Has anyone ever built a Scott-T transformer ?

Discussion in 'General Electronics Chat' started by gte, Jul 14, 2010.

  1. gte

    Thread Starter Active Member

    Sep 18, 2009
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    I need to build a step transformer (or buy one) for my automotive lifts. The lifts are 3 phase 240 and my house of course has 2 phase 208. I've seen the solid state transformers and the rotary ones, but it looks like a Scott-T transformer would be ideal for me ... if not I will probably purchase a rotary one because of the efficiency.

    Thoughts, designs, ideas?

    Thanks!
     
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I don't suppose upgrading your electrical hookup is an option?

    That should be the best, simplest, and in the long run possibly the cheapest solution.

    Do you do much arc-welding?

    That would be another good reason to upgrade your hookup. If you are doing any amount of Oxy-Acetylene brazing but not much or any arc welding, that might indicate that you could start gearing up for arc welding at the same time as you get a three phase connection put in.

    When you check on this - also make certain you check on the commercial power rates,
     
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  3. schmitt trigger

    Active Member

    Jul 12, 2010
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    I've done some low power (~100 watt) Scott-T mainly for experimenting purposes. Works like a charm, but since the turns ratio on one of the two transformers is completely non-standard, you'll have to find a shop who will build you a custom one.
    The phases must also be relatively balanced, which in your case should be of no concern.
     
  4. gte

    Thread Starter Active Member

    Sep 18, 2009
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    Hi, I've upgraded to 400amp, but 2 phase is all I can get.

    I do also have a tig and a mig welder, which is the other reason I went to 400amp.



    How did you find someone to build one of these for you? I believe the maximum amperage is 30 amps, so that'd put me around 7000 watts :)


     
  5. schmitt trigger

    Active Member

    Jul 12, 2010
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    There was a small factory that would make parts for appliance manufacturers. They had some coild winding machines and stocked magnetic steel and wire. The mandrel was not very large, and limited the coil size I could wind. Thus the small wattage rating.

    For the transformer calculations, I used Eric Lowdon's Practical Transformer Design Handbook. A must-have if you are interested in transformers and chokes.

    Unfortunately, a few years ago the plant was outsourced to the Far East and closed down.
     
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  6. gte

    Thread Starter Active Member

    Sep 18, 2009
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    It sounds as if it may be easier to just buy the rotary 2 to 3 phase converters?


     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Ahh! Power upgraded already but they wouldn't provide a three phase hookup. That is pretty common. My Father had a similar problem with his garage. Unless you are near a major power routing, or in a business park they won't talk about hooking you up for something like 3 phase without paying a serious deposit to cover an engineering assessment = non refundable if they end up quoting $150000 to upgrade your power connection and you tell them thanks alot but no.

    I might be wrong but when you talk about the rotary converter you are talking about a 3 phase alternator that is powered by a 2 phase motor?

    Efficiency may be higher but I would first check with the manufacturers for the lifts to see if they can provide or spec 2 phase motors and adaptors to modify the lifts.

    Instead of 2 phase motors to power 3 phase alternators to power 3 phase motors - it would be nice to just replace the 3 phase motors.

    I am asking with a likely response being that this is already checked.

    2 phase 208VAC motors of the same power as 3 phase 240 VAC could be about twice as big and 3 or 4 times as heavy. They might have to fit in a space where they just wouldn't fit and if they did fit they would create enough extra heat to reduce reliabity. Reliability even lower than that lost by just dropping a phase. 3 phase is industrial quality for a reason.

    The options might be to derate your lifts. If you bought lifts good for 10 tons it would be a shame to settle for never using them with more than 6 tons.

    But maybe the motor location is open enough that it will allow a 2 Phase motor as long as you don't mind tripping over it. (I have seen lifts that I am imagining yours are similar too, with adaptions for a big lump of motor that was probably 2 phase. I had no reason to take too close of a look but I can still picture it.)

    Now the only problem is warranty which is why I would go back to the local service agent for the lifts. It will be nothing new to them I bet. They might even offer you some trade in on the three phase motors. They will also guarantee that your warranty will be honored after modification.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    1) In my experience, getting 3-phase power in American residential
    areas is virtually impossible, and even at commercial locations (e.g., an international airport) it can be very expensive. Farm areas may be an exception, depending on many factors.
    2) I used a rotary converter for 25 years in a shop. Never any problems. My converter was simply a 3-phase motor with a capacitor and voltage-controlled relay to get it started on single-phase 220v. The converter was 5 HP; the largest driven motor was 2 HP. I used a newer Baldor motor (so I could lift it), but was told by several sources that an old, heavy-iron, 3-phase motor would be better. Those are available at scrap yards.

    John
     
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  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Should have checked first. (very bad habit sorry.)
    Basically running an Idler 3 phase motor on two phase with motor capacitors to tie the two powerconnections into the third phase to create a shadow power feed. Then parallel working motor to the idler.

    A 3 Phase rotary power converter seems like a good idea but it just wasn't what I thought you were talking about.

    I learned something so I'm happy.
     
  10. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I can't help wondering what the power company would make out of this - what impact it has back into the lines because you are pulling your two phase from 180 degrees to 120 degrees phase angle. It seems a bit risky but seems is not saying that it is.

    It might show as lower than actual power at your meter if that matters.
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    That is true, and modern ones probably will include VFD drive, which is really nice to have. Nevertheless, for driving a 2HP or greater mill and/or other equipment, a rotary converter made from an old heavy-iron motor (or even a more modern used motor) will probably be cheaper, unless you need to ship the motor somewhere.

    BTW, the hypothetical PFC issue was a non-issue. My unit was installed at a commercial site and never caused any question. It was at that aforementioned international airport (RST) in the USA.

    John
     
    Last edited: Jul 15, 2010
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    I worked at BWI. Now that was an experience in power.
     
  14. gte

    Thread Starter Active Member

    Sep 18, 2009
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    Hey Retched,

    There are (2) 6000 pound lifts, one will probably be replaced with a 10000 lift later on.

    I looked at the solid state ones, the problem is that they just simulate the 3rd phase for a short time period while the motor first starts and draws a ton of amperage, then they drop the 3rd phase and only provide 2 phase resulting in a lower weight lifting capacity.

    I'm guessing the rotary converter is going to be my best option at this point. I'll just put it up in the rafters, maybe build an enclosure for it if it makes a lot of noise?



     
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