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Looking for split-phase step-up transformer

  1. Jan 6, 2012 #1
    I am looking for a step-up isolation transformer (not an autotransformer) with an input of 120V and an output of 240V where the ground reference for the secondary is half way between the two hot line outputs so each is 120V relative to ground with opposing phase angles so they are 240V relative to each other. If you don't know how a 240V circuit is configured in USA, then you would not understand this need.

    Ideally this would be a small unit, under 200 watts, with a normal USA grounded plug (NEMA 5-15P) with at least some usable output receptacle (NEMA 6-15R preferred but others that work are NEMA 14-15R, IEC 60320 C13, or Schuko).
  2. jcsd
  3. Jan 7, 2012 #2
    If you are using both hot wires from the breaker box to get your 240v doesn't that just mean that you are not using the center tap from the power company transformer's secondary winding? I think the waveforms between the center tap and each end of the secondary winding are only out of phase insofar as any center tapped secondary would be. That is, you are simply looking at different parts of the same continuous wave simultaneously. It's not truly a multiphase system because there is only one wave. For genuine multi-phase I think the power company would need an extra set of windings for each phase.

    What is your purpose? Are you trying to convert pseudo-multiphase to genuine multiphase? It sounds to me like you are just describing a very common 120/240 volt transformer. You just need the 240 volt winding to have a center tap and many of them do. That will give you a choice between 120 volts and 240 volts from the secondary. In series you'll get 240. In parallel you'll get 120. Neither Mouser nor Ebay should have a shortage of those.
  4. Jan 9, 2012 #3
    Yes, the purpose is to have the 120/240 volt system. But, I have no intention to use the neutral to get 120 volts, as I already have that it just about every outlet. I want to get 240 volts. But I don't want the "euro up convert" that gives 240 volts relative to ground.

    I'd like this in a completely enclosed and portable box with a corded plug for the 120 volt side (NEMA 5-15P), and a NEMA 6-15R outlet I don't need more than 100 watts. I'd prefer it be pre-made like that instead of having to make one.

    I found only "open" transformers on Mouser. I want to avoid that. I could not find anything like this on Ebay. Very few transformers even had 240 volts of some kind. Everything that's in the form of an enclosed 120V to 240V step-up is for Australia/European type power (240 V L-N).
  5. Jan 9, 2012 #4
    If I understand correctly, you just need a 110V isolation transformer that has three identical winding. Use one as primary and the other two in series and ground the middle.

    Or find a 220V to 110V with center tap on the primary, or the other way around fine a 110V in and 220V center tap out. Or one with 4 identical winding............You get what I mean!!!

    These are quite common!!! You testing us?

    Good place to order is:

  6. Jan 9, 2012 #5
    Yungman mentioned Digikey. You could also try Newark and Arrow. So this is really a sourcing question then. Not a technical one. If you can't find one and have to make your own keep in mind that you need to downgrade the VA rating somewhat due to the greater heat from being in an enclosure. Since it will run slightly hotter it may also increase your copper losses slightly.

    I'm not sure what you mean by "euro up-convert". A transformer is a transformer. It trades voltage for current. The US is in fact a 240 volt 60 hz country. The only difference is that we use a center tapped secondary winding on the pole pig to give us the illusion of 120 volts at our outlets. It's really center tapped 240 or the equivalent of two 120 volt windings in series. Another way of stating the difference is that our electricians wire up our outlets with only half voltage.

    If all you want is 240 volts at a standard outlet you can do that by just moving your return 'neutral' wire from the ground bus to an adjacent breaker. Ideally you would use a nonstandard plug design though so that you can't accidentally plug in a 120 volt appliance in that outlet. You could then label the outlet "240 volts". I get the impression though that you want something portable. Although for that purpose I don't see why you would want a dual output. If you want 120 volts you can just unplug your portable step up transformer and plug the device directly into the outlet. All you would need is a 120/240 step up transformer with a high enough VA rating for your application.
  7. Jan 9, 2012 #6

    jim hardy

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    what you're asking for is marketed as "isolation transformer"
    i found one in a junkshop for $3 a few months back.

    they can even be had with a shield between the windings...

    there are many sources, here's one


    but if you're at all handy,
    the raw transformer can be bought for less than $100 and put in a box


    outlets are available at any hardware or home supply store
    put a fuseholder and slow blow fuse in primary .

    but as stated by others, in US house wiring is exactly as you describe, 240 volts with neutral as the centertap.. check on 230 volt 15 amp outlets next time you're in hardware store.
    Last edited: Jan 9, 2012
  8. Jan 9, 2012 #7

    jim hardy

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    search on buzzwords from links like this.
    http://www.isobox.com/hospital_grade_toroidal_W.htm [Broken]
    Last edited by a moderator: May 5, 2017
  9. Jan 11, 2012 #8
    What I mean by "euro up-convert" is the kind of transformer marketed to run European appliances in America from the 120 volt outlet. Not all transformers are alike. Most of the "traveler" transformers are actually autotransformers. Some may be isolation transformers. But they are all either ungrounded secondary, or end grounded secondary (e.g. the output hot is 240 volts relative to ground). Obviously you do understand the difference between 240 volts normally found in North America, and 240 volts found in the Europe, Australia, etc. I have my doubts about a lot of other people.

    What I want is a transformer that has an output emulating the North American 240 volts, NOT the European/Australian 240 volts (already have that).

    It's not that simple. Moving the neutral to a breaker (NEC requires it to be a two-pole breaker) is not the solution unless that circuit is dedicated to the outlet intended for 240 volts. You would not want to energize other things on the same circuit that expect 120 volts, with 240 volts. The proper outlet type for that in North America is designated NEMA 6-15R or 6-20R for 15 and 20 amp circuits.

    Yes, I want something portable. But no, I do not want a dual output. I only want the 240 volt output. However, I do want the proper grounding circuit. The NEMA L6-15 has 2 blades for the 240 volts, and a round pin for grounding (NOT intended for current carrying purpose). Maybe you thought I wanted that as a means to get 120 volts. I want that so I can get the proper grounding, only. But that ground does need to be connected to the center point in the secondary winding to make the "separately derived system" have a ground reference. A C13 outlet would be an acceptable alternative.
  10. Jan 11, 2012 #9
    I actually looked at those Hospital grade units a while back. All of the ones with 240 volt output use the European grounding, not the American grounding.

    As a side note, IMHO, hospitals are doing it wrong with regard to patient safety by using the standard American 120 volt system. They SHOULD (IMHO) use a 60-0-60 volt system for 120 volt devices, and a 120-0-120 volt system for 240 volt devices. The reason is capacitive leakage puts a slight 60 or 120 volt "charge" (often called phantom voltage, but there is some current associated with it, too) on the device, and hence also on patient attachments. Such devices do have all kinds of shielding and isolation to prevent reduce such risks. But, unless they use plastic enclosures, and add a shielded isolation transformer with a center-grounded secondary, they are not doing as much as they can.

    The 2nd Allied link did have one 175VA transformer that could be reversed to do what I want. But I want to avoid having to build my own due to all the work required in getting the proper enclosure and installing it, so it can be portable. I do already have the outlet (a duplex NEMA 6-20R).
    Last edited by a moderator: May 5, 2017
  11. Jan 11, 2012 #10


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    This should not happen if and electric screen is used between the primary and secondary windings. afaik, any true 'isolating' transformer would have one of these.
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