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  1. #1
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    Default rewire 3 phase motor to single phase

    i heard some one here have done it . is there any down side on this mod ? i think cost about 200~$300 so it is cheaper than buying inverter.
    regards
    Peter

  2. #2
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    Quote Originally Posted by thorens View Post
    i heard some one here have done it . is there any down side on this mod ? i think cost about 200~$300 so it is cheaper than buying inverter.
    regards
    Peter
    Too many downsides to mention... those methods usually rely on capacitive phase shifting, like an RPC.. on the other hand inverters are less than $200. ( If you're Joe, then he get's em for $50 )

    So inverters are cheaper and better..

    Ray

  3. #3
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    Quote Originally Posted by RayG View Post
    Too many downsides to mention... those methods usually rely on capacitive phase shifting, like an RPC.. on the other hand inverters are less than $200. ( If you're Joe, then he get's em for $50 )

    So inverters are cheaper and better..

    Ray
    thanks Ray.
    the lathe i'm have doesn't have delta connection so those cheap inverter is out . any other way ? i can only run this motor on 415v 3 phase
    Peter

  4. #4
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    Quote Originally Posted by thorens View Post
    thanks Ray.
    the lathe i'm have doesn't have delta connection so those cheap inverter is out . any other way ? i can only run this motor on 415v 3 phase
    Peter
    1. I guess you could open up the motor and bring out the star point, then re-wire to delta, hook up a VFD and re-wire the controls.

    2. Buy or build a phase converter that will generate 415 3 phase, commercial units are available for a few thousand..

    3. Run on reduced power by running a 240V VFD as per option 1, but with the motor still in star.

    Ray

  5. #5
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    Quote Originally Posted by RayG View Post
    1. I guess you could open up the motor and bring out the star point, then re-wire to delta, hook up a VFD and re-wire the controls.

    2. Buy or build a phase converter that will generate 415 3 phase, commercial units are available for a few thousand..

    3. Run on reduced power by running a 240V VFD as per option 1, but with the motor still in star.

    Ray
    Ray.
    if the motor can rewire to delta internally that should be superb , i guess i should open it up and see .if it indeed can rewire to delta then sure i can run it with the VFD .but as you said even in star model i still can run the 240v VFD but how much torque will i lost if run the motor this way?
    Peter

  6. #6
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    Quote Originally Posted by thorens View Post
    Ray.
    if the motor can rewire to delta internally that should be superb , i guess i should open it up and see .if it indeed can rewire to delta then sure i can run it with the VFD .but as you said even in star model i still can run the 240v VFD but how much torque will i lost if run the motor this way?
    Peter
    I expect you would get about 1/2 power, at half voltage... but my logic for that conclusion is a bit convoluted.. The current drawn is dependent on the rotor slip, and at reduced voltage the slip increases, which tends to increase the current as the voltage drops, at some point things saturate, the slip increases to 100% and the current increases to the locked rotor current. So, you would be limited in speed range more than if it was connected as delta, but the lathe would still turn and it would work.. just not full power.

    Ray

  7. #7
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    Quote Originally Posted by RayG View Post
    I expect you would get about 1/2 power, at half voltage... but my logic for that conclusion is a bit convoluted.. The current drawn is dependent on the rotor slip, and at reduced voltage the slip increases, which tends to increase the current as the voltage drops, at some point things saturate, the slip increases to 100% and the current increases to the locked rotor current. So, you would be limited in speed range more than if it was connected as delta, but the lathe would still turn and it would work.. just not full power.

    Ray
    THanks gaian Ray.
    look like it not the end of the world if one doesn't have 3 phase power installed.
    Peter

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    DISCLAIMER

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    or moderators for advice offered by members posting replies
    or asking questions regarding electrical work.
    We strongly advise contacting a Licensed Tradeperson for all electrical work.
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  9. #9
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    Peter, I've done this a few times before you could buy VFDs (BVFD) it was the only option back in the 70s....
    The power you loose is about the same or more than running the motor in star configuration on a 240V VFD! And the motor will run 'rough' - that is noisy. It also involves a timer to change over from starting capacitors to run capacitors (or do it manually when the motor reaches operating speed - I've done both equally successfully). The other disadvantage is that the motor will have SIGNIFICANTLYT less starting torque. On a friend's Sheraton lathe (his name is Peter too and he lurks here on the forum ) we have to turn the chuck by hand to start the motor in high speed ratio ro it blows his fuse....
    Lastly, there is nothing like the soft start and variable spoeed of a VFD.....
    Just don't even consider it.
    There are now 240V to 415V VFDs available in China quite a bit cheaper than previously only available from the UK.Just a bit more trickky to deal with....
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  10. #10
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    Default SIngle phase conversion??

    Hello Thorens
    If your motor is a single speed type, 2 pole or 4 pole, and of fairly standard design, then it will be cheap and easy to find another used motor with star and delta terminals reconnectable easily. Look in the terminal box and if it has 6 terminals with 3 of them linked together then this is Star connected. For Delta connection the links are removed and used to connect the terminals in pairs usually top to bottom. Should be a diagram inside the cover but any leccie will do it in 2 minutes.
    Talk to a scrap yard or any heavy electrical fitters. Most industries replace motors routinely rather than strip them to re-lube or replace bearings. Have never paid more than $5 or $10 for as much motor as you can carry out the gate. Combustor.
    Old iron in the Outback, Kimberley WA.

  11. #11
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    Quote Originally Posted by jhovel View Post
    Peter, I've done this a few times before you could buy VFDs (BVFD) it was the only option back in the 70s....
    The power you loose is about the same or more than running the motor in star configuration on a 240V VFD! And the motor will run 'rough' - that is noisy. It also involves a timer to change over from starting capacitors to run capacitors (or do it manually when the motor reaches operating speed - I've done both equally successfully). The other disadvantage is that the motor will have SIGNIFICANTLYT less starting torque. On a friend's Sheraton lathe (his name is Peter too and he lurks here on the forum ) we have to turn the chuck by hand to start the motor in high speed ratio ro it blows his fuse....
    Lastly, there is nothing like the soft start and variable spoeed of a VFD.....
    Just don't even consider it.
    There are now 240V to 415V VFDs available in China quite a bit cheaper than previously only available from the UK.Just a bit more trickky to deal with....
    Thanks.
    I just checked some chinese VFD and indeed they are now have some with 380v output .but not sure if it can output 415v .
    regards
    peter

  12. #12
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    Hi Peter. The output voltage relates directly to the input voltage. With 220V going in, the output is 380V (as in parts of Europe and Asia) and with 240V in it will be 415V.

    Cheers,
    Joe
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  13. #13
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    Default

    most 380v motors will run ok on 415v as does most 440V

    also and for info only I asked my motor rewinders (just last week) on what it would cost to find the star point on an old Crompton Parkinson 0.75hp 3ph 415v star and reconfigure to delta?
    about $70-80 + gst was the price...assumes no drama such as bearings ok , end shields ok etc etc.

  14. #14
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    Default the choice of VFD

    guys.
    so time to decide which one to go for. my lathe is 1H/P and there is choice of either 1.5kw VFD or 4Kw unit . price not much different .
    i thinking getting the 4Kw unit since you lose power when running VFD . what do you guys think?
    Peter

  15. #15
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by thorens View Post
    guys.
    so time to decide which one to go for. my lathe is 1H/P and there is choice of either 1.5kw VFD or 4Kw unit . price not much different .
    i thinking getting the 4Kw unit since you lose power when running VFD . what do you guys think?
    Peter
    It sounds like you don't understand the power losses associated with using VFDs

    If the motor is correctly configured to run at and receives the correct voltage and frequency it will not develop significant less power running under a VFD.

    If the motor is configured incorrectly and attached to the wrong voltage/frequency it will not be able to develop it's optimum power whether it is connected direct to a 3 phase supply (i.e. no VFD) or to a VFD.

    Provided the VFD can provide all the power that a motor is capable of drawing, using a much high power rated VFD won't make any difference in terms of motor power losses.

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