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  1. #1
    bunnybungo is offline Plastic
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    Default Help, how can I run 3phase motor with single phase supply

    Good day to all,

    I would like to ask if there is a way for me to run a 3phase 7.5hp conventional lathe machine without using RPC? A friend of mine before told me that he sucessfully run a three phase motor by just using switches and capacitors.

    If it is possible, can I ask for a simple diagram for me to follow.

    Please also include if there is a need for a contactor and timer.

    Many thanks to all!!!

  2. #2
    johnoder's Avatar
    johnoder is online now Diamond
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    Default

    I do it this way, but it's 15 HP. Naturally, I lose about a third of the 15HP since I am using no run capacitors.

    How To Make An Old 15HP 3 Phase Go

    Large wiring helps make for zero light dimming.

    Not shown in the photos is the heavy duty disconnect/safety switch rated for 60 amps with 50amp time delay fuses installed.


    John Oder

  3. #3
    toolnut is offline Stainless
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    Default

    The answer to your question is yes. The question you will have to answer is: Are any of these methods practical for me?"

    1. Basically you can use JohnNoders idea and rig up a disconnect & some start capacitors which allows you to start the motor with the capacitiors then allow it to single phase. This will cost you about 41% of the nameplate power (divide the rated power by the square root of 3). You should expect about 59% of the nameplate power to be available for your use.

    2. buy a commercially made static converter with only start caps and get the same results as John by paying a higher price

    3. buy a commercially made static converter with start and run caps. Supposedly these will give you 80% of the nameplate power (they can be finicky)

    4. build your own static with start caps and run caps. Same results as a commercial unit but cheaper and you can use better components

    5. buy a H-A-S static converter & have your motor converted to 12 leads. This gives you about 98% of nameplate power but the catch is the cost to convert your motor to a 12 lead motor (may require a motor rewind--depending on the condition of the winding's insulation)

    6. buy a VFD, single phase input and 3 phase output, 230 volts. This is a superb way to run the lathe but the VFD will be costly. Check with www.driveswarehouse.com and look for Polyspede VFDs. you will need a 7-1/2 hp rated VFD

    7. Change the motor for a 5 hp single phase motor or a 7-1/2 hp single phase (harder to find than a 5 hp but they do exist)

    8. Use Joe H's autotransformer method. This works great for one motor. You can find information and posts by Joe one this forum. Just search for "autotransformer" or "transformer RPC" and you should find something, or post asking about it and he may answer.

    All of these methods assume you have only one motor on the lathe. If you have more than one then build the RPC and be finished.

    You will eventually understand why so many of us build RPCs so we can run the machine without modifications and still get full power from it.

  4. #4
    LatheWerks's Avatar
    LatheWerks is offline Aluminum
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    Default

    ^thanks for the detailed answer, this will help me

  5. #5
    JST's Avatar
    JST
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    To state the painfully obvious in a painfully obvious way, may I suggest you read the "sticky" at the top of the forum topic list.

    The title is "rotary phase converter designs and plans".

    It is just barely possible that it might have some application to your question.................

  6. #6
    neilca is offline Aluminum
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    The original post says he is looking for an alternative to a RPC. Reading the sticky would probably be futile to his pursuit.

  7. #7
    JST's Avatar
    JST
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    Quote Originally Posted by neilca View Post
    The original post says he is looking for an alternative to a RPC. Reading the sticky would probably be futile to his pursuit.

    In case it has escaped your thoughts..........

    The rotary converters are STARTED by a system that is exactly what the man wants..........................

    That should be obvious.............

  8. #8
    jim rozen is offline Diamond
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    "The rotary converters are STARTED by a system that is exactly what the man wants"

    Pony motor?



    Jim
    Last edited by JST; 06-20-2008 at 10:42 PM.

  9. #9
    JST's Avatar
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    Quote Originally Posted by jim rozen View Post
    "The rotary converters are STARTED by a system that is exactly what the man wants"

    Pony motor?



    Jim
    It WOULD work, but there MAY just be one or two OTHER ideas in there......

  10. #10
    bunnybungo is offline Plastic
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    Thank you for those who read and posted replies on my question.

    I have already tried to run my lathe motor using a static based setup and manage it to run.

    But after a few times of forward and reverse testing my capacitor smoked out.

    The Lathe motor is 7.5HP and I used 1175-1410mfd 110/125VAC Capacitor, please advice which is the most appropriete values to use.

    I will post my block/schematic diagram later. Thanks to all


  11. #11
    jminer99er's Avatar
    jminer99er is offline Plastic
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    I could be mistaken, but should the starting caps be rated for 220-250v?

  12. #12
    bunnybungo is offline Plastic
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    The reason I use the value mentioned above is because I have seen an identical machine and using that capacitor value. He told me that he is using that value of capacitor for more than 5years.

    But, I have read somewhere in these forum states that:

    -should use 100mfd per hp

    -use START capacitor of 370V rating

    -add a bleeding resistor per capacitor

    Are all of this correct? Can someone confirm.

    Thanks a lot


  13. #13
    JunkyardJ's Avatar
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    Exclamation Why would you think a 125v cap is OK on 220-240v?!?!

    Ok, common sense, you're trying to run a 220v 3 phase motor off of 220-240v. Did you really think somehow the cap would live with that kind of a overvoltage??? You're LUCKY it didn't EXPLODE! I dunno how many MFDs is appropriate, but the VOLTAGE should be MORE THAN 220v. I think the voltage is 330v minimum for caps used on 220v stuff for a safety margin. Honestly, as long as the voltage rating is OVER the operating voltage, it doesn't matter how high the voltage rating is. I'd get the highest voltage rated cap you can afford. They get more expensive the more voltage and MFDs they are rated for. DON'T SKIMP on the voltage rating! Higher voltage rated caps will last longer, and handle more abuse than lower rated units. Capacitors can fail EXPLOSIVELY, throwing shrapnel, hot oil soaked paper, and foil EVERYWHERE. I've had caps bigger than a pop can explode, and it sounds like a SHOTGUN going off! So, make sure your next cap is RATED for the voltage, and is for AC, not DC!!! Otherwise it can go KABLOOEY!

  14. #14
    bunnybungo is offline Plastic
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    Actually, I am very hessetant to use the value I've posted, but the thing is he just give me that extra capacitor for me to try, and for me to know if the lathe machine is working. So I did hehe

    When I am using the capacitor I hide it under the cabinet of the Lathe so when it blows no one will get hurt.

    I manage to run the lathe, even tried reversing and forwrding, so now I know that the lathe is working.

    But my next mission is to make it working for its intended use.

    Any inputs will be appreciated

  15. #15
    JunkyardJ's Avatar
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    Arrow You only need one bleeder resistor.

    It doesn't take long to bleed off 1000 mfd, so a small resistor should be adequate. You could use a couple 1/2w 150k ohm resistors, or 75k 1w. I don't think it would hold a charge for more than a couple minutes even with just one resistor. So, according to the formula you posted, you'd need a 370v (minimum) 750 uf cap to start the motor. If that's the formula you read, I'd say it's probably right. If it worked with a 1500mfd cap, it will probably run without that much. If that was only for the start circuit, it shouldn't matter that much. The problem you'll run into is it won't start if the cap is too small, from the way I understand it.

  16. #16
    peterh5322's Avatar
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    General rules ...

    Start capacitors are rated for the RMS line voltage ... 250 VAC caps for 240 volt lines, 125 volt caps for 120 volt lines.

    Run caps are rated for the peak applied line voltage, plus a 10 percent safety factor ... 370 volts for 240 volt lines, 740 volts for 480 volt lines.

    Alas ...

    Some SPC manufacturers use 125 volt start caps in their standard duty SPCs and 250 volt start caps in their heavy duty SPCs.

    Therefore, a standard duty SPC is really an accident waiting to happen, while a heavy duty SPC is really a standard duty SPC.

    A real heavy duty SPC should have a 250 volt start cap and one or more 370 volt run caps, and possibly also one or more 370 volt power factor correcting caps.

  17. #17
    bunnybungo is offline Plastic
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    If I am going to use
    2pcs 500mfd / 250v
    or 2pcs 1000mfd / 110-125v

    Would this be sufficient enough?
    What particular value is needed for bleed resistor? Will I need 1 resistor only or 2 resistors?


  18. #18
    JunkyardJ's Avatar
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    Arrow The 500 mfd caps will work.

    I'd see if you can find a 250 mfd cap, or close to it. That way you can tune in the value to get it to start. Get 2 500mfd caps, and a 250 or so, all 250v. If you get 110v caps, you'll have to put them in series. That cuts the MFD value in half. With 2 500 mfd caps, and a 250, you can make 750 MFDs which is what the formula calls for. Or you can step up to 1000, or put them all on to make 1250, which is what you have now. This isn't an exact science, capacitor values can vary BIG TIME from what's on the label. You'll have to experiment to see what works. You may find that with a big load on the lathe, it won't start with 750 mfds, you KNOW it starts with 1400, but it's more than required more than likely.

  19. #19
    bunnybungo is offline Plastic
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    Default Added Block diagram

    I am attaching a block diagram that I have created, please post input for me to correct all errors or potential problems that this may encounter.
    Attached Thumbnails Attached Thumbnails sphasev1.jpg  

  20. #20
    bunnybungo is offline Plastic
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    I am attaching a block diagram that I have created, please post input for me to correct all errors or potential problems that this may encounter.


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