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  1. #1
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    Default How to reverse a Cap start motor with only 3 wires a terminal box?

    Guys,
    I have a Cap start lathe motor wired as below:
    IMG_0002 (Small).JPG

    Normally a cap start motor and reverse it direction by reversing ONE of the windings ( 4 wires in terminal box ) This motor only has 3 and has me stumped.......
    Also, cap is rated at 125V. I thought it should be at least 250v?!

    I need to find a new rotary switch as well. Needs to be 3 pole with FOR, OFF and REV.
    IMG_0005 (Small).JPG


    Steve


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  2. #2
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    York, North Yorkshire UK
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    Default

    Quote Originally Posted by Sterob View Post
    Guys,
    I have a Cap start lathe motor wired as below:
    IMG_0002 (Small).JPG

    Normally a cap start motor and reverse it direction by reversing ONE of the windings ( 4 wires in terminal box ) This motor only has 3 and has me stumped.......
    Also, cap is rated at 125V. I thought it should be at least 250v?!

    I need to find a new rotary switch as well. Needs to be 3 pole with FOR, OFF and REV.
    IMG_0005 (Small).JPG


    Steve
    You definitely need to get at the junction of the two windings to include the ability to reverse the motor. I would look on the back of the terminal board to see if there are two wires connected to the common terminal. I would use a two pole change over switch with a centre off and add a no volts relay
    for safety in case of a power outage and you forget to turn the motor off.

    HTH.

  3. #3
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    Jul 2012
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    Griffith NSW
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    435

    Default

    Not all capacitors are disengaged once the motor is up to speed. Ive got a motor pretty similar to yours, and I had to remove one end of the motor housing to find where the wires were distributed to the two coils and my junction box looked just like yours. I went to the non-pulley end (with the cooling fan) and pulled it apart from there and thats where the wiring was done, but there is nothing to say it couldnt be done at the other end.

  4. #4
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    melbourne australia
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    Default How to reverse a Cap start motor with only 3 wires a terminal box?

    Quote Originally Posted by Sterob View Post
    Also, cap is rated at 125V. I thought it should be at least 250v?!
    I guess the voltage drop across the cap is less than 125V. The difference would be dropped across the start winding. Might be worth confirming that with a motor rewinder though.

    The terminal box on your motor is identical to the one on my pedestal drill motor. Assuming it's the same type of motor, it does have a centrifugal start switch.

    Chris
    Chris

  5. #5
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    Default

    For a bonus point. If it is wired as per Steve diagram and there is in fact a switch on the starting(?) winding. Why would they bother to bring three wires out?

    Stuart

  6. #6
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    Hi Steve,

    As a general principle you only need to reverse the polarity of the start winding to reverse the motor,

    In this case you might be struggling, it looks like a non-reversible high voltage motor with low voltage start winding, they do this to reduce the voltage rating on the start cap. Basically the start winding is tapped off the center of the run winding and therefore is half voltage.

    If you can find the other end, and bring out the extra wires, it might work.

    Not sure why there is an extra terminal, as Stuart said why is it there? It might be just a connection point for the start cap.... it's not multipole ( 2 speed? ) is it?... over to you Stuart

    Regards
    Ray

  7. #7
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    Quote Originally Posted by Stustoys View Post
    For a bonus point. If it is wired as per Steve diagram and there is in fact a switch on the starting(?) winding. Why would they bother to bring three wires out?

    Stuart

    Yes, maybe there is no switch. I speculated there. I'm starting to think this motor was not mean't to have reverse on it at all. I think the owners may be mis-informing me.
    Its a fairly simple chinese lathe ( 12 x 24 )
    IMG_0004 (Small).JPG

  8. #8
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    Quote Originally Posted by RayG View Post
    Hi Steve,

    As a general principle you only need to reverse the polarity of the start winding to reverse the motor,

    In this case you might be struggling, it looks like a non-reversible high voltage motor with low voltage start winding, they do this to reduce the voltage rating on the start cap. Basically the start winding is tapped off the center of the run winding and therefore is half voltage.

    If you can find the other end, and bring out the extra wires, it might work.

    Not sure why there is an extra terminal, as Stuart said why is it there? It might be just a connection point for the start cap.... it's not multipole ( 2 speed? ) is it?... over to you Stuart

    Regards
    Ray
    I don't want to modify it if its wasn't meant to be reversible. I just wanted to get revers working if it was meant to do that.
    No, its a single speed 4 pole motor, 1 hp.

    edit: Just had a thought...If the wiring WAS modified for the Australian market, The 4th wire may have been tucked back inside the motor. ( for what ever reason....)?

  9. #9
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    Hi Steve,

    You should be able to get a wiring diagram for that, is there a model number or something that you can see.

    Regards
    Ray

  10. #10
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    Hi Steve,
    If you do much thread cutting you'll want reverse sooner or later.

    Hi Ray,
    Assuming Steve's diagram is correct except M6 is indeed a center tap of the run winding.
    Lets say
    V=active
    U=neutral
    M6=neutral
    What if we swap to V=neutral and U=active, leaving M6=neutral.
    Havent we reversed things?
    I really have no idea and am just having a stab. Dont try this at home

    Stuart
    Attached Images Attached Images

  11. #11
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    Hi, Where do the mains connect to, and is there a diagram inside the terminal cover.
    Regards
    Hugh

    Enough is enough, more than enough is too much.

  12. #12
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    Conder, ACT
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    Default

    DISCLAIMER

    No liability is accepted by UBeaut or the Wood Working Forum's administrators
    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.
    WARNING

    Information supplied within posts is not to be considered as detailed formal instructions to complete a task.
    Members following such information do so at their own risk

  13. #13
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    Hi Stuart,

    Short answer, there are so many ways it could be done, it's hard to tell....

    Long Answer, ( after digging out Rosenberg..)

    The most common type of capacitor start motor has the run windings at the bottom of the slots and start windings over the top, but displaced 90 electrical degrees, so the start poles are midway between the run poles.

    The run windings will happily work either way, forward or reverse, the only thing is which way you kick it... The start winding polarity I think determines the direction that you kick it... so reversing the run winding polarity might not reverse the direction.

    I'm going to guess that the extra terminal is for the external start button.

    Regards
    Ray

  14. #14
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    Quote Originally Posted by RayG View Post
    Hi Steve,

    You should be able to get a wiring diagram for that, is there a model number or something that you can see.

    Regards
    Ray

    Hi Ray,
    Yes, I've got a wiring diagram in the manual but it appears to be in-correct.
    It shows a 4 wire motor. It also has control wiring which the lathe does not have. It only has a rotary 3 pole switch. It appears to have 3 positions but I can't be sure as most of it is missing....
    I'm guessing the the wiring was changed for the Australian market?
    IMG_0004 (Medium).JPG

  15. #15
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    Default

    Quote Originally Posted by Stustoys View Post
    Hi Steve,
    If you do much thread cutting you'll want reverse sooner or later.

    Hi Ray,
    Assuming Steve's diagram is correct except M6 is indeed a center tap of the run winding.
    Lets say
    V=active
    U=neutral
    M6=neutral
    What if we swap to V=neutral and U=active, leaving M6=neutral.
    Havent we reversed things?
    I really have no idea and am just having a stab. Dont try this at home

    Stuart
    I have no idea if they have ever cut any threads on this Lathe. If I had to guess, I'd say no, as they are Farm Boys, with no formal training. ( I've known them a long time. )

    The 'centre tapped winding' theory is interesting. I've never heard of that in single phase motors before. ( had not much exposure to them. )

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