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Cap is Crap!

Discussion in 'Commercial' started by Zman, Jul 19, 2017.

  1. Zman

    Zman Hanging by a Thread

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    Last call today.Young ,460/3 Phase Carrier RTU.Both Stages calling, One Condenser Fan spinning wrong way, both Compressor Contactors pulling in/out by the second,not staying engaged. Kill the Power,check some wires,turn on,all running normal.Short time later,shut Power off,turn back on,back to Fan running backwards.I condemned the Dual 5 / 370 run cap for the fans,but since it jumps off the Contactors for common,can this also make those cheap 2 Pole Contactors act up?I say say yes,someone in the Company with more experience,says nah! 460 volts on all phases was good.No loose wiring. I've seen bad caps spin fans backwards before,just never seen them effect the contactor also,or self correct briefly after shutting off power.Any thoughts appreciated! Going back tomorrow with new cap and replaced the Contactors that were taking a beating.
     
  2. Texas-Tech

    Texas-Tech Official Geezer*

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    duhh
    The only thing that will affect a contactor pulling in and dropping out is the control side. Contactor don't care what the voltage is as long as you got 24. Now if you have a power issue, and the transformer was seeing a drop in voltage it could cause trouble.
     
    crackertech likes this.
  3. Rather B Riding

    Rather B Riding Behind bars

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    Contactors jumping is almost always in the low voltage circuit. Voltage drop. Bad coil on one of the contactors possible short. PSC motors running backwards I usually replace motor and cap.
     
  4. knave

    knave Undeterred

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    Just had one running backwards the other day, with a good capacitor.
     
  5. Zman

    Zman Hanging by a Thread

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    Just weird.When that fan spun wrong,both Contactors acted up.But when the fan spun correct,all was well. I wasn't sure if a bad cap would back feed thorough the jumper off the Contactors,making them act up. I'll have to double check volts in and out of transformer tomorrow.Thanks.
     
  6. Rather B Riding

    Rather B Riding Behind bars

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    I was at the counter picking up a new motor years ago and an older tech gave the best explanation I've heard for the PSC motor running backwards. What he said was air moving across the blade had the motor spinning backwards while it was off and when energized the power of the current couldn't stop it and changed the polarity of the motor. Made since to me but I always figured if its spinning the wrong direction its bad. Its not like I can reverse phase rotation on a single phase motor. Heard a lot of techs say they can fix it but I've tried their ways and it doesn't work. New motor and cap always works.
     
    knave likes this.
  7. knave

    knave Undeterred

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    This one started up slowly, in the wrong direction.
     
  8. Zman

    Zman Hanging by a Thread

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    Yeah,heard that also.The first stage fan pulls the 2cd stage fan backwards before it has a chance to energize. But I have seen bad caps run motors backwards. This cap has 2 commons in the middle and a 5mfd on each side,1 for each fan,so if the cap was bad you would think it be both sides ,and consistently work backwards,and not self correct temporarily like I had. I'll have to recheck all voltages tomorrow.
     
  9. Rather B Riding

    Rather B Riding Behind bars

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    608
    How did this turn out? I started thinking about this today, on a roof with carrier RTUs. The 2 stage carrier units start both fans with a 1st stage call so the other fan isn't spinning backwards. Both fans are powered off the 1st stage contactor. I always thought it was poor engineering. If 1st stage goes flat the low pressure switch interrupts power to the contactor. When you get a 2nd stage call, you get 2nd stage compressor with no fans. Usually by the time the tech gets there its a flat 1st stage circuit and a dead compressor on 2nd stage circuit.
     
    knave likes this.
  10. Zman

    Zman Hanging by a Thread

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    Replaced the Cap,let it cycle several times ,all was well.One side of that cap must of been bad.I shut off yesterday,so on arrival today I tested as was,and Fan ran backwards,after starting slowly,again.But the New cap seemed to fix it. I still have low reads on the low volt side,20. But Contactors weren't acting up,so I left for now,too many calls to get to.Checked transformer without a load,27 volts,so it ain't that.Took the Nest tstat out of the equation,still low volts.Something is drawing down the 24. Got to back to the same building for some other units,so I'll do more investigation.
     
  11. jays 65

    jays 65 New Member

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    unit may have been banging on and off high pressure switch with fan running backwards
     
  12. Zman

    Zman Hanging by a Thread

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    I thought the same,but My Boss who is a good tech,said if it Was HP,there would be a delay,not an instant on/off. But then again,He was wrong saying it wasn't the cap.But that was over the phone advice,not being on site isn't always easy to see what the tech is seeing!
     
  13. Texas-Tech

    Texas-Tech Official Geezer*

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    Have seen a bad compressor drop the voltage 40volts while it was trying to run, no telling how a bad cap could affect things.