Overloading a transformer

Discussion in 'The Projects Forum' started by kubeek, Jun 5, 2006.

  1. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Let´s have a transformer, with the core rated for 400W. The wires are rated for higher current, than normally would be, for example two times higher.

    What happens, when I load the transformer with a load that should take 600W, if the voltage stays. Will it deliver the 400W the core is reated for <- the voltage would drop, or deliver 600W with overheating the core?
    Or possibly something else?
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
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    It will work as long as all the smoke stays inside. When the smoke that it runs on escapes, it will stop.
     
  3. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Smoke from WHAT?
     
  4. radiosmoke

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    May 30, 2006
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    LOL from the transformer.
     
  5. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    Hey! Stop laughing...

    I meant from what exact part of the transformer, when the windindgs are made for the current. Understood?
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Well.. consider what would happen when a core that is designed for 400W operates at 600W and gets saturated. No more nice dI/dT.
     
  7. kubeek

    Thread Starter AAC Fanatic!

    Sep 20, 2005
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    can you be more specific? I never studied electronics as a school subject, so what does "nice dI/dT" mean?
    Does it mean the "voltage drops -> 400W at output" way, or the "core starts to heat" way?
     
  8. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    The current would increase when the xformer tries to deliver 600W of power, which would cause the core to saturate when the maximum magnetic flux density is reached. Once the core is saturated and no more increase in magnetic flux is possible, the inductive property of the winding changes. Winding doesn't act inductive anymore, and starts to look resistive. The winding current, which was previously controlled by the inductive (mutual) of the windings, is now ony limited by the resistance of the winding copper wire. The current would increase very quickly (exponentially with constant voltage, instead of constant dI/dT), overheats the winding and sends smoke signal out.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    The smoke comes from the burning insulation which is overheated by the nice hot conductors. We colloquially refer to this as "letting the magic smoke out of the wires." It is considered bad form.

    A wise rule of thumb is to choose components rated at least 20% higher than the anticipated worst case. Some should be rated at least 100% extra. (Filter capacitors are one example).
     
  10. Erin G.

    Senior Member

    Mar 3, 2005
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    Cooling transformers can usually squeeze some extra current out of them. Take this example:

    13.8KV / 161KV step up (oil-filled) transformer supplying a city distribution grid. The city's load has increased over the years, and the voters have yet to approve a new power plant. The transformer is at max load, and on particularly hot summer days, it's in an overloaded condition. Solution: Add cooling fans to the outer cooling fins of the transformer. The oil circulates using covection, and the fans increase the cooling efficiency. The result is that we've acheived another 3 to 5 mega-watts out of the transformer (above and beyond it's rating) without "letting the smoke out".

    Though we have used this trick in a number of settings (usually 480VAC & up), it's pretty impractical for the smaller transformers I believe you're addressing, kubeek.

    Hope the info helps,...
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    You've got me curious now, Erin. How small of a transformer would be practical for oil cooling? One KW? A few KW? Is it a matter of cost & trouble versus percent increase?
     
  12. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Hey Gadget, I think they all missed the joke.
     
  13. Gadget

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    Jan 10, 2006
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    Yea. I get that at work a lot as well. A Technician amoungst Electricians.
    "It's hard to soar with eagles when you work with Turkeys"..
    (Although Turkeys rarely get sucked into Jet intakes....)

    (They're not a Bad lot really)... ;)

    BTW, is Transformer Oil Still full of PCB's...???
    I Know (in my roll as a moderate greenie) I nearly fainted when the Boss said he'd spread a heap of the stuff (waste xformer oil) over his driveway, and access road (dirt/gravel) to Keep the Dust down......
     
  14. Erin G.

    Senior Member

    Mar 3, 2005
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    Tranformers are rated in kilovolt-amps, or KVA. Though you can calculate the KW of a transformer, it's not a very practicle measurement of transformer load rating. I don't know where the exact thresh-hold between air- and oil-cooled tranformers is, but most of the low voltage (480VAC, 3 phase) oil cooled models I've seen have been in the 300KVA range and up, which comes out to about 361 full load rated amps.

    When you start getting into those kinds of loads, internal arcing becomes a concern. The oil acts as an insulator to prevent the coils from arcing against each other or the transformer case (enclosure), as well as acting as a cooler for the windings & core. There are fins installed on the outside of the transformer, and the oil circulates through the fins and case via convection. When the transformer loads up, the oil inside the case gets hot and begins to move towards the outside to the fins, forcing the cooler oil in the fins back into the case. When the transformer is loaded the oil is constantly circulating, removing heat from the case and dissipating heat through the fins.

    Putting fans on the fins significantly increases the effiency of the cooling, and actually "raises" the KVA rating of the transformer a bit. Since this topic has come up, I took a stroll through my facility and noted some of the KVA ratings on the tranformers. One name plate really got my attention. It's a 14KV to 161KV step-up / distribution tranformer, with fans installed on the fins by the manufacturer, G.E. There are two KVA ratings on the name plate: 78,000KVA for "self cooled" (fans off), and 96,000 KVA for "forced cooled" (fans on). That's quite a difference!

    Gadget: New transformer oils no longer have PCBs in them. However, since the PCBs have become such a concern, every oil filled transformer is required to have an information sticker stating "Contains PCBs" or "Contains no PCBs". Just last week I had to put a bunch of the No PCBs stickers on some of our transformers. Glorious work, hanging info stickers.

    BTW, I got the joke, just hadn't heard it in long while.
     
  15. radiosmoke

    Member

    May 30, 2006
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    Heres a link to the EPA on PCBs and transformers. After 1977 PCBs could no longer be manufactured. However those containing PCBs can still be used but come under EPA Regulations.
    Click here
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
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    When I worked at the shipyards, a crew of six of us spent two days pulling all the PCB containing lighting ballasts from a decommissioned oil tanker. Somebody's regulations clearly stated that she couldn't be towed to India and recycled into razor blades while PCBs were still in place.

    Erin - thank you for the information!
     
  17. kellys_eye

    New Member

    Apr 24, 2006
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    errrr, I think you mistake PCBs for PCBs

    who's gonna tell him?
     
  18. thingmaker3

    Retired Moderator

    May 16, 2005
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  19. Gadget

    Distinguished Member

    Jan 10, 2006
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    No, He's quite right. Many old Flouro Tube Ballasts contained PCB's and had to be replaced. There are still a lot of Flouro fittings out there with the old ballasts still in them.
     
  20. legac

    Well-Known Member

    May 4, 2005
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    That would be workable for a very short time. Otherwise the Xformer will be over heated.
    Cheers
     
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