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Electrical head scratcher's got me stumped!

ratioratio Posts: 1,563Member
edited May 17 in THE MAIN WALL
Ok, I've got two ERVs, OEMed Reznors, that keep blowing fuses. Always A & B fuse, never C. 12A time delay fuses, 6 and change running amps, 480 volts. Two 3 HP blower motors and one wheel motor fed via an xfrmr on A & C. It happens more often than once a week in warm weather, getting less frequent as the temperature drops. One does it twice as often as the other. Fed from different panels, from different switchgear, from possibly different services, from (I imagine) the same primary. Industrial, three or maybe 4 2000A services. We dropped coin on a DM-5 & it said that the voltage holds steady & yup we hit 30-40A when the fuses let go. The motors meg out better than my Fluke 287 can read, winding resistance is ±0.1 Ω of each other. There's no burns that indicate a reoccuring fault, it happens with the wheel motor completely out of the circuit (Stakons unplugged).

Where could I be getting 40+ A of fault current? The motors start fine, a damper interlock staggers them. The place runs many million dollars worth of equipment, I can't believe that there are significant power quality issues. It's two separate units on different ends of the building, although they are sisters off the same line, one digit apart on the serial number.

Like I said, I'm stumped.

Comments

  • JUGHNEJUGHNE Posts: 4,549Member
    How about a motor bearing heating up and loading the motor.
    It is cooled down by the time you look at it again.
    But eventually it seems it would completely seize up and not become free again.
  • ratioratio Posts: 1,563Member
    Mechanical defect of the motor is about the closest thing to a probable cause that I have right now. Contra-indicating it, however, is that it's two separate units electrically unrelated until we get to the primaries. Also, the same two fuses always, and the same two on both units. And I replaced a motor, because the bearings felt poorer than I'd expect. Oh, and they're only about a year old now.

    Since it can't be the motor I swapped, I've got orders to pull the other one & take it to a rebuilder & have them go over it with a fine-toothed comb. I don't really want to, because I'd have to pony that 70# motor across the roof, but I can't think of anything better just now.

  • ratioratio Posts: 1,563Member
    When I had the power logger on it, the voltage ran steady, well within tolerances, before, during, and after the fuses blew. The current held steady until they blew, which was over, start to finish, within three seconds. Normal current, visible peak of 30-40+ amps, no current. The shortest logging interval is 1 second, although the logger should capture sub-cycle transients.

    The motors turn freely by hand & run fine when I'm watching(!) & I had no trouble dialing in the airflow when I commissioned them.

  • Jamie HallJamie Hall Posts: 8,534Member
    It's a pain -- but I'd go over the wiring with a fine tooth comb. Or better yet a magnifying glass... and see if I could find any evidence of arcing.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • icy78icy78 Posts: 175Member
    Can you switch all 3 phases around, keeping same rotation. It may give you a clue, or not, if now C phase fuse is one that opens. Can you meg the wiring, disconnected from the motor? Also what Jaime said.
  • JUGHNEJUGHNE Posts: 4,549Member
    Is the control voltage from a different source?
    If the controller drops the contactor out for just a second and tries for a restart immediately while you have motor spinning, (Back EMF being produced by free spinning??? WAG).

    So a time delay to restart on the starter control circuit might prevent this, if it is the case.
    Did you monitor the line or load side of the contactor?
    And a lot can happen in less than 1 second.

    Maybe something is up with the interlock circuit?

    I would try a lot before carting that beast up and down, and then still have the same problem...…..never looks good :#
  • Harvey RamerHarvey Ramer Posts: 2,156Member
    It may seem to simple, but check all the wiring connections. A faulty connection can do exactly what you describe.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,181Member
    @ratio

    If the whole unit is protected as a group 2 motors @ 3 hp + the small wheel motor the calculation would be as follows using NEC code amps:

    1 3 hp motor at 4.8 amps + 2d 3 hp motor at 4.8 amps + wheel motor (I am guessing 2 Amps for the wheel motor for this example)= 4.8 +4.8+ 2 + 25% of the largest load (1 of the 3 hp motors) =4.8 + 4.8 + 2 +(.25 x 4.8)=12.8 amps so your fuses look ok.

    I am assuming that all the motors have internal thermal overloads or do they have motor starters??.

    I am assuming the motors are wired for the correct voltage (if dual voltage motors) Why are they not tripping overload instead of blowing fuses?? I am just thinking out loud here.

    I am assuming that loose connections or bad motor starter contacts can be ruled out?? since it's two separate units fed by different sources. Almost seems like they are going single phase. Maybe slap a ICM 450 ($100) phase monitor on it and see what that will do.

    Are the motors getting hot?

    Is this fed from a grounded or Ungrounded (not likely) system?
  • ratioratio Posts: 1,563Member
    I'll try and answer your responses in order:

    @Jamie Hall, I've done a few checks of the harness and connectors. It looks fine, there's no indication of a single fault, let alone many high-current faults that blow a Bussman LP-CC-12 fuse.

    @icy78, I've thought about swapping the feeds around. I did meg the wires back to the contactors when I megged the motors, everything showed greater than 2.2GΩ at 1000 V test voltage.

    @JUGHNE, control voltage originates in an RTU located 8 ' away, maybe 20' of 18/2 as the wire runs. It's tapped directly to the fan contactor on a unit that runs the fan continuously during occupied times and has a jumper on the OCC term. It shouldn't ever drop out. I clamped a motor while I manually ran it & dropped it out for a sec & it didn't seem to spike any higher than a normal startup, but I could try that live with the whole unit running. The first run with the power logger was on the load side of the fuses, but the second was on the line side, the voltage varied IIRC only a volt or two, well within the ±10% we're promised, although it was a little hot. Still not too hot that even +10% would have exceeded the nameplate max input voltage.

    @Harvey Ramer, checking the connectors was the third, fourth, and ninth thing I did. I would have expected it to either weld closed or blow open by now though.

    @EBEBRATT-Ed, yes it's one set of fuses for the whole unit. two 3HP blowers, one 240V heat recovery wheel powered via step-down xfrmr (not involved, as I noted earlier), ≈40VA control xfrmr (even though the control signal is wet contacts from the associated RTU), two contactors & a relay. we run at about 5-6A, stagger start the two 3HP blowers via a damper interlock. I don't believe the motors have internal overload, I think they're being protected via the fuses. The motors are wired for the right (high) voltage—I checked that during commissioning. I slapped a 4k$ power quality logger on it, that the company purchased because of my incessant whining about getting one. Power is 2000A 277/480V solidly grounded wye, standard industrial power, new build a little over a year ago, in an old industrial neighborhood, so infrastructure should be beefy even if the power company has been deferring maintenance for decades. Also, many millions of dollars of equipment runs fine in this facility.

    Motors get hot enough that you don't want to keep your had on them too long, but nothing that makes me go "Huh!". Heat related issue is indicated since it happens more often in the summer, but wouldn't a short keep getting worse, or go away, after 10+ faults?

  • Jamie HallJamie Hall Posts: 8,534Member
    To go back to the arcing hypothesis -- it would be something somewhere in the factory wiring, not what you did. I'd be looking for somewhere in there where A or C (I think those are the ones you said blew) are just a smidge too close to each other, or to a ground. High humidity will aggravate it, as will higher temperature. You could easily get enough current to blow any fuse from an arc at that voltage, and it might not leave much evidence.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ratioratio Posts: 1,563Member
    I'll go over the harness some more. I think it's got to be in the unit somewhere, that's the only thing that would make both units do the same thing. My gut likes a motor fault, but I can't understand how I could have an identical fault on two separate units, and that it's minor enough that a megger wouldn't pick it up yet big enough to blow the fuses in <3 seconds without any noticeable sign before. Maybe I'll get a few temp loggers & track the motor temps as well.

    There is one difference between the two motors—the exhaust fan has a high static drive kit installed, but not the supply fan, the reason being that the supply blows into the RTU return but the exhaust sucks through a ¼ mile of ductwork. However, they were balanced via the mfgr's ΔP tables, traversed at the outside air intake to verify, and the amp draws are similar & under the nameplate amps.

    I originally was going to replace the exhaust motor, as that one should be the one that's working harder, but when I got the doors off, the supply motor bearings felt worse than the exhaust so I swapped that one instead. Maybe I shouldn't have second-guessed myself.

    There's gotta be something that I'm missing, but I can't for the life of me think of what it might be.

  • icy78icy78 Posts: 175Member
    Possible the contactors have carbon tracking?
  • Jamie HallJamie Hall Posts: 8,534Member
    icy78 said:

    Possible the contactors have carbon tracking?

    Certainly something to look for -- and that could support a major arc without leaving much of a trace. Would get worse over time...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • JUGHNEJUGHNE Posts: 4,549Member
    If there was a wink out in power and an immediate restart without benefit of time delay on second motor would that pop the fuses? You could simulate that condition maybe.

    If deadheaded on air flow, in or out, it seems the OL would open.

    FLIR viewer could show a lot of invisible problems.
  • Solid_Fuel_ManSolid_Fuel_Man Posts: 1,057Member
    I would defiantly swap phases around and see if the two problem phases follow or stay the same. Two consecutive units most likely means 4 consecutive motors etc. Also can you fuse each motor separately? That way you may be able to pinpoint which motor is faulting. I'm not sure if a time delay would be beneficial, but is a cheap troubleshooting tool.

    Divide and conquer! Fuse as many things separately temporaly as you can. No compressor loads just fan motors?
    Master electrician specialising in boiler and burner controls, multiple fuel systems, radiant system controls, building controls, and universal refrigeration tech.
  • ScottSecorScottSecor Posts: 171Member
    I agree with the possibility of contacts being worn and arcing out, especially if the contactor cycles often.
  • JUGHNEJUGHNE Posts: 4,549Member
    Or monitor voltage drop across L1 to T1 contacts etc after long operation.
    This works for me with AC contactors if running for hours and makes the call to replace or not. Any drastic voltage showing get replaced.
  • ratioratio Posts: 1,563Member
    Thanks for the suggestions. I peeked at the contactors, but they look fine & I doubt they've seen triple-digit operations yet—the only time it goes off is when the fuses blow. I'll try the short dropout test, but IMHO it'd have to be in the one-cycle range to be a contender; and the main breaker would be complaining about it if that were the case, high-end Eaton switchgear. (As an aside, I had to get the Eaton guy out to the job to reprogram a 1000 A breaker that had the long-term trip set to 200A—we had over 800A of connected load just in RTUs!) TBH, I haven't gone to see if the breaker has an alarm in it.

    I'll verify the contacts & swap the phasing around & see what changes. I should be able to move one motor clockwise and one motor counterclockwise, that might tell me which motor is faulting with only one new set of fuses. It costs me about $50 for the two fuses. I have a dish of them that I showed the office when I went to ask for the power logger!

  • JUGHNEJUGHNE Posts: 4,549Member
    Does the contactor drop out when the fuses blow?
    I thought it got coil power from the RTU.
  • ratioratio Posts: 1,563Member
    Hmmmm, I can't recall now. The controls are kinda odd. The ERV is commanded on via applying 24vac to a set of terminals, but the control panel itself has a 24vac xfrmr too. The startup docs tell how to hotwire it with its own xfrmr for testing.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,181Member
    If they are three phase motors you need overload protection on the motors. You can't protect a group installation without overload protection. Can't do it with fuses. All motors need short circuit protection (fuses or breaker) and overload protection (thermal overload). It's either built into the motor (it will say it on the name plate) or you need motor starters.

    Sounds like the motors are pretty hot. Are they mounted in the airstream?
  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,181Member
    I can't find that model on line @reznor. Is this a factory package or a field built up unit?
  • ratioratio Posts: 1,563Member
    It's a rebadged Reznor XBWS downflow, I think size 28 or 36. Attached is the schematic, installed accessories are the freezestat and the supply damper which is interlocked properly.
  • Harvey RamerHarvey Ramer Posts: 2,156Member
    Are the motors driving the fans via pulley's? If so, belt tension might be to tight?

    Most of my head scratcher electrical problems have been ridiculously simple once I found them.

    Any branches taken off the three phase? Like convenience outlets for the RTUs? Someone might be plugging something in and blowing the fuse. This would be downstream from the xfmr of course. But I have seen secondary low voltage trip the high voltage primary side of an xfmr.

    I had a metal worker in a machine shop once that kept blowing a fuse. It was full of control panels, relays, fuses, contractors, timers and about 10,000 wires going in every direction. Plus a manual that was written in a foreign language. I was feeling slightly overwhelmed with the customer peering over my shoulder while I was doing my best to look like I had it under control. As good fortune would have it, in about 15 min I found a spot in the wiring harness that had rubbed through and was periodically shorting to ground.

    Sometimes you only have voltage to ground while a machine is in operation.

    99% of the time, the hardest electric problems turn out to be the simplest. Look for easy.
    Ramer Mechanical
    ramermechanical.com
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • ratioratio Posts: 1,563Member
    Thanks, @Harvey Ramer, I hadn't considered belt tension as a possible cause. They're not banjo-string tight, but they are nice and snug, like you'd expect for a 3 HP motor. I suppose they don't need to be that tight, since they're only going to start one or twice a year. That would explain why one does it more often than the other, but still doesn't seem to fit with the duration of the fault, <2 seconds, I would expect that to be an increasing overload until the fuses open.

    There are a few single phase loads, but all internal to the unit, which has screwed on doors, and the roof is not very busy, so I don't think it's caused by something someone is doing.

    Well, I've got a list of things to look at before I have to lug the motor across the roof. Thanks for the ideas & suggestions. I'll certainly let everyone know when I find the problem; & what it ended up being.

  • GBartGBart Posts: 546Member
    Maybe I missed it if you did it or it was suggested but you may want to follow the wiring all the way back to the panel and check any and all connections, typically you'd think a high amp draw would show up, you could also have a funky contactor where every once in a while the contacts float off center when off and when they close they don't mate properly.
  • JUGHNEJUGHNE Posts: 4,549Member
    Blower bearings....pillow blocks....being belt drive?
  • hvacfreak2hvacfreak2 Posts: 451Member
    edited May 18
    I've been trying to follow this and I haven't had anything to add other than what has been mentioned. I would throw on some cheap delay on make timers on the control side of the contactors to prevent rapid make / break of the contactors from a malfunctioning controller or loose damper actuator or end switch. Along with the ICM 450 that was mentioned that would cover most anything outside of the unit.
    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

  • EBEBRATT-EdEBEBRATT-Ed Posts: 4,181Member
    A fairly cheap diagnostic would be to add a separate fuse block and split the motors apart as far as fusing goes. Fuse each one at 10 amps.

    The motors must have thermal protection built in as there are no starters or overloads on the drawing.
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