3 phase wiring/capacitor question.
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  1. #1
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    3 phase wiring/capacitor question.

    Hello all,

    I have a question that maybe someone here can help me with.

    I am making a converter and for some reason it does not work on a motor I have.

    I hook L1 and L2 to the motor wires. I hook my capacitor bank to L2 and the third leg of the motor.

    I put power to it and hit the momentary button to fire the capacitors.

    The 5hp motor Fires up and away it goes. Everything is great.

    I also used it in the same way to start another 5hp motor, no problems.

    Now, I step up to a 7.5 HP motor and it will not start it. My first thought was that I need more juice from the start capacitors but I am already running plenty by my calculations.

    The start capacitors are 216-259 MF each and I have four of them. Using the low end of 216, I have 864 Mf total and that divided by 7.5HP would be 115 MF per HP. That should be plenty to start this motor. It does make a difference but still does not get the motor up to speed to continue running.

    I though maybe the motor was bad so I took it to work and hooked it to the converter at work and it fired right off, no problem.

    I took the cover off the work converter to see what capacitors were in there. There are four of them and they are mounted in a manner that does not allow you to see the writing on them. They are in there with silicone holding them in place. I don't want to yank them out because the run capacitors have had the information sanded off so the start capacitors probably do as well.

    Anyone have any suggestions as to what the problem could be?

    I need 7.5HP to run my 5HP air compressor.

    Thanks.



  2. #2
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    Re: 3 phase wiring/capacitor question.

    Try this website for RPC questions. (RPC, Rotary Phase Converter)
    http://www.practicalmachinist.com/vb...splay.php?f=11
    Last edited by Norcal01; 07-12-2008 at 10:36 PM. Reason: Added info.

  3. #3
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    Re: 3 phase wiring/capacitor question.

    Let me get this straight. this is a static phase convertor without an ider motor right?

  4. #4
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    Re: 3 phase wiring/capacitor question.

    You have the capacitors in parallel right? Just checking, not trying to be a prick.

    I would guess that it has to do with the motors inrush current, that can be 6-10 times the motor's full load current. So at 230V FLA is 23A according to NEC 430.250 the inrush current could be as high as 2.3kA. Obviously its for milliseconds, but maybe that could do it?

    Sorry if im telling you what you already know!


    Andrew

  5. #5
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    Re: 3 phase wiring/capacitor question.

    Norcal01,
    Thanks, I will check it out.

    Tresi,
    Kinda, once I get the motor starting correctly it will become the idler and then be attached to my air compressor. No point connecting it up to the compressor until I get it running/starting properly first.

    TGaky321,
    Parallel is correct. The basics are always a good place to check first.

    As for the inrush current, you are not telling me something I already know. Actually I guess that you are saying that the initial current demand is high and that may be the problem but since it worked at my job without problems on a 30A circuit and does not work at home on a 50A circuit I would think that this would not be the problem in this case.

    I wonder if putting the capacitors on a different leg would make a difference. I would not think so but wonder what else it could be. I do not have run capacitors between legs but plan to do that after getting things going to balance the legs.

    Any more help you guys can provide would help. I will try changing legs around and see what happens.

    Bob



  6. #6
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    Re: 3 phase wiring/capacitor question.

    But can you start it by hands? that would address the doubts about initial current. Second if you can hear any noise and fill silght vibration of the motor - that would indicate polarity problems.
    Last edited by Nomand; 07-14-2008 at 10:17 AM.

  7. #7
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    Re: 3 phase wiring/capacitor question.

    I can't start it by hand. I pulled a rope a bunch of times and it will just not start.



  8. #8
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    Re: 3 phase wiring/capacitor question.

    Quote Originally Posted by Bob Warner View Post
    I can't start it by hand. I pulled a rope a bunch of times and it will just not start.
    Any 3ph AC motor will work from one phase with 1/3 of power. If your motor is operational then connecting 1ph and neut to any of 2 legs must run the motor (may be with a little push). Further is there a possibility then windings are assembled incorrectly? Looks like you have either broken windings or windings are working in wrong sequence... Still if there is a wrong sequence you should be able to hear some low humming noise (can be very low) and fill some little vibration on the shaft...
    Last edited by Nomand; 07-14-2008 at 01:21 PM.

  9. #9
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    Re: 3 phase wiring/capacitor question.

    OK, I got REALLY redneck this morning and tried something. First, there is a little hum and vibration, you can tell there is power present.

    My son put his four wheeler on blocks (Told you it was redneck), I put a large pulley on the shaft and we spun the heck out of the shaft using the tire against the pulley and then put power to it, NOTHING. I reversed two wires in case we were going the wrong way and tried again, NOTHING.

    I brought it to work and hooked it to the converter at work and works just fine, even with the big pulley on it. I reversed the wires and it runs backwards just fine.

    Kinda weird, huh?

    Not sure what to do next, maybe take the converter from work to the house and see if it works there.



  10. #10
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    Re: 3 phase wiring/capacitor question.

    Bob,

    Maybe skip the converter-idler attempt and just get a VFD to run the machine(s) directly.

    re: the capacitors. Without trying to disassemble things, you should be able to
    just hook a decent multimeter up to the capacitor part of the circuit/converter/starter and turn the multimeter to "Capacitance" and measure the value that's actually there. You might have blown/marginal cap(s) that let you squeek by with the 5hp idler but don't let you crank up the 7.5hp beast.

    Be careful if applying voltage to a motor and it doesn't turn. That puts the full short-circuit voltage through the motor windings without any back EMF to resist it, so you are running in a locked-rotor max voltage and current condition.

    Good thing the motor didn't 'kick' with the 4-wheeler rednecking against the pulley. That could have kicked or launched the 4-wheeler!

    Oh, and if you can spin up a 7.5 hp motor by hand with a rope, remind me to not get on your bad side. Hopefully you made sure the rope pulled completely loose from the motor before hitting the 'start' circuit and putting juice to the motor. Man-rope-7.5hp motor with juice=hurt/dead man

    When you rednecked the motor, you just applied the two 'hots' to the motor on two leads and didn't have the capacitor-start bank there, right? Once the 3-phase motor is spinning up to speed, it should run (at reduced power and efficiency) with just the two hots from 1-phase (US 240V household stuff).

    Maybe your 7.5hp motor is wired for 440V and you are trying to feed it from your house/shop 240V?

    The general guideline seems to be 100 micro-F (uF) per HP for the start circuit. I think your caps might be borderline/insufficient for the 7.5HP motor, and just borderline/OK for the 5HP motor. You do have AC rated caps, and they are minimum 240V rated, right? If not, you would most likely have blown the caps by now.

  11. #11
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    Re: 3 phase wiring/capacitor question.

    Quote Originally Posted by Bob Warner View Post
    OK, I got REALLY redneck this morning and tried something. First, there is a little hum and vibration, you can tell there is power present.

    My son put his four wheeler on blocks (Told you it was redneck), I put a large pulley on the shaft and we spun the heck out of the shaft using the tire against the pulley and then put power to it, NOTHING. I reversed two wires in case we were going the wrong way and tried again, NOTHING.

    I brought it to work and hooked it to the converter at work and works just fine, even with the big pulley on it. I reversed the wires and it runs backwards just fine.

    Kinda weird, huh?

    Not sure what to do next, maybe take the converter from work to the house and see if it works there.
    Do not put any pulleys! - sounds dangerous with 7.5 HP. If motor operates a little push by hand quite enough.
    How many wires do have coming out of the motor? Can you post picture of tag plate? What circuit at home do you use - 110 or 208 or 240 (it is different from 208 - 240 is 1 phase, 208 is 3 phase)?
    Since you said it worked at work - I am pritty sure we are dealing with mixed up polarity - and not in motor.
    And again does it work at home from 1 phase and neutral - 110v will do. At least it must start rotating somehow.
    Last edited by Nomand; 07-14-2008 at 07:36 PM.

  12. #12
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    Re: 3 phase wiring/capacitor question.

    Now that im thinking about this, i dont get how your getting 3ph by just using one cap bank?

    I kind of glanced at this website, I dont know if you have seen it.
    http://home.att.net/~waterfront-wood...econverter.htm

    It seems like static phase converters are application specific. It depends on the impedance of the motor.

    Maybe that will help?

    Andrew

  13. #13
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    Re: 3 phase wiring/capacitor question.

    Quote Originally Posted by tanky321 View Post
    Now that im thinking about this, i dont get how your getting 3ph by just using one cap bank?

    I kind of glanced at this website, I dont know if you have seen it.
    http://home.att.net/~waterfront-wood...econverter.htm

    It seems like static phase converters are application specific. It depends on the impedance of the motor.

    Maybe that will help?

    Andrew
    What you see on this web site is so called balanced converter. This would deliver about 90% of 3phase if everything setup right. If you remove starting capacitor and one of the line capacitors you will get most common converter delivering about 75% of 3ph. And starting capacitor in most cases unnecessary due to natural unbalancing of the rotor.
    My feeling that in Bob's case he either connecting lines to wrong wires or his capacitiors are actually too big. Any of these conditions violate polarity if the phases. The whole idea behind the capacitor is to provide phase shift 120deg. Apparently he is shifting either too much or too little. And different wings are falling into counterphase.
    But first thing first - he must manage to start the motor from 1ph or as MoonRose called 2 hot lines. And then adding capacitors try to bring motor to full power.

  14. #14
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    Re: 3 phase wiring/capacitor question.

    Here is what I have.

    Wires are connected as follows:

    7-1
    8-2
    9-3
    4 through 6 all tied together


    Line 1 connected to 9 & 3
    Line 2 connected to 8 & 2
    Capacitors connected to L2 and 7 & 1

    I turn on the power and hit the push button to fire the capacitors.

    The motor tries to spin up but never gets fast enough to run and starts to slow down so I shut off the power.


    This exact same setup on the 5HP motor fires it instantly.
    Last edited by Bob Warner; 07-15-2008 at 11:13 AM. Reason: Stupid Attack



  15. #15
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    Re: 3 phase wiring/capacitor question.

    Quote Originally Posted by Bob Warner View Post
    Here is what I have.

    Wires are connected as follows:

    7-1
    8-2
    9-3
    4 through 6 all tied together


    Line 1 connected to 9 & 3
    Line 2 connected to 8 & 2
    Capacitors connected to L2 and 7 & 1

    I turn on the power and hit the push button to fire the capacitors.

    The motor tries to spin up but never gets fast enough to run and starts to slow down so I shut off the power.


    This exact same setup on the 5HP motor fires it instantly.
    Disconnect capacitors while keeping connected L1 and L2 - it should run with slow steady speed.
    Herewith I am attaching possible wiwring of your motor wth 3ph. So, what you mentioned above looks correct. Try to run without capacitors.
    Attached Images Attached Images  
    Last edited by Nomand; 07-15-2008 at 11:45 AM.

  16. #16
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    Re: 3 phase wiring/capacitor question.

    I got it.

    This thing was driving me nuts. I decided that there had to be something wrong with the motor. I decided to take the case apart, decided if it was bad I could scrap the copper.

    As I started taking it apart the number 3 wire felt funny and I could that it was broken inside the insulation. The wire must have touched when it ran with the three phase converter at work. I repaired the broken wire and it works perfectly fine now. Starts up immediately.

    This is the type of thing that can really throw you. It does me anyway because I KNEW I had things connected correctly. I KNEW I had the right amount of capacitors. I KNEW I was doing everything right, but it would not work.

    Sometimes you just got to get away from it and let it go, then go back when you are not mad at it. I was mad at this for a while and could not make sense of the lack of results.

    Now I can go on to the rest of the build.

    Thanks to all that helped,

    Bob



  17. #17
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    Re: 3 phase wiring/capacitor question.

    Glad it's all working.

    And all that trouble because of a broken wire ...

  18. #18
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    Re: 3 phase wiring/capacitor question.

    Thats cool that its finally working, sometimes the stupid things will get you for a while.


    I remember when I was younger we were working on a machine that would make the headliner material for Chrysler's. We had moved it across the factory, when we went to start up a small auxiliary feed motor wasnt working, I was going nuts, checking phases, checking connections, voltages etc etc. It unded up being a tripped heater in the contactor. All it needed was a push of a button!

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