Output inductor buzzing in a high power UPS - Page 1
Forum section: electronics / repair

author: Pentium100
author_rank: Frequent Contributor
timestamp: December 15, 2019, 08:20:02 am
content: Hi,I have a Borri B8033 UPS rated at 40kVA. The UPS is working, but the output filter inductor of one phase is buzzing rather loudly, it probably is indicating some kind of problem. The noise is lower when the load is higher.My LCR meter shows higher inductance for that inductor, compared to the other two phases (it also shows lower Q at 10kHz).The driver module for the output IGBT shows signs of getting hot (electrolytic capacitors were kinda bad, the board itself is slightly discolored), but I have no idea which came first - something happened to the driver board and that messed up the inductor or the driver board is getting hotter because of the inductor (or it may be not related). The output filter capacitors are OK.The buzzing inductor measures:f,Hz L,uH Q ESR100 860 6 01k 790 12.5 0.3910k 706 7.69 6.76100k 510 2.54 126Another inductor measures:f,Hz L,uH Q ESR100 730 5.8 01k 680 11.67 0.3310k 615 9.05 4.25100k 470 2.83 103.8The third inductor measures:f,Hz L,uH Q ESR100 737 5.79 01k 662 13.2 0.3310k 600 8.7 4.26100k 451 2.75 103So, the buzzing inductor has much higher inductance than the others. It looks exactly the same, so I think that there is some kind of fault.What kind of fault could cause the inductor to buzz and to measure higher inductance value?

author: wraper
author_rank: Unknown
timestamp: December 15, 2019, 08:31:37 am
content: If you are measuring this in circuit, measurement difference probably is caused not by inductor. I suggest looking for faulty film capacitors.

author: Pentium100
author_rank: Frequent Contributor
timestamp: December 15, 2019, 08:37:26 am
content: Quote from: wraper on December 15, 2019, 08:31:37 amI measured the inductors out of circuit, well, with the output module removed and the inductor still in the chassis, there could have been some sense resistors, but I do not think so.

author: wraper
author_rank: Unknown
timestamp: December 15, 2019, 09:42:07 am
content: Quote from: Pentium100 on December 15, 2019, 08:37:26 amSwap 2 inductors and see how it works.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 15, 2019, 07:10:30 pm
content: Quote from: Pentium100 on December 15, 2019, 08:37:26 amAre the output capacitors part of the output module that you removed? I'm not familiar with that specific model UPS but a lot of UPS that size that I've worked with, have the film capacitors mounted in the chassis usually in close proximity to the output inductors.It would be very unusual for an inductor to increase in inductance, in fact I don't really see how that would be possible. The more likely failure scenario would be a decrease in inductance as the result of failing insulation on the windings causing shorted turns.

author: Pentium100
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:38:27 pm
content: Quote from: TheMG on December 15, 2019, 07:10:30 pmYes. Everything is on the module, only the input and output inductors are on the chassis. I measured the output capacitors and they measure fine.I've been wondering what can cause the increase in the inductance as well, or at least the inductance to be measured higher. Maybe the core is a bit loose? But then an airgap would probably decrease the inductance, not increase it. It also could be that this inductor was always higher inductance for some reason, but it's too much of a coincidence IMO.The inductors have air flow cooling them, but still get quite warm. The buzzing one did not seem to be hotter than the other ones (then again, I checked this some time after shutting the UPS off). I waited for them to cool down and then measured again, with the measurements staying pretty much the same.I wonder how else I can test them. I will try swapping two of them and see if the buzz stays with the inductor.

author: Pentium100
author_rank: Frequent Contributor
timestamp: January 02, 2020, 07:29:37 am
content: I managed to remove the inductor and found out that a couple of core laminations are loose and can move a few mm (see attachment). The coil former (not sure about the term, the plastic part with the winding) can also move up and down on the core.I guess now I have to figure out how to glue the inductor so that its parts do not move.That inductor is rated at 700uH, but because of the loose lamination, it measures as higher inductance. If I put the lamination back to its place, the inductance drops.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2020, 04:30:00 pm
content: Heh, that'll do. Hold it in place and squirt some superglue under it?Tim

author: Pentium100
author_rank: Frequent Contributor
timestamp: January 02, 2020, 08:08:12 pm
content: I used epoxy. Glued the few places where I saw the laminations were loose, put the inductor back in the UPS and now it's silent.