What's up with my HVAC fan?

Discussion in 'General Electronics Chat' started by strantor, Dec 25, 2015.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    One might think that a man holding the job titles that I have in the past (Field Service - motor controls, et. al.), should be an "expert" on all things motor, but that isn't the case. I'm single phase impaired. Every motor I have ever dealt with in a professional capacity was 3phase or DC, or stepper, or anything-else-but-single-phase. When a single phase motor problem finds me, it always makes me feel stupid. Today's culprit is in my very own attic. Sounded like someone was stumbling around drunk up there.

    Here's what my HVAC fan is doing (video):
    https://drive.google.com/file/d/0B3Jz02jC_rNgaGtvSkh4Tk4xTkU/view?usp=sharing

    Here is a folder of related photos, including wiring diagram:
    https://drive.google.com/folderview?id=0B3Jz02jC_rNgeEY2NGxVOVFQTWs&usp=sharing

    If you can't see the video, it's acting kinda like a 3 phase motor acts with a missing phase. Lots of ugly humming; makes 1 rev, stops, makes 1 rev, stops, ...

    The fact that it's acting like a single phased 3 phase motor initially made me suspect the run cap (the component responsible for generating the "3rd phase" of a single phase motor). I took it off and tested it by the means I have here at the house on Christmas Eve (no LCR or ESR meter). It reads 19.9uF by my fluke DMM and I charged it up to 260V with my megger, held a good charge for a long time. Megger checked for shorts to case, none. Cap is not bulged or leaking, and does not get hot while plugged into the powered circuit.

    AFAIK the only thing that could be wrong with the cap is high ESR which i can't test for, but I thought that would make the cap overheat, and it isn't, so I started to suspect the motor. The motor turns very freely by hand, no bearing issues; I can spin it by twisting the shaft with 2 fingers, and it stays spinning for a long time after. No electron leaks, meggered the motor too. motor windings read 14, 17, and 20 ohms for (high, med, low). The motor draws 3.8 amps, whether it's running as it should be (I did get it to run for a while by letting it cool down for a couple hours) or whether it's doing the buzz dance in the video above. Unlike a single phased 3 phase motor, I would expect the amp draw to skyrocket but it doesn't.

    I tried spinning the fan as fast as I could by hand and then applying power; when the power hits it, it immediately snaps to a stop and starts doing the buzz dance. The motor is getting 238V to the leads and I read 232V across the cap while it's running.

    Is this more likely a bad cap or a bad motor?

    @#12 BATSIGNAL
     
  2. #12

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    Nov 30, 2010
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    Bad motor winding. Lack of air flow is causing the overheating because it's design is dependent on massive air flow.

    The way to test the cap is to use its Xc. Connect it directly across the 240 volt power line and measure the current. This method bypasses all the weaknesses of low voltage meters, etc. Measure the actual line voltage, measure the actual current flow, calculate the uf's.

    There are only 2 parts: The cap and the motor. Pick one.
    Assuming you tested for shorts from windings to case. Of course if you have any continuity there, the winding is obviously shorted to the case.

    Edit: My 0.2 HP motor uses 1 amp when up to proper speed.
     
    Last edited: Dec 25, 2015
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  3. strantor

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    Damnit. I was hoping you'd say capacitor.

    Winding is definitely not shorted to case; i megger'd that and it's good. But winding shorted to winding is still in play.

    Odd though (to me) that it's drawing nameplate amps through all of this.
     
  4. #12

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    Nameplate amps at stall speed is absurd. It should take a big start surge, as in, locked rotor amps. About 7x the run current to start.
     
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  5. strantor

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    Yeah, that's what i would expect too. any theories about why it's not doing that?
     
  6. strantor

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    Maybe it isn't a shorted winding, but a winding that developed a high resistance? That might explain the low starting torque, and low locked rotor amps.

    Not sure though, how a motor winding just "develops" a high resistance. I can only think of a bad connection, in which case I wouldn't need to replace the motor, just fix the wiring hopefully.


    BTW it's working like a charm today. I left it off all last night so it wouldn't catch fire and burn me to death in my sleep. Woke up this morning (MERRY CHRISTMAS BTW) and my wife had turned it on, no strange sounds, no sluggish starts, just operating as usual.
     
  7. #12

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    Wait a minute. I'm too busy breaking my arm by trying to pat myself on the back for answering you within 6 minutes at 3 AM on Christmas Day!

    Now that it's NOT 3 AM, we can think about the, "less likely" scenarios and some test methods.
    I had a fan relay un-solder itself from the circuit board because the motor was beginning to get stiff bushings, therefore more amps than usual. I re-soldered the relay, polished the bushings, re-oiled, and the motor lasted 3 more years before the armature walked through the bushing far enough to cause a laminate crash. You have the right (and the ability) to search for intermittent connections, fatigued solder joints, etc. Running a private cable of 240 VAC to the motor will take all the other components and connections out of the circuit so you can test the motor. You can let the motor run continuously, all day. If it isn't already broken, running all day won't hurt it.

    You will know to turn the power off before you go to sleep, you will know if it runs all day, and you will know if it doesn't. You should probably know the ultimate answer by sunset today...or you can enjoy your family on Christmas Day and leave it until tomorrow.
     
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  8. #12

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    I never argue with a motor about whether it has a winding (or several windings) open or shorted, or some combination of the above, probably because I can't spend all day in some stranger's attic picking nits. The motor runs or it doesn't. It has a proper 240 VAC supply or it doesn't. The capacitor passes the right amount of current or it doesn't. It's usually that simple. When the customer asks me why it chose to break today or why it shorted instead of going, "open", my brain instantly turns into oatmeal. I get the MF running and write the bill. I leave the, "why" of it to Madam Babushka and her crystal ball.:rolleyes:

    You have all day, or all week, to fuss with this, if you want to. You are equipped to find the one cubic millimeter where the real fault is. If you find it, I still won't know the, "why" of it. My only concern is to get the machine running as quickly as possible. If I want to do an autopsy, I can take the corpse home with me, but it only takes a few to get permanently frustrated with trying to trace a bunch of very tight windings while sweating like a pig in a heat index of 103F. That's what I have to do to best serve my customers. You have the luxury of chasing down the real defect, if you want to.

    If you have to take it apart, don't forget to wash the squirrel cage. Plain old kitchen detergent and water, with a tooth brush.;)
     
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  9. strantor

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    Thank you #12 for your gratis after hours + holiday consultation. Sorry, the date and time didn't really enter my conscious mind. Actually we were in the middle of some early gift opening on Christmas Eve when i started hearing the racket in the attic, and so ended that chapter of the day. I imagine my wife probably said something like "well, there he goes. I guess I'll see him tomorrow. Let's go to bed now kids."

    After I spent the evening in the attic and didn't definitively drill down the answer, i posted here, still oblivious to the goings on of the rest of society.
     
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  10. #12

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    The main reason I wear a watch is that I forget what day it is.:p:oops:
     
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