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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

    :-p

    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

    Averagesupernova

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    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

    NascentOxygen

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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

    NascentOxygen

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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

    NascentOxygen

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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

    NascentOxygen

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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

    NascentOxygen

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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

    Averagesupernova

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  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

    Bobbywhy

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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.

    http://www.phaseconverterinfo.com/phaseconverter_static.htm

    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:

    http://www.elec-toolbox.com/images/fig1-8.gif

    http://2.bp.blogspot.com/_vfmOyxDCr...IGrAiDA/s1600/3_Phase_Converter_Schematic.png

    http://img.photobucket.com/albums/v246/silverdoctor/UKconverter.gif

    Sources:

    http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

    http://www.circuitlab.org/2011/12/schematics-3-phase-converter-miller.html [Broken]

    http://www.practicalmachinist.com/v...e-phase-converter-home-united-kingdom-102960/

    Are these the correct schematics for such converters?

    ****

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

    http://img407.imageshack.us/img407/8593/3phaseto1phaseconverter.png [Broken]

    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 by a moderator: May 5, 2017
  21. Apr 13, 2012 #20

    Averagesupernova

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    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.
     
  22. Apr 13, 2012 #21
    Perhaps it is possible that the transformer can be designed with a wattage rating that is higher than the input power of the 3-phase supply to avoid overloading and enable all the power from the 3 phases to be converted into a combined 1-phase output but it is correct that a pair of wires can be connected to a single pair of terminals from a 3-phase circuit and 1/3 of the power can be used as the 1-phase output. The 3-phase to 1-phase converter circuit diagram was made by the thread originator purely for curiosity's sake and not for any purpose.
     
  23. Apr 13, 2012 #22

    Averagesupernova

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    My point is that you cannot just throw 3 windings, one from each phase, on the same core. They will fight each other.
     
  24. Apr 13, 2012 #23

    jim hardy

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    The first one is what i was describing.

    The rest are more complex devices that start with single phase and make a third phase by shifting angle with capacitors. Note the first one starts with three phases and ends with xame three phases.
    Point being, two vectors define three points and if those points are vertices of an equilateral triangle they are same geometry as three phase phasors.

    Second one appears to employ a motor , and that scheme actually works fairly well. My neighbor has one in his home workshop. It actually uses the motor as an induction generator for part of each cycle to make third phase.

    Third one is similar to second but with improvement of a step-up transformer. Works quite well for running motors. i've done it myself. Draw the phasor diagram and you'll see it.
     
    Last edited: Apr 13, 2012
  25. Apr 13, 2012 #24

    NascentOxygen

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    It represents in principle a method to convert from 3ϕ to single phase: you rectify the 3ϕ and use that DC to power an inverter. But I would need to examine his "transformer" arrangement very closely (at a low testing voltage) before anything else, as it is unusual. You might consider using an AC→DC→AC inverter like this if your need was for a single phase frequency that was different from that of the mains 3ϕ, or if it were essential that you must load all 3 phases equally (this could be a power authority condition). Otherwise, why not just connect your single phase motor (or its associated transformer) between any two of the phases from your 3ϕ supply?
     
    Last edited by a moderator: May 5, 2017
  26. Apr 13, 2012 #25
    There are a few ways to convert power from a single phase AC to three phase AC. Nowadays, the most popular way is to use a single phase inverter/rectifier that converts the AC voltage and current waveforms into DC. The DC link voltage is usually regulated by the rectifier. A typical rectifier that is capable of bidirectional power flow is a simple H-bridge. A full bridge is also needed to obtain high power factor -- the drawn current is also sinusoidal.

    Now, we have the DC link to which we connect a three phase inverter. The simplest topology are three phase legs. The middle of each leg is connected to one phase of the utility via an inductor. It is not possible to connect two voltage sources without a magnetic element such as inductor.

    The three phase currents are regulated via current control algorithms by controlling the duty ratios of the three legs (PWM scheme). This topic has been rather mature for the past ten years. To obtain maximum efficiency and power factor, each phased current is a scaled version of its phase voltage. This way there is no reactive current in the system.

    These two systems are called back-to-back AC/DC/AC system and it is a voltage sourced system.

    To convert power directly from AC to AC you would use a cycloconverter. These guys can handle high currents and voltages as they are mostly based on SCRs and triacs - current sourced converters.

    A picture of a three phase inverter is here: http://techno-fandom.org/~hobbit/cars/training/800/m112.jpg

    Single phase inverter (bottom part): http://solar.smps.us/grid-tie-inverter-schematic.png

    And here are two back-to-back VSC systems: http://ars.sciencedirect.com/content/image/1-s2.0-S0960148111006550-gr1.jpg [Broken]

    Source: power electronics engineer
     
    Last edited by a moderator: May 5, 2017
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