Probing UPS inverter output with scope - Page 1
Forum section: electronics / beginners

author: caspercba
author_rank: Newbie
timestamp: July 27, 2018, 02:04:12 pm
content: HI all,My father runs a clinical lab (blood tests etc) and one of the autoanalizers halts due to random failures in optical sensors (not the same sensor every time). We've checked everything we could, test the sensors, calibrate, and everything is fine. But having a failure in random sensors once a week seems that the issue is not the sensors but something wider like the power supply.It has been suggested by the technician that there might be a power quality issue. This is the setupMAINS (it think its 3-phase) -> big UPS (5-10 KVA) -> machine (COBAS 8000 Modular analyticshttp://www.cobas.com/home/product/clinical-and-immunochemistry-testing/cobas-8000-modular-analyzer-series.html). To get an idea of what it looks like, have a look at the attached imageNOTE: The UPS is running under 80-90 percent loadFollowing his advice, id like to have a look at the UPS inverter output and see how the output sine wave looks like.My test equipment:Rigol 1054zI've read quite a lot about how to probe mains with the scope, and im confused.Solution 1: Use a stepdown transformer: Im worried that this solution might modify the sine wave and smooth out the signal, hiding the real problem.Solution 2: Connect two probes (one to neutral) and one to hot wire, then use differential to see the wave.Solution 3: Unground the scope, use only one channel (Idont want to do this).Any hints suggestions are very welcome. Im a student in EE in my third year and I honestly have no idea how to do this. Ive already watched Dave's video about not-blowing the scope, but I still dotn know how to perform this jobThanks for your help.Gaspar

author: borghese
author_rank: Regular Contributor
timestamp: July 27, 2018, 02:22:45 pm
content: Use a battery powered oscilloscope

author: Cliff Matthews
author_rank: Unknown
timestamp: July 27, 2018, 02:45:37 pm
content: Quote from: borghese on July 27, 2018, 02:22:45 pm?? He should stay away from 3-phase mains and phone the company that originally installed these in the health lab. At very least, a call to rent a power quality monitor/logger from a science instruments company could have been attempted.. Interesting the UPS model was not given? Pues bien, te deseo suerte con las cosas electricas - saludos de Canada (sin ofensa ninguna).Me ciento culpa dandote nada, talvez aquihttps://www.paginasamarillas.com.ar/buscar/q/analizador-electrico/

author: agehall
author_rank: Frequent Contributor
timestamp: July 27, 2018, 02:54:05 pm
content: Quote from: caspercba on July 27, 2018, 02:04:12 pmDO NOT EVEN TRY TO DO THIS. Honestly, if you don't know how to do it and have to ask, this is not something you should be doing and you would probably not be able to understand the output anyhow.OT: Why is everyone and his brother always insist on plugging their scope probes into mains voltage to view waveforms!?

author: Fsck
author_rank: Super Contributor
timestamp: July 27, 2018, 02:57:56 pm
content: Please don't do this if you don't know what you're doing.

author: caspercba
author_rank: Newbie
timestamp: July 27, 2018, 03:26:25 pm
content: Thanks to eveyrone for their honest advices.The UPS model is missing but i will have its model/brand tomorrow probably.Mr. Swedish, don't worry, my brother is not involved, so calm down. Thanks for your constructive? reply.I don't think I will do the probing myself since it is dangerous but I wonder if the output of this ups , being of the online type ( fed 100% from batteries) could be proved with the probe ground .... In practice this would be like a battery powered device... Isn't it?Cheers, again thanks for the quick advice!

author: roberts.tech
author_rank: Contributor
timestamp: July 29, 2018, 02:46:08 am
content: I test inverters and ups units all week long at my company. I have a portable isolated Tektronix scope and I also use an old standard line operated scope on my bench. On the bench with the old scope I always make my measurements with a small 120 to 12 volt transformer to isolate the scope. I can see most of the problems right through this transformer such as distortion on the sinewave and the 40khz switching of the mosfet transistors if the output filter is not working well. For higher frequencies I have to switch to my digital isolated scope.Yes the output of the your ups is derived from the battery but then the normal wiring conventions will be followed from there and it will be just like line voltage.While it is possible to measure line voltage with a standard scope if you know what you are doing it is dangerous because of the lack of isolation. So I am not recommending it.Get on ebay a buy a used portable scope like the Tektronix THS720A and you will have no worries.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2018, 06:27:00 pm
content: I have no difficulty seeing the high frequency "steps" on the output of my sine inverters using a step down transformer to isolate the oscilloscope and this is where I would start just for safety. There will not be any power line disturbances which would affect something without also being visible through a step down transformer.A couple of times I set my oscilloscope to accumulate mode and monitored the output of an online UPS for days at a time but never detected any glitches which was as expected.

author: ferdieCX
author_rank: Regular Contributor
timestamp: July 29, 2018, 07:04:52 pm
content: Quote from: caspercba on July 27, 2018, 03:26:25 pmAlmost for sure, the output of the UPS is wired with neutral, one or three phases and ground. It also has a Bypass relay that allows to feed the load direct from the distribution line, automatically in the event that the inverter fails, or manually for maintenance purposes.The output of the UPS behaves like the distribution line of the power company.

author: Gregg
author_rank: Super Contributor
timestamp: July 29, 2018, 08:10:49 pm
content: Don’t lose sight of the bigger picture; that is the whole electrical power system from the utility power connection to the equipment that is intermittently failing. Look for poor connections all along the way. Check for voltage drop across breakers and especially transfer switches.There could be something like an air conditioning unit that sends a nasty inductive spike into the line ahead of the UPS. Just because they use the word “uninterruptable” in the name doesn’t mean they have covered all possibilities. If your father is running the lab, I’m sure he could have the UPS maintenance company check out the UPS. For hospital situations I’m sure regular maintenance is a requirement. UPS maintenance companies should have all of the testing equipment to safely test the high voltage output. Try to be there when they do maintenance and try not to be a pest; most technicians are willing to explain things to a student that shows the right interest and asks intelligent questions. Stay far back and quiet when they have the live high voltage open; respect the fact that there is zero tolerance for mistakes when working on live high voltage.Another possibility is something connected to the output of the UPS injecting harmonics or spikes into the line, not the fault of the UPS. What you really need is a power quality recorder instead of your scope.