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3 phase to single phase

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  • 28-01-2018 2:05pm
    #1
    Paddymc18
    Registered Users Posts: 6 ✭


    Hi I picked up a compressor and the motor is 3 phase and I need it to run on single its a 4kw motor 400v I have changed it to delta and added a capacitor and whrn I plug itin the motor kicks and the the switch on the board trips


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    #2
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Assuming you can get it working with a capacitor to create the 3rd phase, what size breaker is it tripping? It might take 50 to 100 amps attempting to start, and if it does not quickly start turning, that current will be sustained.


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    #3
    Paddymc18
    Registered Users Posts: 6 ✭Paddymc18


    Bruthal wrote: »
    Assuming you can get it working with a capacitor to create the 3rd phase, what size breaker is it tripping? It might take 50 to 100 amps attempting to start, and if it does not quickly start turning, that current will be sustained.

    Hi thanks for replying

    I live in ireland btw I looked at the trip switch I just says 230-400v


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Paddymc18 wrote: »
    Hi I picked up a compressor and the motor is 3 phase and I need it to run on single its a 4kw motor 400v I have changed it to delta and added a capacitor and whrn I plug itin the motor kicks and the the switch on the board trips


    A 3 phase supply should have 3 phases that are electrically 120 degrees apart. Using a capacitor you have essentially created a 2nd phase. What have you done to create the 3rd phase? Another consideration is that the starting current for all 3 phases is now being supplied from an MCB on just one phase. Therefore the starting current will be considerably larger.

    The best chance for getting this to work from a single phase supply would be to use an inverter.


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    #5
    Paddymc18
    Registered Users Posts: 6 ✭Paddymc18


    2011 wrote: »
    A 3 phase supply should have 3 phases that are electrically 120 degrees apart. Using a capacitor you have essentially created a 2nd phase. What have you done to create the 3rd phase? Another consideration is that the starting current for all 3 phases is now being supplied from an MCB on just one phase. Therefore the starting current will be considerably larger.

    The best chance for getting this to work from a single phase supply would be to use an inverter.

    Iv seen people on youtube doing this but the motor they had was smaller and I dont have the money for a inverter nor do I want to buy one and on the cover for the junction box it says monophase so it must be possible to run it im just lost about this capacitor situation im wondering do I need a higer rated ųf capacitor


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    #6
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    A 3 phase supply should have 3 phases that are electrically 120 degrees apart. Using a capacitor you have essentially created a 2nd phase. What have you done to create the 3rd phase? Another consideration is that the starting current for all 3 phases is now being supplied from an MCB on just one phase. Therefore the starting current will be considerably larger.

    The best chance for getting this to work from a single phase supply would be to use an inverter.
    Adding a capacitor does give an extra phase. And it is in effect 3 phases, L, N, and the capacitor making the 3 phases. It's crude though, and creates imbalance in the motor especially as the load varies.

    The windings are probably 400v too, so 230v will only give 1/3 power output even if the capacitor was operating the motor at its ideal phase displacement at its normal load level.


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    #7
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Bruthal wrote: »
    Adding a capacitor does give an extra phase.

    Yes, I did say that.
    And it is in effect 3 phases, L, N, and the capacitor making the 3 phases.

    Ok, I see what you mean, but I still don't see this working.
    The OP would be better off with a single phase motor.
    The windings are probably 400v too, so 230v will only give 1/3 power output even if the capacitor was operating the motor at its ideal phase displacement at its normal load level.

    Well he did change it from star to delta, so theoretically that would cancel out the fact that the line voltage is 230V instead of 400V.


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    #8
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    Ok, I see what you mean, but I still don't see this working.
    The OP would be better off with a single phase motor.

    It can work alright. 4kw is a bit on the big side though and probably wont qork well, especially with 230v on a 400v motor. I used to install commercial dishwashers for a few months, one or two had 3 phase pumps, and using 2 capacitors, get them to run on single phase. The load was linear so it worked well enough.
    Well he did change it from star to delta, so theoretically that would cancel out the fact that the line voltage is 230V instead of 400V.

    Yes but a motor with 400v windings is at full output in delta (400v across each winding)
    In star, the same motor would have 230v across each winding, giving 1/3 output, (hence star/delta starters to reduce starting currents).

    Connect 230v 3 phase in delta to that motor, and it is the same as 400v connected to it in star.

    If the motor has 230v windings, then it will be full power in delta with 230v 3 phase. Id say it is 400v windings though.


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    #9
    Mississippi.
    Registered Users Posts: 283 ✭✭Mississippi.


    Bruthal wrote: »
    It can work alright. 4kw is a bit on the big side though and probably wont qork well, especially with 230v on a 400v motor. I used to install commercial dishwashers for a few months, one or two had 3 phase pumps, and using 2 capacitors, get them to run on single phase. The load was linear so it worked well enough.



    Yes but a motor with 400v windings is at full output in delta (400v across each winding)
    In star, the same motor would have 230v across each winding, giving 1/3 output, (hence star/delta starters to reduce starting currents).

    Connect 230v 3 phase in delta to that motor, and it is the same as 400v connected to it in star.

    If the motor has 230v windings, then it will be full power in delta with 230v 3 phase. Id say it is 400v windings though.


    At 400v 3 phase star the 400v is across two windings whereas 230v 3 phase delta it is across one winding just, it would have the same power just much higher current draw.

    4kw is very big for bridging out with capacitors though, buy several and experiment as in the end the fine tuning is critical.


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    #10
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Mississippi. wrote: »
    At 400v 3 phase star the 400v is across two windings whereas 230v 3 phase delta it is across one winding just, it would have the same power just much higher current draw.

    1.7 (√3) times the phase current in the lines for the delta setup.

    Each actual winding is the same voltage and current in both scenarios there.


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    #11
    Tuco88
    Registered Users Posts: 1,051 ✭✭✭Tuco88


    You would be better getting a drive long term seeing its on a compressor, usually a hard working unit...

    You can get it to run on single phase with a suitable rated capacitor across a winding. Which you have done.

    The problem is the starting current on a motor that size is a problem. Never mind under load.

    A 3p pillar drill years ago. It was defo not 4kw.
    A capactor fixed on the motor by the terminal box, in use all the time across a winding.

    I added a second for starting, that was taken out by a timer. A stop/start station was used and an old analog timer that was the size of an mcb that was placed inside it did the job. Too much capacitance will get the windings very hot.


    In the end you have a 4kw compressor at 1/3 its power output?

    You can buy a tidy 2hp (~1.5kw) thre is also a 3hp available in single phase. I take its a air compressor for tools?


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    #12
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Tuco88 wrote: »
    The problem is the starting current on a motor that size is a problem. Never mind under load

    The starting current is the same no matter what the load, brought about by a stopped rotor at the instant of switch on.

    It just takes longer to reduce to normal levels, the longer it takes the motor to get up to speed, which is affected by a loaded motor starting alright. Just as a matter of interest again.


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    #13
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,595 Mod ✭✭✭✭2011


    Bruthal wrote: »
    The starting current is the same no matter what the load, brought about by a stopped rotor at the instant of switch on.:(

    If you supply a 3 phase motor from single phase the phaser sum of starting currents from all 3 phases will be drawn from the single phase supply. Therefore the starting current will be larger.


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    #14
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    2011 wrote: »
    If you supply a 3 phase motor from single phase the phaser sum of starting currents from all 3 phases will be drawn from the single phase supply. Therefore the starting current will be larger.

    Obviously.

    However, my.point was that the load on any given motor does not change that motors starting current. Only the duration it takes to reduce to normal running current. Which I mentioned was just as a matter of interest.

    Of course, a motor starting with a full mechanical load on it does increase starting current problems, as tuco suggests.


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    #15
    Tuco88
    Registered Users Posts: 1,051 ✭✭✭Tuco88


    I find i forget everthing now its really annoying. Anyway if anyone is into reading Jim Cox has those practical books he has one excellent one on motors i picked up years back.


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    #16
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Tuco88 wrote: »
    I find i forget everthing now its really annoying. Anyway if anyone is into reading Jim Cox has those practical books he has one excellent one on motors i picked up years back.

    You were correct anyway in saying the starting current can cause more problems with heavy loads on the starting motor.

    One other possible thing that can increase the starting current of a given.motorto higher than a stopped rotor is if it is still rotating one way when it is started in reverse direction.


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