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Probing UPS inverter output with scope
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caspercba:
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 setup

MAINS (it think its 3-phase) -> big UPS (5-10 KVA) -> machine (COBAS 8000 Modular analytics http://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 image

NOTE: The UPS is running under 80-90 percent load

Following 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 1054z

I'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 job

Thanks for your help.

Gaspar
borghese:
Use a battery powered oscilloscope
Cliff Matthews:

--- Quote from: borghese on July 27, 2018, 02:22:45 pm ---Use a battery powered oscilloscope

--- End quote ---
?? 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 aqui https://www.paginasamarillas.com.ar/buscar/q/analizador-electrico/
agehall:

--- Quote from: caspercba on July 27, 2018, 02:04:12 pm ---Im a student in EE in my third year and I honestly have no idea how to do this.

--- End quote ---

DO 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!?
Fsck:
Please don't do this if you don't know what you're doing.
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