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Question about single phase to three phase conversion

  1. Apr 5, 2012 #1
    What type of circuit can convert single phase electric power into three phase electric power? Can the circuit diagram be posted in this thread?
  2. jcsd
  3. Apr 6, 2012 #2
    single phase motor, pulley going to three phase generator


    you would need a circuit with capacitors to make up the energy that doesnt exist at the single phases frequencies.
  4. Apr 6, 2012 #3
    I fail to see how the capacitors will 'make up energy' that doesn't exist.
    Typically converting single phase to 3 phase is done with a rotary phase convertor, or a solid state switching device such as used in variable frequency drives. Both devices have been covered here on PF. Do a search.
  5. Apr 7, 2012 #4


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    Are you wanting to do much more than demonstrate the 3 sinusoids on a CRO, or operate a tiny 3ø model motor?

    Otherwise, you are talking about a heavy duty DC to AC converter producing 3 phases, and that's just to drive a small domestic 3 phase water pump.
  6. Apr 7, 2012 #5
    Can a diagram for a solid state converter be posted here? Variable frequency drives can adjust output frequencies but how can they be used to cause two separate phase shifts in the single phase current to produce a 3-phase output?
  7. Apr 7, 2012 #6


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    You'd basically have 3 DC-AC converters in one big box, all synchronized so they stay perfectly in step and precisely 120° out of phase (one leading, and one lagging, the third).
  8. Apr 7, 2012 #7
    And what keeps the three currents out of phase? Is there some kind of electronic timing mechanism that delays currents so that when the first phase is on, the other two phases are off and the same applies to the other two phases?

    Perhaps electronic timer switches are connected to each output to time when the outputs will stay on and off.
  9. Apr 7, 2012 #8


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    An oscillator can be designed (using R's, C's and transistors or op-amps) to provide the three low-power sinewaves with correct phase differences. These in turn control high power amplifiers.
    I assumed you'd be wanting 3ø sinusoids. If it is good enough to have 3 rough stepwise approximations, then you can use a switching arrangement and this is more efficient. But it is rare that a 3 phase application will accept squarewaves, the most efficient of all to generate.
  10. Apr 7, 2012 #9

    jim hardy

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    take 6 volts from a doorbell transformer
    phase shift by 60 degrees with RC as suggested above and invert

    now you have two 6 volt sinewaves 120 degrees apart, draw the phasor diagram.
    label one A and one C and the circuit common B.

    Now you have three phase open delta with phase B "grounded"

    run it through a power amplifier (stereo boom box amp) and into a small three phase transformer and you have a few watts. I used that to make a 3 phase 260V source for checking instruments in the power plant. But i used the LM12 power operational amplifier which is obsolete and cost $80 apiece. Use something more modern, one of those fifty watt TDA series audio amps.
  11. Apr 7, 2012 #10
    Can the diagram for this type of design be posted so that the exact parts and their location in the circuitry can be identified?
  12. Apr 7, 2012 #11

    jim hardy

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    my old drawings are in DCCAD which no longer runs under windows.
    If i can find a hardcopy will scan it into jpg and post.
    But it really is as simple as described. I used LM324 for phase shifter and LM12 for power amp. Found a small 3 phase transformer, will post its part number when locate it. I have a couple in the barn.

    I used +/- 30 volt supplies for the LM12 opamps, +/-12 for LM324..
    Be aware this is a linear approach so it's very inefficient, of absolutely no use for running motors or illumination
    But it worked great for fooling a three phase controller circuit into thinking it's powered up.

    What is your goal?
    If you want it to run motors, search on three phase power for home shop. Machinist supply stores sell them, check Enco. The basic approach is run a fair sized three phase motor on single phase with no load and it will make the other two phases for you. So you parallel your 3 phase machine with that larger free running motor. It's not perfectly balanced but it'll start and run well.
    The home shop machinists buy a box with capacitor and relay to get the first motor started. 3 phase motors are cheap at the salvage yard because so few people have 3 phase at home.

    old jim
  13. Apr 7, 2012 #12


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    Sure it can, provided it is already in the public domain.

    Google will find around 25,000 such circuits. We'll let you have the honor of posting a link to one that you like the look of, then we can discuss some of the finer points of it. After all, you are the one who's doing this homework. :wink:
  14. Apr 12, 2012 #13
  15. Apr 12, 2012 #14


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    That is not a 1-phase to 3-phase converter, not in the usual meaning of the term. What you are referring to is a dodgy arrangement that allows a small 3∅ motor to run on a single phase supply.
    Because they are generating only 2 phases, not 3. It's a rough and ready compromise, but it works because motors can run off 2 phases, though at reduced power.

    So are you interested in operating a 3∅ motor off a single phase?
  16. Apr 12, 2012 #15
  17. Apr 13, 2012 #16
    No, just interested in figuring out how to do it. Is it possible to combine 2, 2-phase converters and suddenly end up with a 3-phase converter?
  18. Apr 13, 2012 #17


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    Static Phase Converters

    The simplest type of phase converter is generically called a static phase converter and has been in use for nearly one hundred years. This device typically consists of one or more capacitors and a relay to switch between the two capacitors once the motor has come up to speed. These units are comparatively inexpensive. They make use of the idea that a three-phase motor can be started using a capacitor in series with the third terminal of the motor. It then runs with essentially two of the three windings powered.

    It is almost guaranteed that a static phase converter will do a poor job of balancing the voltages on the motor. Unless motors operated on static converters run only for short periods or deliver significantly less than half of their rated output, they will be damaged from overheating.

    Some manufacturers of static converters make a simplistic statement that since two of the three windings are powered, the motor will be capable of generating two-thirds its rated power. This is misleading and could lead to damage of the motor. If the motor were loaded anywhere near two-thirds its capacity, it would be permanently damaged in short order.

    The only good thing that can be said of static phase converters is that they are inexpensive. They can only operate motors, and only single-motor loads. The motors must be limited to light loads and intermittent use.


    Now, if you still want to build and use one of these, you may get more technical details here: http://www.homemetalshopclub.org/projects/phconv/phconv.html
  19. Apr 13, 2012 #18

    jim hardy

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    Look up "open delta"

    and do this thought experiment:

    Draw a three phase delta connected transformer winding.
    Focus your attention on voltages at A and C wrt B.
    Observe that if you remove the single winding that's between A and C you do not change the voltage between A and C, it is defined by the other two windings' amplitude and phase.

    So it is possible to use two voltage sources to make three phase.
    It is in fact done quite often using two single phase transformers. Saves the cost of one transformer. Be observant of power poles in your daily travels and you'll probably see such a setup.
  20. Apr 13, 2012 #19
    There are some circuit diagrams here for the open delta converter:








    Are these the correct schematics for such converters?


    Additionally is this diagram for 3-phase to 1-phase conversion correct:


    The 3 power phases can be connected to inductors which will be placed in proximity to a larger inductor that is the same size as the sum of the volume of the 3 smaller inductors. The power will then be transferred through electromagnetic radiation and the inductors will thus serve as a transformer with all the energy being fed to one inductor. Because of the overlap of currents coming from the 3-phase power supply, the output inductor can be connected to a rectifier which will then be connected to an inverter to produce a 1-phase output.
    Last edited: Apr 13, 2012
  21. Apr 13, 2012 #20
    Bararontok, where did you get that schematic? I would like to see the smoke pour out of that transformer when hooked up. Incidentally, why is it needed to 'convert' from 3 phase to single phase? Just hook on to a pair of wires from a 3 phase source and you have your single phase. Unless you want to provide more power on the generated single phase than one phase of the original 3 can provide, I see no reason to have any kind of convertor.
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