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Thread: Capacitors on transformer secondary

  1. #1
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    Mar 2005
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    Capacitors on transformer secondary

    We've got a dual-conversion split-phase(120/208 transformerless) Powerware 9170+ 18 kVA UPS that is feeding an isolation transformer (208 to 120/240)configured as an SDS. The transformer was added in an attempt to eliminate common-mode noise from the UPS (may not be an issue in a server farm, but a VERY BIG issue in our application, with includes 15-20 radio broadcast and production studios). While the common-mode noise is dealt with quite nicely in this arrangement, there's still objectionable high-frequency noise showing up on the secondary of the transformer, noise that is quite evident in our low-level, high-gain audio circuitry.

    Ideally, PowerWare should clean up their inverters and filter them better... but of course, we have to get them past the first step (admitting they have a problem) before we can get them to work the program . Based on the discussions to date, I don't see that happening in the near future.

    I know the best way to filter this noise is with a good EMI filter between the UPS and transformer. I also know that I've run out of budget, so I'm experimenting with a few alternatives. I've found that a couple of 30-60 mfd capacitors from the secondary legs to ground do a very credible job of shunting the objectionable noise to ground and reducing it to an acceptable level. I've thought this through and can't think of any problems that would be created by these caps, but then again I don't deal with capacitors on power lines all that frequently, so I may be forgetting something.

    Any reason why these two capacitors on the secondary could create a problem?

    Thanks!

    D.

  2. #2
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    Re: Capacitors on transformer secondary

    30 mfd sounds a bit large to me. They must be non polar or they would have exploded, right?

    Is this noise in the audio range?

    You might try ferrite cores slipped over each conductor. This adds a bit of inductance in series. If Radio Shack doesn't have the cores, you can find some in a defunct light dimmer. Or, you might try an iron or steel nut to see if that helps. Such an inductor upstream from the caps ought to help.

    Be forewarned though that the current level may be high enough to saturate the cores in which case they become useless.

    [ April 29, 2005, 10:40 AM: Message edited by: rattus ]
    Don't mess with B+!
    (Signal Corps. Motto)

  3. #3
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    Re: Capacitors on transformer secondary

    Originally posted by rattus:
    30 mfd sounds a bit large to me. They must be non polar or they would have exploded, right?

    Is this noise in the audio range?

    You might try ferrite cores slipped over each conductor. This adds a bit of inductance in series. If Radio Shack doesn't have the cores, you can find some in a defunct light dimmer. Or, you might try an iron or steel nut to see if that helps. An inductor upstream from the caps ought to help.
    The caps I'm using are oil-filled caps designed for AC applications (I'm smart enough to not try putting an electrolytic across a power line!)I actually started with much smaller capacitor values, and when they had no effect I stepped out back and yanked a couple of 30 mfd @ 660VAC caps from a defunct Ferrups.

    I haven't determined the exact frequency range of the noise -- my "good" oscilloscope is out at one of my sites and I haven't gotten there yet. The 'scope I'm using still lets me see it, but isn't really well calibrated. By speeding up the trace and increasing the gain, you can magnify the slope of the sine wave and see the noise quite clearly. It's not in the RF range... if it were, smaller caps would have more effect than they do. There's a noticable difference between 15 mfd, 30 mfd, and 60 mfd (more cap = less noise), which tells me it's probably in the same range as the audible noise made by the inverters. Of course, it's hard to listen to the noise when it's riding on a 60 hz sine wave! I'll be looking at it with the "good" 'scope today and make a better determination.

    One way or the other I don't think it's high enough in frequency to benefit from a ferrite -- a 60 hz transformer would be pretty amazing if it could pass RF unattenuated!

    D.

  4. #4
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    Re: Capacitors on transformer secondary

    Essentially you're building your own filter.

    Add an inductor in series to your capacitor in shunt and the response curve will increase from 6 db per octave to 12 db. The cut off frequency has to be calculated though.

    I tried to find a good web site for you (and there are a few good ones) but it's getting harder all the time to find things. I found a lot of coffee filter sites.

    Try AC filters, or second order response and stuff like that.

    Edit: If you're primarily suffering from high frequencies your filter circuit wont need to be as massive as it would if you were after lower frequencies.

    Edit: And I can't spell.

    [ April 29, 2005, 02:14 PM: Message edited by: physis ]
    Sam, San Francisco Bay Area

  5. #5
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    Re: Capacitors on transformer secondary

    Physis:

    Stop worrying about your spelling. Your making me self conscious (I had to correct two words already. What would I do without spell check?)

    I would have thought your high end audio equipment would have eliminated that noise in the DC power supply. After all, it eliminates the 60 Hz noise (at least it should!! Maybe some of the 60 Hz is getting rolled off by the broadcast amp??) So why won't it eliminate the higher freq?

    Is your transfromer sheilded?

    The only problem I can think of (and I'm not sure it is a problem) is what is the resonant frequency of the transformer and capacitor, and what would happen if this resonant frequency were to somehow appear in the circuit?

    Steve

  6. #6
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    Re: Capacitors on transformer secondary

    Steve, you have to look at low pass, band pass and high pass filtering.

    It's pretty common to have some level of low frequency descrimination for 60 Hz. Having an issue with higher frequencies will be somewhat outside of the typical concern.

    Ever run a guitar amp at max gain from a generator? Blueckh.

    I know, I'm sick of writing "I can't spell" too.

    Maybe I'll replace it with "error A", it would be the same though.

    Edit: Error A

    [ April 29, 2005, 03:39 PM: Message edited by: physis ]
    Sam, San Francisco Bay Area

  7. #7
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    Re: Capacitors on transformer secondary

    And inverters and stuff? I'm sure they're not making sine waves.
    Sam, San Francisco Bay Area

  8. #8
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    Re: Capacitors on transformer secondary

    Actually, this one makes pretty good sine waves. It oughta for the price. Of course, it also oughta make CLEAN sine waves at this price...

    D.

  9. #9
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    Re: Capacitors on transformer secondary

    I'm sure the load is large enough to make your filter pretty "honkin'" But if you have a sufficient understanding of filter design you can save some "honkin'" dollars too.
    Sam, San Francisco Bay Area

  10. #10
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    Mar 2005
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    Re: Capacitors on transformer secondary

    Oops. I thought it looked different today -- and as it turns out it did. Somebody turned on the breaker for the fluorescent lights -- there's one fixture in each studio on the UPS panels, going back to the days before we had a generator and were running longer-term on battery. These days, we only need anough battery to get the genset running and switched over.

    Of course, those fluorescent fixtures add their own little collection of noise. With those off, I can see more clearly what the UPS is generating. The noise from the UPS is in the 10-20 kHz neighborhood, and definitely responds well to the 30 mfd caps. Actually, so does the noise from the fluorescents.

    D.

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