the potential ground, just a volt fraction, will create a current in the shield known as a ground loop, acting as a transformer, being the primary the probe shield and the secondary the signal carrier. the central wire. This 60 hz noise will distort the analyzed signal.

Another advantage is, as you observe RedOctobyr, the elimination of a possible, destructive short circuit if the ground clip is connected to a hot terminal with the generator ground connected.

Using this procedure I took the waveform coming from my Mosa generator 7 KW 220V 60 HZ on November 2017. I used two 10x probes invert and add functions in my old analog scope. 640 V peack to peak. This generator produces a really clean waveform.

With the lamps, also you are right RedOctobyr , for simplicity I mentioned 3 equal lamps but if it is not a problem, applying Ohm's law, is possible to calculate the voltage distribution using different power lamps, at more power (watts) less resistance, less voltage drop and viceversa.

A safer way would be using high value resistors (100K, 200K,etc), limiting the current to safe values, but I think that this is not the scope of this forum.

...And why not, use both methods: Direct and xfrmr, then compare results. 

I hope this clarify certain doubts. Thanks for your attention


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## RedOctobyr (Aug 8, 2018)

Thanks, that's great info! 

I'm certainly no expert, but the sine wave from your generator seems impressively smooth, to me. From what I can figure out, it is not an inverter generator. It would be interesting to see the outputs from other non-inverter generators, but that information doesn't seem to be very common.


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## Osviur (Sep 7, 2015)

Hi RedOctobyr,
Thaks for your thanks. My generator is an italian Mosa diesel generator, with a Ruggerini 420 cc engine and a 7 KW asynchronous generator 220 60Hz 3 phase output. It remains unused because here is very very rare a power outage and it only runs about ten minutes each month or so with a 6 KW load. Starts in the first turn, always. Here, ambient temperature never reaches less than 12° C (53° F) so it helps to a fast start. I bought it, used, 3 years ago and never has been used by me in a real situation and hope not to need it ever.

Regards.


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