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Thread: Some pearls of wisdom

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    Some pearls of wisdom

    Taken from a forum where one member is trying to sell a compressor(3 phase)


    It could be changed to single fase yes but that means you have to change the motor. The question is can you get a strong enough single fase motor to work on that compressor? Any pics perhaps?

    no, 3 phase motors can run on single phase, just wont be as powerful


    Thats the problem. So it wont be any good running it on single phase. Best would be to change motor to single phase motor then.

    no man hond you dont understand. its just going to take longer to fill the tank. its a compressor unlike a normal e/tool which you need the power to do the work

    OK Whatever

    on single phase that thing is going to pull lank amps and probably trip the power.

    You can go to about a 3kW motor on single phase normal outlet (using the correct curve breaker) before it will trip, more on a welding outlet.

    Im kinda thinking a 500l compressor will use more then a 3kw motor. D curve breaker will work you are correct.


    And the prize goes to...... (from someone that builds and sell generators for a living?


    Running a 3-phase motor on single phase will burn it, no matter what the scientists on this thread say.

    Star-Delta starters (to lower start-up amps) start a 3-phase motor by rotating it on single phase and then switching over to 3 phase once peak RPM is reached, this allows less current draw on start-up than normal Direct-On-Line starters (DOL).

    You all wanna see what happens when a star-delta malfunctions and it gets stuck in single phase? It ain't pretty.

    So hou op kak praat, dit gaan nie werk nie.

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    Bronze Member dfsa's Avatar
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    Quote Originally Posted by greasemonkey View Post
    So hou op kak praat, dit gaan nie werk nie.
    Gevaarlike Forum wat jy daar het. Enigste ou wat weet wat hy van praat. Hy praat nie KAK nie.

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    Gold Member Martinco's Avatar
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    It is quite obvious the all the fundies on TFSA have been leading us mortals by the nose and caused us to spend thousands on buying new motors when we could have used the old ones in the fashion described above !

    Even Eskom can save millions on supplying single phase only. ( Less copper to be stolen as well )

    These guys are going to start a new industrial revolution !

    On a more serious note. A three phase motor will actually run on a single phase if you connect only one of the phases to 230 v but you have to start the motor by hand ( in any direction ) and the amount of power in very small.
    So technically the guy is correct but not practical.
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    Bronze Member dfsa's Avatar
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    Quote Originally Posted by Martinco View Post
    On a more serious note. A three phase motor will actually run on a single phase if you connect only one of the phases to 230 v but you have to start the motor by hand ( in any direction ) and the amount of power in very small.
    So technically the guy is correct but not practical.
    Nou begin ons ook KAK praat: On a technical note: Yes you can run a 3 Phase motor with Single Phase input up to about 2,2KW. You simply go and buy a VSD with SAPWM vector control. These drives take a Single phase 220vac input and supply a 3 Phase 0-400Hz output.

    Now back to the KAK praat forum: This guy now need to go and buy this VSD and it should run on his existing motor, but in real terms he should first go and buy another motor 3 Phase 220vac. In that case I would rather go and dump the compressor motor and get a 220vac single Phase motor that can handle the Load.

    Ohh forgot, he would not need to go buy a D Curve breaker as he will have a soft start. At least he will safe himself a couple bucks on the CB.
    Last edited by dfsa; 31-Jul-12 at 01:58 PM. Reason: D curve breaker

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    Bronze Member mikilianis's Avatar
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    Some Info On the subject

    Running 3-phase motors on 1-phase Three-phase motors will run on single phase as readily as single phase motors. The only problem for either motor is starting. Sometimes 3-phase motors are purchased for use on single phase if three-phase power is anticipated. The power rating needs to be 50% larger than for a comparable single phase motor to make up for one unused winding. Single phase is applied to a pair of windings simultanous with a start capacitor in series with the third winding. The start switch is opened in Figure below upon motor start. Sometimes a smaller capacitor than the start capacitor is retained while running. Starting a three-phase motor on single phase. The circuit for running a three-phase motor on single phase is known as “ add a phase ” or various other brand names. “ Add a phase ” supplies a phase approximately midway ∠ 90 o between the ∠ 180 o single phase power source terminals.

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    Gold Member Martinco's Avatar
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    Quote Originally Posted by dfsa View Post
    Nou begin ons ook KAK praat:
    Nie so nie dfsa, ek het dit al op verskei male gedoen. Kyk na pos bo en sien hoe.
    Martin Coetzee
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    Bronze Member dfsa's Avatar
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    Quote Originally Posted by Martinco View Post
    Nie so nie dfsa, ek het dit al op verskei male gedoen. Kyk na pos bo en sien hoe.
    Ek Praat maar engels vir ander mense: OK so how long did the motor run after you done the Phase angle and what torque did you manage to get out?

    Let us forget at this point about over heating. ( You have a 3 Phase 400vac motor that you now apply 1 Phase 230vac )

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    regarding star delta. star is 230 volt 3 phase not single phase.

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    Bronze Member dfsa's Avatar
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    Quote Originally Posted by bergie View Post
    regarding star delta. star is 230 volt 3 phase not single phase.
    Bergie you need to read tha whole post Mate. Just a note: Are you sure it is not perhaps a 400/660 motor? Oops there go the 230vac Star connection.



    Quote Originally Posted by mikilianis View Post
    Some Info On the subject

    Running 3-phase motors on 1-phase Three-phase motors will run on single phase as readily as single phase motors. The only problem for either motor is starting. Sometimes 3-phase motors are purchased for use on single phase if three-phase power is anticipated. The power rating needs to be 50% larger than for a comparable single phase motor to make up for one unused winding. Single phase is applied to a pair of windings simultanous with a start capacitor in series with the third winding. The start switch is opened in Figure below upon motor start. Sometimes a smaller capacitor than the start capacitor is retained while running. Starting a three-phase motor on single phase. The circuit for running a three-phase motor on single phase is known as “ add a phase ” or various other brand names. “ Add a phase ” supplies a phase approximately midway ∠ 90 o between the ∠ 180 o single phase power source terminals.

    I don't think the person or yourself have tryed to modify a Motor like that. Firstly you would need to do some intense calculations to select the correct capacitors. Now you need to add the cap switch and disconnect the start cap at the right time. You also most probebly will have to select a F class motor ect. Now you switch on the power and find the motor is unballanced. Just maybe the cap also go pooof.

    Quote Originally Posted by Martinco View Post
    Nie so nie dfsa, ek het dit al op verskei male gedoen. Kyk na pos bo en sien hoe.
    Dude no disrespect, but I really would like to see a 2.2kw motor run like this at capacity. It will run with no load, but sorry not at full load. I would like to know what you did about the Heat. Your motor will cook in no time.


    This looks like a problem that many people might have "Single phase supply with Three phase equipment" Hmmmm maybe there is money to be made here.

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