Oscilloscope waveform - APC UPS - Page 1
Forum section: electronics / beginners

author: bb512
author_rank: Newbie
timestamp: November 08, 2017, 07:31:18 pm
content: I have a large APC UPS I'm trying to test. Looking at a scope, I wanted to see how the wave form looks while it is on battery. Using a decent scope (Hantek DSO5072P) I'm able to see that my house current is fairly clean, a little flat on the peaks of the sine wave, but ok.But when I switched to battery things got really strange. It seems that this model of APC UPS (DLA3000RM2U) cuts the ground leg in the power cord when on battery, and puts out half the voltage on the hot and the other half on the neutral (rear panel plugs). Yup, I get about 55V on the hot and 55V on the neutral outputs (not the power cord) while on battery. Other models don't do this. There is a FAQ at APC that says this is to prevent backfeeding.Anyway, I want to scope both output legs now to see the waveform. Probing the hot leg only shows a pretty ugly sine wave, and only 55V. I'm not sure the proper way to connect both probe channels to the unit while on battery. I assume from the APC article that while on battery, the earth ground is disconnected. Should I just put one probe each on the hot and neutral without the ground clips on the chassis? Or both ground clips on the chassis? The DSO is new, still learning how to set it up. Any help would be appreciated!!

author: gardner
author_rank: Regular Contributor
timestamp: December 08, 2017, 07:26:22 pm
content: I just had an issue with my 25-year-old UPS and had to replace it, which got me hunting around for ideas and I found this thread.I am curious how you've gotten on with your project. If you have not yet blown up your scope, I would recommend that you look at Dave's video on this.I think you would want to run your UPS via an isolation transformer and use a differential probe on the scope to get a proper picture of mains-related stuff on your scope.I have not heard of any inverters doing what you describe. It is a safety requirement that earth remain continuous throughout. If you are simulating an electrical outage by pulling the plug, then obviously the earth is disconnected. Again, I am not sure if this applies to UPSs, but inverter transfer switches must disconnect BOTH hot and neutral from the mains when transferring, and simultaneously connect (bond) the output neutral to the earth/ground. It would be a serious problem to generate any significant earth referenced voltage on the neutral.If you are measuring the waveforms on your scope relying on the mains-earth-reference inside the scope, the strategy that the load transfer uses for neutral bonding may make things look weirder than they really are.

author: A Hellene
author_rank: Frequent Contributor
timestamp: December 08, 2017, 07:45:27 pm
content: Here you are, bb:The operation ofAPC UPS families, a couple ofAPC schematicsandan opinionthe on APC acquisition by Schneider Electric.-George

author: Electro Detective
author_rank: Super Contributor
timestamp: December 08, 2017, 10:13:04 pm
content: To see that weird 110 volt UPS generated waveform sort of 'safely' between the two line pins, try Differential Mode using both scope channels and both probes at 10x,without the alligator/croc/ground/earth/BANG! clips connected> remove the suckers and put electrical tape over the area so nothing can touch the probe shieldsThe scope sensitivity should be at 5v per division, or more by backing off the CAL fine tune,so with the probes set to 10x you see 50 volts or more per division on screen.IIRC with Smart APC units, the two 55v legs may/should be coming off the transformer outputs and isolated/fully floating, with either some or no reference to earth/ground/neutral at your house switchboard,AND OR a centre tap connection on the UPS output transformer.A decent TRMS multimeter set to Autorange AC or 300 to 600 volts AC manual range(and GLOVES and protective EYEGLASSES) can tell you a lot about this arrangement before diving in looking for DSO eye candy with scope probes.The old school APC Smart series power quality is usually a decent sine wave with good peaks and may actually be better than the usual flattish (?!) incoming mains,have tested with sensitive audio and video gear in the past and no problem.It's been a while since I played with UPS units, so please do your homework/research first before zapping something based on any heresayGood luck and play safe,best policy for you, your scope... and the pocket

author: xani
author_rank: Frequent Contributor
timestamp: December 08, 2017, 10:24:41 pm
content: Quote from: A Hellene on December 08, 2017, 07:45:27 pmHuh, that explains a lot, our pre-aquisition (~15 years old) units are chugging along nicely, we just replace batteries every few years

author: Electro Detective
author_rank: Super Contributor
timestamp: December 08, 2017, 11:48:54 pm
content: Same here, fit new batteries, check the charge voltage and or software settings, load test/calibration = back in businessEdit: a fan clean, dust off, connectors/components check, visual inspection,and perished rubber feet upgradehelps keep these vintage beasts kicking goals on power outages