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Snubber for a 3kVA 240V to 110V transformer

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  1. Jun 13, 2016 #1
    Hey... how's it going?

    I've built a step down box to run 110 VAC appliances off incoming 240 VAC (Malaysia). The only components in the box are a cooling fan (240 V) and a great big transformer. It's a single phase 30A output aluminium cored 240V->110V 50 Hz transformer that weighs about 25 kg.

    I have no idea at all what the inductance of the transformer is and I have no idea what sort of an inductive kick it'll give if the incoming 240 VAC is interrupted for any reason... but I feel I really should have some kind of protection. This is especially important since it's running expensive scientific equipment as well as a PC.

    Do I need kickback protection? How should I implement it? Should I use a MOV or a TVS or an RC snubber? Or a combination?

    Would anyone be able to recommend some specific components... or maybe just indicate some rough specs?

    Thanks!

    - Leo
     
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  3. Jun 13, 2016 #2

    berkeman

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    Welcome to the PF.

    I would think that as long as you used surge-protected 110V power strips at the output of your step-down transformer, that your devices should be reasonably well protected... I would also put a switch and fuse/breaker in the Hot lead at the power input to your transformer.

    http://ep.yimg.com/ay/fireadesource/industrial-6-outlet-power-strip-with-surge-protector-6.jpg
    industrial-6-outlet-power-strip-with-surge-protector-6.jpg
     
  4. Jun 13, 2016 #3
    Hi berkeman,

    I can't get 110 V power strips where I live... we only have 240 V kit here. Also, from reading reviews, surge protection in power strips seems a bit hit and miss. Some are good, some fail silently and leave you without protection. There's also the issues that one bit of equipment draws up to 20A and needs a dedicated (fat) socket and also that I've already got some nice sockets for the rest. Circuit breaker? Well, effectively, there is a breaker on the hot side of the transformer... because it's on a dedicated 20A circuit and there's a breaker in the distribution panel.

    Are there some discrete electronic components I could use to do my own surge/kickback protection? MOVs? TVS?
     
  5. Jun 14, 2016 #4

    anorlunda

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    If your scientific equipment is so expensive, then it deserves decent protection better than consumer grade or home made. I suggest a commercial/industrial surge protection device from Allen Bradley, ABB, or whoever supplies such stuff in Malaysia. MOV or TVS devices are the correct way to go. Those consumer devices probably use MOV or TVS. But the device needs to be properly engineered; I would not recommend a home brew because your equipment is expensive.

    But think also of personnel safety. In case of a low side short circuit, that transformer could destroy itself, and it might draw so much current that the 20A breaker could not interrupt it Put a container of some sort over the transformer to prevent molten metal droplets from spraying around the room. An automatic fire extinguisher in the room might also be prudent.

    Also for a simple backup, on the low side two pennies separated by a piece of tissue paper will reliably break down at 1500 volts, and can be made to sink very large currents when connected to the source via a fused connection. (Don't forget the fuse or you will create the kind of short circuit that destroys the transformer.). 1500v won't save your electronics, but it could prevent something worse.

    .
     
  6. Jun 14, 2016 #5

    jim hardy

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  7. Jun 14, 2016 #6
    Hi anorlunda. I'd prefer a DIY option since it'll be a lot cheaper, and surge protectors are designed to protect against infrequent surges, not regular inductive kicks. Good call with pennies but I think that shouldn't be necessary for the 110 V side since the transformer is isolating and the gigantic iron core should smooth any transient spikes coming in, right?

    Jim, thanks for the links. If I'm putting MOVs in parallel, should I spec some at 130 V and some at 140 V and then some at 150 V ? Or all at the same voltage? Do I take it you don't recommend TVS or an RC snubber?
     
  8. Jun 14, 2016 #7

    jim hardy

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    Actually the transformer will propagate a transient that's imposed on the primary on over to secondary .
    That's why i prefer those MOV's to RC snubber, they offer some protection against lightning coming in through the power lines.
    Staggering them is not a bad idea.

    An RC snubber would handle the inductive kick
    http://www.digikey.com/en/product-h...lier/quencharc-arc-suppressor-snubber-network
     
  9. Jun 14, 2016 #8

    berkeman

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    Interesting parts, thanks Jim! :smile:
     
    Last edited by a moderator: May 8, 2017
  10. Jun 14, 2016 #9
    I've done some quick calculations and it seems an RC snubber can generate quite a bit of heat. A 0.5 uF capacitor would limit 110 V @ 50 Hz to about 17 mA. This would make 1.7 W of heat in a 100 Ohm resistor. But the units come as a whole and they're specced for up to 600 VAC, so I guess heat dissipation is already designed for.

    As for the MOVs, I'll get a few at different breakdown levels and wire them all in parallel with the RC snubber and the transformer output.

    I'll go hunting today and see if I can find them.

    Thanks very much for your help, Jim!
     
  11. Jun 14, 2016 #10

    berkeman

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    You may want to monitor your 240Vrms source voltage for a while as well. If the variation in source voltage is more than the standard +/-5%, you will need to be careful to not use too low of a rating on the transient voltage limiter components. :smile:
     
  12. Jun 14, 2016 #11
    Good point... it's normally about 243 V on each phase.

    We did have a scary situation last month. During a storm, 30 second long surges came about once every 2 minutes for half an hour... lots of long lasting surges. My multimeter said 290 V, but the lights were flickering (brightly!) so I don't think it was a clean waveform. The AC went crazy too and it blew up a phone charger.

    I figure maybe one of the phases was arcing to neutral and the other two phases got a voltage boost. Sound plausable?
     
  13. Jun 14, 2016 #12

    berkeman

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    Yikes!
     
  14. Jun 14, 2016 #13
    Welcome to Malaysia...

    It's worse in Indonesia though!!
     
  15. Jun 14, 2016 #14

    jim hardy

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  16. Jun 14, 2016 #15

    jim hardy

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    Wow. Pick a fuse that'll open when that sort of voltage saturates your transformer.

    See also "overvoltage crowbar circuit"
     
  17. Jun 20, 2016 #16
    I bought the following three varistors (130 VAC, 150VAC, 175 VAC):

    http://my.element14.com/epcos/b72220s0131k101/varistor-74-0j-130vac/dp/1004282
    http://my.element14.com/epcos/b72220s0171k101/varistor-98-0j-175vac/dp/1004352
    http://my.element14.com/epcos/b72220s0151k101/varistor-85-0j-150vac/dp/1004289

    ... and this RC network:

    http://my.element14.com/roxburgh/xeb1201/rc-network-250v-0-1uf-flying-lead/dp/1187659

    All of them are wired in parallel straight across the 110 V side of the transformer.

    Jim, good call with the fuse. At the moment, it's on a dedicated circuit with a 20 A breaker. A 13 A fuse should be ok.
     
  18. Jun 20, 2016 #17

    jim hardy

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    Given the substantial size of your transformer
    i'd use several of those snubbers
    you want to absorb a goodly fraction of your magnetizing current for a fraction of a cycle
    that'll reduce how often the varistor has to do it
    notice CDE makes Quencharcs as large as 1 uf 22 ohms
    http://www.cde.com/resources/catalogs/Q-QRL.pdf

    overkill ? probably
    i mention it really just to get you thinking about what goes on during transients

    here's a fun sales video.
    It shows how a preposterously undersized snubber is ineffective (15 amp load and a 0.1uf snubber)
    but a varistor , as you are using, does the job
    http://www.arcsuppressiontechnologies.com/arc-suppression-facts/nosparc-in-action/
     
  19. Jun 20, 2016 #18
    Hey, Jim... I had no idea how to spec the snubber, so I chose whatever was in stock and affordable. If the MOVs absorb the turn-off kick, that's great. This kit is designed to run continuously so powerdown should only happen once every two weeks or so.

    I've put a 13 A fuse in series with the incoming 240 V. The inrush current makes a nice 'DOONK' sound but it doesn't blow the fuse.

    As long as I don't turn off too often, I figure I should be ok now.

    Thanks for the really helpful replies, everyone!
     
  20. Jun 20, 2016 #19

    jim hardy

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    About one time in ten you will power up right at the sinewave zero crossing and that's what gives the worst case inrush.
    Keep spare fuses. If you blow one occasionally that's okay ..

    old jim
     
  21. Jun 22, 2016 #20
    Here is a succinct statement of snubber design from a well-respected supplier. (It even matches the way I learned and used it.:smile:)
     

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