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  1. #1
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    Default Clarifying 3ph Motor connections

    I am pretty confused by all the comments I have read about different 3ph motor connections. I would like to try and make sense of it all so I can hook up/have hooked up my CY lathe soon. Its 7.5 HP motor will be driven by a 4.5kW VFD.

    Anyone, feel free to set me right on any point.

    3Ph motors have 3 field(?) Windings?

    Each of these have 2 wires for a total of 6.

    These wires can be connected in Delta or Y.

    What speeds do these connections give?

    My recently acquired piston grinder has a 2880rpm 3ph motor. How is it connected?

    A number of people have suggested that my CY lathe motor may not be able to be connected via a VFD as it is a 2 speed motor.

    It has 6 wires coming from the motor out to the connector box. This suggests to me that I can connect it any way I want. Am I wrong in this?

    Thanks in advance.

    Dean

  2. #2
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    Hi Dean,

    I have limited knowledge/experience with 3 phase motors. I have however successfully converted two 3 phase motors from star to delta. One for my mill and one for my lathe. The lathe VFD conversion is yet to be done though.

    Until recently I have never seen a 2 speed 3 phase motor. I not so long ago acquired a band saw. I did a VFD conversion on that. It had a 2 speed (1440/2800rpm) 3 phase motor. Running it on the VFD proved unsuccessful as it was very low on torque/power and as such I swapped it with an 3 phase motor.

    In my quest for knowledge on how these 2 speed motors are wired ( wanted to convert it for VFD like the others) I found a number of wiring diagrams. While I cannot be sure, it seems that these motors would be quite difficult to convert to a purely delta configuration. I have found a number of sites but this one seems to be the easiest to follow:
    http://www.practicalmachinist.com/vb..._c-p_motor.jpg

    Also, the speed at which a motor spins is dependent on the number of poles. 2 pole motor is 2880 rpm, 4 pole motor is 1440 rpm and a 6 pole is 750 rpm or there a bouts.

    There are a number of people here that would know much more than me on this subject. I'll wait to see what they have to say.

    P.S. I think you can run your 2 speed motor on a VFD but you may find it will lack a significant amount of power. It will still run though I would imagine.

    Cheers,

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    I'd say that 3 of the wires are the ones used for one speed and the other three for the other speed.
    The ones I have seen usually have one speed wired along a row of connections and the other speed along another row.

    If the motor is capable of being configured as ∆ or Y "at the connection box" there is usually some indication of this either on the motor info plate or the inside of the connector box or its cover.

    If there is no indication about this then it's most likely connected internally as a Y and there is no way you can convert it to ∆ at the connector box. It will have to be opened up and the coil connections rewired internally.

  4. #4
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    Thanks Simon. Now what I don't understand is how this layout differs from normal Delta or Y motors and how the poles are laid out.

    Dean

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    DISCLAIMER

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    We strongly advise contacting a Licensed Tradeperson for all electrical work.
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  6. #6
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    Hi Dean,

    I assume your VSD has 240V 3 phase output?
    Most smaller 3 phase motors(3hp and lower) are wired for 415V star and 240V delta. Although not all motors have the star point bought out so they can be easily wired for delta.
    2 speed motors are different and above 3hp motors may well be wired for star start delta run.



    There was a thread not long ago talking about the theory of converting a series delta, parallel star motor 2 speed motor to parallel star, parallel star. This would mean you where wired for 240V on one speed and 415V on the other...............Its still just a theory as far as I know.

    Some say you shouldn't use two speed motors on a VSD as you aren't meant to have switches between the VSD and the motor. You can have switches...... you just have to be sure not to switch them while the VSD has an output.


    A "normal" motor in delta doesn't have two windings in series.

    Stuart
    Last edited by Stustoys; 26th Dec 2013 at 10:28 PM. Reason: spelling

  7. #7
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    As the disclaimer suggests get someone in who knows.


    But this rough sketch will show roughly what the difference is.
    2spm.jpg
    Excuse how rough my sketches are. The easiest way to see what you have is to do a continuity test on the 6 connections. A normal 3 phase will give you a reading between A1 an A2, B1 and B2, C1 and C2 and nothing between them (A,B and C ) the 2 speed motor ( Phase shift ) will give you readings (different values) between all 6 connections.
    I hope this helps but if it just adds to the confusion I hope it does enough to convince you to call in the experts.
    Regards
    Hugh

    Enough is enough, more than enough is too much.

  8. #8
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    Dean,
    Can you post a photo of the motor nameplate?
    That may give some indication of the winding configuration.
    Cheers,
    Bill

  9. #9
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    Stuart. Thanks for that info. I am not planning on having 2 speeds if I can help it. I was hoping to use the VFD for that.



    Quote Originally Posted by A Duke View Post
    As the disclaimer suggests get someone in who knows.


    But this rough sketch will show roughly what the difference is.
    2spm.jpg
    Excuse how rough my sketches are. The easiest way to see what you have is to do a continuity test on the 6 connections. A normal 3 phase will give you a reading between A1 an A2, B1 and B2, C1 and C2 and nothing between them (A,B and C ) the 2 speed motor ( Phase shift ) will give you readings (different values) between all 6 connections.
    I hope this helps but if it just adds to the confusion I hope it does enough to convince you to call in the experts.
    Regards
    Thanks Hugh. That will help me to work it out.

    I hope this helps but if it just adds to the confusion I hope it does enough to convince you to call in the experts.
    Not as easy as it might seem. I actually don't expect much success in finding an "expert" who will both know anything about this whole situation and be willing to come out here. It is my understanding that any of the more knowledgeable electricians in the area are not interested in small domestic or difficult jobs.

    I am currently just trying to work whether the existing motor will do to get this lathe going. No point in calling out a sparky if I haven't got a motor that will work.

    Dean

  10. #10
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    Hi,
    A lot of the sparkies I know and have worked with would run a mile if confronted with this sort of problem, that's why I said expert, you have to get the right guy [I mean person].
    Good luck and don't electrocute your self.
    Regards
    Hugh

    Enough is enough, more than enough is too much.

  11. #11
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    Hi Guys,

    Does this help...

    <http://en.wikipedia.org/wiki/3-phase_induction_motor>

    .
    Best Regards:
    Baron J.

  12. #12
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    Quote Originally Posted by Oldneweng View Post
    I actually don't expect much success in finding an "expert" who will both know anything about this whole situation and be willing to come out here. It is my understanding that any of the more knowledgeable electricians in the area are not interested in small domestic or difficult jobs.
    A sparky isn't necessarily an expert in this case. With motors most of them just connect and "fault find" (that is, say "I think the motor is buggered" or maybe "The motor tests out ok but..."). You need to talk to a motor rewinding place. The easiest way to get a working motor is probably to get them to rewind it in the style you want - at one stage I had a motor fault and they explained that they could probably find the issue for me but by the time they did it would cost more than cutting off the windings and redoing it, so it got rewound and the fault vanished.

    Michael

  13. #13
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    I just had a look on Google Earth to see where Wimmera was located, and it appears to be a long way from anywhere. Basically western Victoria, west of Horsham.
    I now understand Dean's problem of finding a knowledgeable electrician anywhere within cooee.
    My suggestion is for Dean to contact the local electrical wholesalers and ask for a recommendation, or call into a nearby factory that has an on-site electrician who is experienced in motor control gear. Carrying a slab in the boot could be beneficial.
    (Just realized how big a 7.5hp motor is, not really an option to take it around to the local factory).

    As others have said, be very careful with electricity, it bites.
    Good luck in getting everything going the way you want it.

    Alan...
    Last edited by Uncle Al; 27th Dec 2013 at 10:32 AM. Reason: HP not Kw

  14. #14
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    > I am pretty confused by all the comments I have read about different 3ph motor connections.
    >I would like to try and make sense of it all so I can hook up/have hooked up my CY lathe soon.
    >Its 7.5 HP motor will be driven by a 4.5kW VFD.

    7.5HP * 0.746 = 5.6kW. What is the reason you want to drive it with an undersized 4.5kW VFD? Maybe you have access to a low cost VFD in this size? Get me right, it is possible to do, but it will limit maximum output power to 4.5kW. It may also trip out sooner than expected when braking with a heavy workpiece in a heavy chuck. It is normally a better choice to size the VFD either same as the motor, or one size larger to allow for chopper frequency and temperature derating.

    > Anyone, feel free to set me right on any point.
    > 3Ph motors have 3 field(?) Windings?
    > Each of these have 2 wires for a total of 6.
    > These wires can be connected in Delta or Y.

    So far all correct for a small horsepower single speed 3ph motor

    > What speeds do these connections give?

    The speed is given by the line frequency and the number of poles the motor has. With most small HP 3ph motors, the number of poles is given by the rotor.

    Less common, there are also 3ph motors that can switch the number of poles electrically. These motors have the stator windings split into several sections. These sections can be switched by more or less complex switches to give 2, 4, 6 or 8 poles. More than 2 speeds is getting fairly complex though. And then there are motors that have 2 speeds AND can be wired in Y or delta.


    > My recently acquired piston grinder has a 2880rpm 3ph motor. How is it connected?

    Impossible to tell, it depends. You best have a look under the motor connection box cover (disconnect from mains first!). But please be aware that there may be safety concerns using a VFD for grinders. It is easy to exceed the safe speed of a grinding wheel, a VFD can drive the motor at more than twice the line frequency resulting in more than twice the rpm. You need to make sure this can not happen.

    > A number of people have suggested that my CY lathe motor may not be able to be connected via a VFD as it is a 2 speed motor.
    > It has 6 wires coming from the motor out to the connector box. This suggests to me that I can connect it any way I want. Am I wrong in this?

    Have a look under the connection box cover. If it is a 2 speed motor, and there are only six wires coming from the stator, then you cannot switch it from Y to delta. If it was a 2 speed dual voltage motor, it would have 9 or 12 wires coming from the stator. It can be fairly complex to work out. A 5kW VFD is not cheap, and it is quite a bit of power if things go wrong, definitely not a good idea for learning and experimenting. It is not as forgiving as a fractional HP VFD for a small hobbylathe.

    What I would recommend in your situation (fair distance from an expert shop that could assess suitability), is to sell this motor. Depending on age and quality, it is likely you can get a good price for a 2 speed motor in good condition. Possibly enough to buy a brand new (or certainly a used) stock standard single speed motor. That way you can choose the number of poles (top speed) and the voltage and the size/power. I suggest you buy both motor and VFD from the same dealer, that way you are 100% sure both fit together. You may not necessarily need it to be 7.5HP, it is quite likely that for home shop use half that much is plenty (and much cheaper). It for sure depends what you do, and how big your lathe is, but 7.5HP sounds like industrial grade roughing cuts all day long. Chris

  15. #15
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    A few things to note- I'm sure braking won't be a problem as the CY has a foot brake, and the antrac has 3.75hp in low speed, this is enough to take 3mm DOC's at .25mm/rev, with a negative rake tool. More than enough for the sort of things we do!

    My suggestion was to hook the VFD up to one of the speeds, which one i'm not sure, and run it on 240v but in star. I would think even if the motor power is halved it should have plenty of go.

    Cheers,
    Ew
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

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