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Snubber circuit

  1. Jul 15, 2016 #1
    I have been doing a lot of reading online concerning this circuit but having little luck in my dilemma. I work in the bowling industry the machines use 1/3 HP 115 v single phase induction motors. FLA on these motors is 6 Amp. These motors use capacitors 480-520 MF for braking. This braking circuit is achieved by using 2 NC contacts The motor run circuit uses 2 NO contacts The relay is of course 4PST My problem is: The NC contacts used for braking burn off at least 50% compared to the NO contacts. This is a common problem for years across this industry. The concern has been addressed with the manufacturer to no avail. I guess selling more relays is more important! Does anyone have a snubber/ suppression circuit that would help with this problem? Sure hope someone can help me
     
  2. jcsd
  3. Jul 15, 2016 #2

    Svein

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    I had that problem once. I solved it by inserting an 8Ω 50W resistor in series with the relay contacts.
     
  4. Jul 15, 2016 #3

    anorlunda

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    Last edited by a moderator: May 8, 2017
  5. Jul 15, 2016 #4

    berkeman

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    Do these circuits typically use zero-crossing detection for voltage (closing the contacts) and current (opening the contacts)?
     
  6. Jul 15, 2016 #5
    I tried a solid state relay they didn't last long at all. I you have ever gone bowling you would see that the motors are cycling on and off a lot!. These machines on a busy day will run 10 hrs. straight. A compact arc suppression circuit is what I need. Using a rated TVS in the circuit might also help.What do you guys think.
     
  7. Jul 15, 2016 #6

    anorlunda

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  8. Jul 15, 2016 #7
    20160715_163238.jpg As you can see the 2 middle NC contacts are shot
     
  9. Jul 15, 2016 #8

    jim hardy

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    Can you show that braking scheme ? Does it try to reverse the motor ?
    Can you tolerate say a half second delay before applying the brake?

    Are you free to experiment ? Perhaps a couple amps of DC through the motor would be an equally effective brake.
     
  10. Jul 15, 2016 #9
    Yes I have a schematic will try my camera on my android. I will get back .Thanks
     
  11. Jul 15, 2016 #10
    I added as a thumbnail. Blue is the braking circuit Red is the run circuit Let me know if picture is ok
     

    Attached Files:

  12. Jul 15, 2016 #11
    I looked this Quencharc up.Interesting. I'm not sure exactly what they mean " put it across the contacts"
     
  13. Jul 15, 2016 #12
    Like this?
     

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  14. Jul 15, 2016 #13

    jim hardy

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    are these the contacts burning up? i only count half enough on the relay picture to agree with the drawing....
    upload_2016-7-15_21-30-23.png

    is it safe to assume the NO contacts labelled "T" (above and left of this snippet)
    and the NC contacts labelled "T" (in this snippet)
    are on same relay? The one you pictured?

    Interesting, i never saw this scheme before - i wonder how it works....
    ...are they switching the start caps from series to parallel and connecting them across the motor run coil, to turn the motor into an induction generator so it'll slow itself down? The two resistors providing electrical load? They must be low ohm... Do they ever get hot?

    Or maybe i dont understand the scheme at all.....

    If i do have it right, then here's a hypothesis for you to talk over with your guys
    ........
    On a start sequence
    after CS inside the motor opens(Centrifugal Switch, opens when motor reaches speed)
    it is the job of those two resistors to discharge those two big starting capacitors.
    >>>>Were one of those resistors to fail open <<<< the capacitor no longer has a discharge path..
    ...the first symptom would be motor doesn't brake as quickly as it should
    ...second symptom would be relay T gets burnt contacts 3 and 8(lower right) but probably not contacts 5 and 10
    reason is
    because as soon as relay T closes its NC to brake,
    any charge that got trapped on the capacitors when CS opened,
    discharges immediately through those two bottom right contacts , and probably at hundreds of amps !
    If you've ever put a screwdriver across a 500uf capacitor that has any voltage on it you probably melted a big chunk out of your screwdriver. I know for i have done that and the flash and noise of the arc is, well, positively shocking..(:rolleyes:,:sorry:, sorry, just couldn't resist)

    To test the hypothesis
    How about placing a voltmeter across the capacitors, from TS20 to 115volt return
    and watch voltage? It should drop to zero almost immediately after the motor reaches speed
    then move meter to TS12
    it should read zero until relay T applies the brakes , at which point it should jump up to probably 115volts and fade back to zero as motor slows down.
    i would use an analog meter so the DMM doesn't get confused trying to autorange.
    If voltage stays high after start your resistors are not discharging the caps
    you'll have to try several start-stop cycles because it's random where in the line cycle CS opens, and by Murphy's Law on first few tries it'll open at just the instant there's no voltage on the caps..

    If you find an open resistor
    then of course replace it,
    and I'd try placing a 12 volt 50 watt or so lamp in series with each of those resistors. Every time the motor brakes you should see the lamp light up ever so briefly while motor decelerates.
    If it fails to light that tells you the resistor has failed open and your capacitors are abusing your contacts.
    Those little Halogen lamps for accent lighting might be an easy fit . Experiment with different wattage to get a noticeable glow, you don't want a bright flash..
    That gives you a visible indication that your maintenance guys can watch for. "Let there be light" seems easy to remember....

    What do you think ?

    What did i miss ?
    (Oh wow, what if CS doesn't open ? Voltmeter test described will show that)

    old jim
     
    Last edited: Jul 16, 2016
  15. Jul 16, 2016 #14

    Svein

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    Ah - I did not read your post carefully enough. So there is a pair of NC contacts in the braking circuit. That takes it into an entirely different area. Breaking a circuit is much harder than you think, especially in this context. You need circuit breakers, not relays. Here is a link to some: http://new.abb.com/low-voltage/products/circuit-breakers/emax2.
     
  16. Jul 16, 2016 #15

    jim hardy

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    Hmmmmm...
    Does that motor ever get "jogged", that is, switched on briefly and then back off before it gets up enough RPM's for CS to open ?
    If the caps don't have time to discharge , it assures the NC contact will destroyed by the mechanism i described above. And in that scenario they aren't even offered a chance to discharge.

    What controls relay T ? Can it "jog" the motor, switching it back off before it gets up to speed ?

    If that sort of operation is possible, try it and watch for outrageous arcing on those contacts when switched off too soon after start.

    I'm beginning to think that's more likely your culprit. Don't know why, just a feeling.
    It would explain why the failure is industry-wide, though.

    old jim
     
  17. Jul 16, 2016 #16

    jim hardy

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    if you find that exaggerated arcing on jog
    and you have the freedom to try something

    Are T's contacts break-before-make ?
    if so , this should work
    ...
    lift and tape wire that goes between TM-1 and TMP-Z
    put a 250 uf motor start capacitor CTM-3 as shown, with a bleeder resistor, maybe 10K 5 watt
    add jumper around lower right NC contacts, C1-32E to 115 volt return C1-34P
    bowlingalley1.jpg

    in other words fix it so CTM1 and CTM2 don't get charged during motor start. That way they can't wreck the relay when motor gets jogged..

    I know it costs you a lot more for the man-hours to replace those relays than for the relays themselves....
     
  18. Jul 16, 2016 #17
    Thanks for getting back. I work nights so if I don't get back don't worry I will. Anyway the T stands for the machine TABLE motor
    There is also a S which stands for the machine SWEEP motor {Not shown] The relays are 4 pole 2 NO & 2 NC The braking scheme is exactly
    as you said. The resistor is a 3 terminal center tap 625 ohm. In my opinion
    using 2 480-520 caps is way over kill. These 1/3 motors only require a 250-320 MFD cap. The schematic is odd because the motor plug has 5 terminals V X Y and Z } V is return[neutral] X is 115 V} Y & Z are the start circuit. This problem with the NC contacts burning off is a problem with all Bowling machines it's a flaw of the design I think. That's why I'm here on this forum. I will try your suggestion tonight. If I could a 12 DC injection for braking makes more sense.
    Motors never jog. Once bowler throws a ball a DATA SENSOR sends a signal to start the machine cycle. Once machine has completed a cycle the machine returns to HOME posistion
     
  19. Jul 16, 2016 #18
    For everyone helping Thanks! I wanted to add just for clarification, The braking circuits main job is to maintain the timing of the T & S motors
    If any of you have bowled you would see the two main components The sweep which sweeps away the fallen pins then there is the
    table [T] which picks the pins up and down. There for the two components must stay in unison.
     
  20. Jul 16, 2016 #19

    jim hardy

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    When they're in series for start , two 500's make a 250... In parallel for braking they make 1000, surely enough to overcompensate the induction motor and turn it into an induction generator.
    Wow 1000 uf at 60 hz is only 2.65 ohms ?
    It's important that relay T's NO contacts open before the NC ones close else line voltage is applied briefly across 2 ohms of capacitance. Relay races are awful troublemakers.

    That voltmeter test will be interesting.
    500uf X 312.5 ohms is a time constant of 16 milliseconds, one line cycle
    so after VS opens, voltage at TM-1 should decay to zero in around 1/10 second
    i'd hook a dc meter there on maybe 250 volt scale and watch a few machine cycles
    if you see dc more than just a blip, there's trouble in River City alright..

    oh - a computer ? see my signature.. Look for occasional big sparks while there's a meter across those caps.
    I'll be happy to be wrong,
    old troubleshooter's maxim: " Before you figure out what it is, you'll figure out a lot of what it ain't".

    old jim
     
  21. Jul 16, 2016 #20
    Braking not breaking

    Thanks I'll check tonight. I'm glad you're working with me on this. I believe what you're saying is what's ocurring. I never understood after so many years why even the new machine use AC motors. at 115 volt
    If they must use AC why not 230 volt. My opinion!
     
    Last edited by a moderator: Jul 17, 2016
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