Transformer configuration to produce 3 phase out of single phase

Discussion in 'General Electronics Chat' started by strantor, Sep 13, 2012.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    A while back, a guy came around asking about a furnace in this thread. He made mention of his "wild leg" 3 phase service that takes 2 wires and turns it into 3 phase for his shop. I thought he was mistaken until he posted the schematic from his electrical provider (attached).
    Looking at the schematic provided, what I see is a single phase (2 phases of a 3 phase system) with a neutral, going into 2 transformers wired in some crazy manner that I don't understand, and supposedly 3 separate phases coming out.

    If that's actually what's going on, could I use this setup to produce 3ph from my 220V household mains?

    Why or why not?

    How could this work? Would it actually create 3 phases that are 120 degrees apart?
  2. vk6zgo

    Active Member

    Jul 21, 2012
    As far as I can see,it is creating 3 phases out of 2,not 1.
    I have heard of this circuit,but not really come across it.
  3. cork_ie

    Active Member

    Oct 8, 2011
    In that particular instance it was three phase from two phase.

    You can get three phase from single phase. There are 2 types, static = cheap but need to be matched to the load & rotary convertor which are much more expensive.

    Static convertors are good enough for many uses and an easy DIY build. I have a schematic from a commercial unit I repaired a few years ago and will try and locate it if you are interested.
  4. cabraham

    Active Member

    Oct 29, 2011
    Key words for search are :

    Scott T connection

    With 2 phases in (90 degrees apart, not 180), any number of phases can be generated.

  5. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    Gentlemen, we are not dealing with 2 phase here. It is 2 legs of a 3 phase system, which if I'm not mistaken, is single phase. 2 phase is an extremely uncommon system consisting of 4 wires; basically 2 seperate single phase systems 90 degrees apart from eachother. That's not what this is. What I see coming into the guys shop is single phase with a center neutral, same as what comes into my breaker panel. Try putting an oscope across 2 of the 3 legs of a 3 phase system and see what you see.
  6. cork_ie

    Active Member

    Oct 8, 2011
    In this instance There are two transformers each supplied by a different phase with respect to Neutral (and ground) So you do have two phase input. It would only be single phase if the three phase supply was in DELTA format, yours is WYE.

    On the left transformer you will have 120V between X2 & X3 and 120V between X2 & X1, so far so good.
    On the right transformer you will have 240 Volt between X1 & X3 - the centre tap is not used. The output here will be 120Deg out of phase with respect to the same outputs in the left transformer.

    Now - you are connecting X1 of the left transformer to X3 of the right tying them together (outlet B). This introduces a whole new scenario and the Lag & lead of the secondary winding inductances is going to affect the phase angles at LH X3 (outlet A) and RH X1 (outlet C) . You will notice that the centre tap of the RH Transformer is not connected so the two secondary windings are NOT the same. This is also the reason why there is 208 V phase to neutral at outlet C. If you draw Vector diagrams of the two secondary windings as configured you will see that it all makes sense.

    This is a classic Scott T transformer as already mentioned by cabraham.

    You can built a static single to three phase converter but you will also need some capacitance.

    Electronic single to three phase converters are commercially available , with all the bells & whistles, i.e. variable speed,soft start, forward & reverse, remote control etc. etc. I fitted one with a 5HP 3 phase motor to a winch about 10 years ago and it cost around £400 at the time
    Last edited: Sep 13, 2012
    strantor likes this.
  7. Biff383

    Active Member

    Jun 6, 2012
    If you want a vfd to run up to 5 amps single phase to 3 phase 0 Hz, (doubtful) to 400 Hz (likely) give me a pm. I'm liking what your doing and will contribute to the cause. Almost free.w
  8. tinkerman

    Active Member

    Jul 22, 2012

    You can produce "3 phase" from standard 240 volt household supply. It's not true three phase. You can use either rotary converter or static. I have built both and are in use in my shop as well as a third rotary unit in a farm application. You will have to reduce the horsepower rating of the motor you are operating in a static converter. Rotary will give you better output and better phase angles. There are more problems to watch for. Unbalanced phase currents which vary with loading and can overheat the winding. It is possible and does work. Not complicated or expensive to build your own, either way.
  9. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    How much power are we talking?
    Depending on your load commercial units could be very reasonable.

    Technically you could pull the power factor about with caps, or possibly caps on one leg and a choke on another but the losses would be horrendous and it would only work properly within a fairly small range of loads.

    Pulling, current lead, a load out of phase with another connected to the same supply is actually very common, virtually all single phase induction motors do it to establish a direction of rotation and facilitate starting. some have multiple capacitors and rotary switches others have secondary windings, usually smaller than the main, with a capacitor connected all the time and many smaller motors designed to be reversible will have two identical windings with a cap in series with whichever winding produces the desired rotation. You can even generate single phase with a three phase motor ... check out C-2C connected induction machine.