When a severe windstorm took out power here for five+ days, I had to run critical household circuits (refrigeration,lighting and a few convenience circuits) on a 8KW portable generator. The UPS power supplies I have on my PC’s whined about low voltage while on gen power, even though two different high quality, true RMS DMM’s indicated there was 117 ~ 119 vAC present. A Sure Test analyzer, however, indicated 106 ~ 109 vAC. Testing was done at wall outlets and directly at the 120 vAC outlets on the gen itself, with identical results.
I’m a rank amateur at using a 'scope, and this may be the only time I’ll ever need one, so value pricing, simplicity and ease of use are primary objectives. I wish to:
determine if the waveform coming from the gen is anything close to a true sine wave, or a modified square wave, and,
what the peak-to-peak voltage truly is.
Id like to stay under $100.00 USD, in a perfect world under $50.00 USD.
Its disconcerting how many false and inaccurate readings have been reported across forums concerning power measurements. This forum is absolutely riddled with them. I too would be interested in an economical method of accurately analyzing VAC being supplied by a generator, inverter or UPS. Also of interest would be to measure THD at the wall socket as current load is increased on the device supplying the power. Reading the buyer ratings on amazon, most of the inexpensive analyzers sold appear to be inaccurate junk when checked against expensive commercial analyzers. I gave up at that point but need to address this sooner or later.
Would you please get back to us and let us know what you find? Even if its a link to another forum, that would be helpful to more than a few of us who aren’t electrical engineers or gurus!
My experience trying to run UPSes on a generator is that most generators have an unstable or slightly out of range frequency, which the UPSes don’t like. A double conversion UPS (converts input AC → DC → back to AC for output) solves this (and basically all other power conditioning problems, and some can even convert 230->110V or vice versa) but is definitely on the expensive side. I’m probably going to get myself one for my desktop and servers after I move but a reasonable one is $600+ and they can easily reach $1000.
A line-interactive UPS will generally be fine +/- at the very least 10% and generally 20%; the one this computer is on can be set to transfer to battery above 144V as the highest setting and 88V as the lowest. That said, when I’ve been running stuff on a cheapish generator, it managed to fairly consistently keep it in the 110-120 range even under varying load (although the frequency was out of range and none of my 3 UPSes worked).
A decent inverter generator will let most UPSes run fine for less money, but are generally only 1-2kW, sometimes with the feature to connect two together, but generally that’s enough to run sensitive electronics with a larger generator for stuff that’s less picky.
As for scopes, don’t have one myself, but most people I know with them like Rigol, they’re fairly hackable as well in terms of unlocking features from more expensive models but that’s a rabbit hole I haven’t gone down (yet…).
Playing with mains is dangerous, especially on an oscilloscope,
since the ground is directly connected and will short if false connected!
You might use differential probes or buy a handheld scope
and use it with batteries, to be on the safe side.
Fortunately your 120V isn’t as dangerous as our 240V, so using a
cheap oscilloscope with a 1/10 or 1/100 probe and not connecting
the ground wire of the probe might work. I wouldn’t do this.
Multimeters:
If mains is generated a RMS multimeter might display incorrect values,
because the circuit or logic is designed for sine waves.
Taking a look at UPS and generator output is really handy to figure out whats going on.
And what may damage your equipment.
A scope is also great, with an inexpensive photo-diode circuit, for looking at flashlight PWM and duty cycle.
There are several inexpensive battery powered O-scops on Amazon.
I would strongly recommend a battery powered version.
Without a connection to the mains there should be no problems with inadvertently creating a short through the O-scope ground and sending the scope up in smoke.
Just don’t use one with the charging cable attached.
Here is one. Don’t know anything about it - just an example.
I appreciate all the replies. For those among us who are more well versed on 'scopes than I, could you please offer advice pro or con on the two small units tested by HKJ below specific to my basic needs:
I like the first one - but - it has a non standard probe connection.
But - it comes with a 10x probe - so it’s OK for 120v - 240v.
But a review on BLF by HKJ, a few reply’s that the scope failed.
The second one is tiny- like matchbook sized. Too small for my taste.
A few more I found on Amazon. I like Amazon for the easy returns the scope is a dud out of the box. Another Amazon scope
I’ve had a few generators over the years. A little over 2 years ago I got this Champion 100519 240v 5000w running, open frame inverter. Check this out! Amazon.com . I’ve had to use it for outages a half a dozen times. And I’ve taken it to a half a dozen job sites for two or four hours at a time. I highly recommend it. They do make a slightly larger one if you really need more power.
Just another thought. When you look at the output waveform - be sure to do it under a test load.
A few 100w incandescent bulbs, in parallel for a few hundreds(s) of watts, is easy to rig up and cheap to make.
I’ve had a few UPSs over the years that looked fine when not powering anything.
But the “simulated” sine wave went to hell when loading it at near the rated output.
For a test load use light bulbs or some other ?? that is pure resistive - No other stuff that might distort the output.
All the Best,
Jeff
Watch the EEVblog video for way more info than you need to know about not killing your scope (or yourself).
Basically with a battery powered scope, there is no scope ground to accidentally create a path - through the scope - from the generator / UPS to mains ground.
As long as you do not make yourself a link in a hot to neutral/ground - it’s safe.
With a mains powered scope - or a battery one with the charger plugged in - NEVER hook the ground connection on the probe to the neutral/return line on the UPS.
That’s the long slot in the120v socket.
If you do - there will be a loud pop and something is going to most unhappy.
Plenty of info on how to do this safely one the Web.
All the Best,
Jeff
The reason I mentioned the inverter generator is because in the end even if you discover , as you suspect, that your power is not clean enough, then what are you going to do? I have a 16year old 6500w surge generator that senses a load and steps up the idle and burns plenty of fuel and will power everything in the house, with some selective control. The problem is when it senses the load has dropped off some the idle steps down and the voltage drops to barely 110. It is loud, burns too much fuel and is heavy and was miserable to take to job sites. 10 years ago I got a smaller 120/240v 4000 starting 3500 watt running champion generator that was much smaller and lighter and burns less fuel and did just about everything I need in the house except run an electric water heater. I don’t think anybody makes a 240 volt capable generator at less than 5000 running Watts now. It turns out it wasn’t quite enough on some job sites. And I still had some issues with slightly low voltage at sometimes under light load when it would step down. The champion inverter generator I mentioned earlier does have an economy mode and it will idle down low when it senses a light load but the frequency and voltage never vary in the house. It is gasoline only, fairly quiet, really efficient under light loads, and manual start, although it starts very easily. It may not be ideal for everybody. At this point it serves me well. Clean power for everything whether it needs it or not. Edit, this is all North American power grid , 60 HZ.
What kind of generator are you using? Is it a whole house system like Generac. Or a honda gas powered. How much load was on it. How many kw is the generator rated at.
The oscilloscope is not going to do anything for you if you dont know how to use it or if you dont know what you are looking for.
BTW someone above said generators and UPS dont work well together. This is simply not true.
8,000w run / 8,500w start Generac portable. Tested both 120v AC receptacles @ the gen’s control panel, no load, so both legs (phases). Same results when testing 240v receptacle. Same results testing both phases at the transfer switch, no load.
I haven’t chased it further. It’s mostly a technical curiosity at this point.
I’m considering replacing the Generac with a Champion 8,000w portable unit that claims & is tested to deliver true sine output.
I do truly appreciate the responses & will update with any & all new developments.
If the Generac is old and left outside in the elements, may be good idea to replace it. These small engines need a rebuild after about 2000 running hours. Some a lot less.
Always garaged. Under a patio canopy when in service, so near-zero weather exposure. Hour meter shows 86.1, so barely broken in.
When I looked at the hour meter I saw I was wrong about power capacity; 8,000 run, 12,000 start.
EDIT: it’s 14 yrs. old, bought new. Model XP8000E.
Pressurized oil system with spin-on filter, so longitivity should be better than the unfiltered splash oiling motors ubiquitous to most portables. That should have little to no effect on the voltage discrepancies noted, though.
So let me ask you. What makes you think you have a problem? The UPS reporting low voltage? What is the voltage as reported by DMM? Are the computers on the same breaker as say a refrigerator or another higher current load?
