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Electrical head scratcher's got me stumped!

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Comments

  • Fred
    Fred Member Posts: 8,520
    Stupid question but do you know that the slow blow fuses are good? Did they buy them all at the same time and maybe got a bad batch/run?
  • ratio
    ratio Member Posts: 2,763
    The first sets were of course factory. After that, two different brands from two different suppliers (& two slightly different current ratings too, but we're back to the nameplate size).

    Perhaps a minute internal winding arc generates enough plasma to occasionally cause a larger arc to ground? Proof positive of alien activity? Maybe I'll try a little tinfoil hat. For the motor, not me.

    SuperTech
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,257
    The foodservice tinfoil is much thicker, therefore much more better.....for keeping the aliens tractor beaaaaaaammmmms from pullllling at the plasma arcs. YES
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    SuperTech
  • ratio
    ratio Member Posts: 2,763
    Another data point: service dept was on site to swap the other motor. Calls & tells me that 5 of the 6 motor fuses I added are blown— all three on one of the motors & two on the other. Keep in mind that one of these motors has already been replaced.

    How do you occasionally blow all three motor fuses‽‽‽ To simultaneously hit the I2t of three separate time delay fuses you'd have to exceed it by some margin, wouldn't you? Is it even possible for a mechanical fault to cause that? And then go away for a while?

    This is going to drive me off the bottle.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,989
    @ratio

    Just lookin back at some of the older posts where I had mentioned overload protection and you said it just had contactors.

    Just to refresh ALL 3 phase motor need 2 things

    1. Code calls it " Branch circuit, short circuit protection" this is your fuses or breaker in this case fuses.
    2. "motor overload protection" 3 was this is done, motor starter with overloads, vfd with overload protection or internal protection in the motor.

    Don't think it was ever mentioned what you have except that you have contactors. for a 3 hp motor code amps is 4.8 so fuse size for each motor is 1.75 x 4.8= 8.4 amps so 10 amp time delay fuses are ok
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,989
    Also fuses protect the wire, contactors etc short circuits and grounds overload protection protects the motor
  • DZoro
    DZoro Member Posts: 1,049
    Sounds like you may have to pull the wires and put some eyes on all the wiring.
    D
  • JUGHNE
    JUGHNE Member Posts: 8,315
    I like DZoro's idea. IIWM I would inspect the conductors between the fuses and the motor. Probably flex/sealtite. It sometimes collects water vapor that condensates and could then dry up.
    Perhaps intermittent insulation breakdown when you are not there. However, these situations will usually torch themselves....or maybe not if the flex is non metallic, (no grounded raceway to short to).
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,257
    You still could be blowing one fuse, which single phases the motor then blows the other two.

    Do you have a way to know if all 3 blew simultaneously?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • ratio
    ratio Member Posts: 2,763
    Found the cause. The Molex connectors throughout the wiring harness were flashing over internally between two poles. It doesn't take much of an arc at 480 volts to blow a 6 amp fuse, so the flashover wasn't very obvious until, well, it was. (I had, at the very onset of the problem, examined them. Apparently, not as thoroughly as I believed.) The real pisser was that the connectors on the unit that didn't blow the fuses as often showed the worst burns, & was 15' from the roof hatch—the one I concentrated my efforts on (because it blew more often) was more like 150' from the hatch. :disappointed:

    For some reason, the mfgr's rep was unable to answer my question about the voltage rating of the connector…

    DZoroSolid_Fuel_Man
  • ScottSecor
    ScottSecor Member Posts: 386
    @ratio thanks for following up. I guess the moral of the story is to be weary of using Molex connectors with 480 volts? We rarely see them on anything over 120 volts.
    Solid_Fuel_Man
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,989
    @ratio ,

    Glad you found the problem. Too bad it was so painful
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,257
    Wow, youd think the manufacturer would know better than put 480 on molex connectors. Seems yours wouldn't be an isolated incident unless there was an OEM change and you happened to get the first batch of units.

    I, personally dont like to see 480 used on much other than motors (which RTUs would qualify). As a long time electrician, I've seen more trouble with 480volt stuff than 208volt stuff. The low current high tension just does strange things.

    Nice find, and thank you for the follow up so we can all learn from the manufacturer's mistake. ...that you had to eat.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
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