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  1. #1
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    Default Motor on air compressor

    Air compressor breaker tripped, seems like motor is bad. It's a 7hp 60gal unit, motor is AO Smith 3450 RPM, 220v. I can rotate belt freely and hear bypass on compressor opening, so there's no resistance to motor starting. When I close the contactor on the pressure switch I hear a very loud humm and vibration which will trip breaker if I leave it on more than a short instant. Motor turns freely by hand. We have local places that rebuild motors, think that's the best way to go, or just buy a new motor? No idea what these cost.
    Kubota B2710, New Holland CM274 front mower, Toro Zmaster ZTR, Ford 908 bush hog, New Idea manure spreader, Swisher trail mower

  2. #2
    Silver Member heviduty's Avatar
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    Default Re: Motor on air compressor

    Does the motor try to turn at all? If it does, you may have a weak starting capacitor. If it doesn't, you may have a stuck centrifugal switch or dirty centrifugal switch contacts. With the power off, see if you can move the switch with a screwdriver or something. You may have to remove the end shield to get at it. If it is sticking, a drop of oil on the shaft / weights / whatever is sticking, may all you need.
    Happy as if I had good sense.
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  3. #3
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    Default Re: Motor on air compressor

    Quote Originally Posted by heviduty View Post
    Does the motor try to turn at all?
    No
    If it does, you may have a weak starting capacitor. If it doesn't, you may have a stuck centrifugal switch or dirty centrifugal switch contacts. With the power off, see if you can move the switch with a screwdriver or something. You may have to remove the end shield to get at it. If it is sticking, a drop of oil on the shaft / weights / whatever is sticking, may all you need.
    My electric motor knowledge is zilch. Talked to the guy at the local place that repairs these things, he told me to smell it. Since it doesn't smell burned and didn't smoke when it tripped the breaker, he said it's pretty sure to just be a capacitor or starting circuit, not a winding, shouldn't cost too much. He said if it was a winding to just write it off and buy a new motor. So I'm on the way over there to drop it off. Glad I had a gear puller or that rusted on pulley would have never come off. Found a replacement motor online for $280, so that's the worst case.
    Kubota B2710, New Holland CM274 front mower, Toro Zmaster ZTR, Ford 908 bush hog, New Idea manure spreader, Swisher trail mower

  4. #4
    Super Star Member LD1's Avatar
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    Default Re: Motor on air compressor

    I'm a little late and you may have already dropped it off, but I would suggest giving it a good spin by hand (in the direction that it runs) and while it is still spinning, turn it on and see if it runs then. If it does.....yep, windings are probabally fine and it is a starting issue.

    Also, do you know if it is an ACTUAL 7HP motor or if it is a "compressor duty" 7HP. Cause there is a big difference in size and cost. A TRUE 7HP is about twice the size and twice the price. What does the nameplate say for FLA (Amps)??
    ".........there is only one way to find out."
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  5. #5
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    Default Re: Motor on air compressor

    It was the starting capacitor. The guy at rebuild shop didn't have the right one to replace it, but he had one that was a different form factor so he connected it and tested the motor, it ran fine and starting torque was good. Charged me $20 for diagnostics. Then it gets interesting. Went to an online site that sells replacement parts for Sanborn compressors, they want $130 for the starting capacitor, that sounds nuts. So I go online at Graingers and they sell an equivalent one for $13.50, so I ordered it. Mission accomplished. thanks for help and suggestions.
    Kubota B2710, New Holland CM274 front mower, Toro Zmaster ZTR, Ford 908 bush hog, New Idea manure spreader, Swisher trail mower

  6. #6
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    Default Re: Motor on air compressor

    Quote Originally Posted by LD1 View Post
    I'm a little late and you may have already dropped it off, but I would suggest giving it a good spin by hand (in the direction that it runs) and while it is still spinning, turn it on and see if it runs then. If it does.....yep, windings are probabally fine and it is a starting issue.
    That's what the guy at the shop did, it spun fine, he took the cover off starting capacitor and the end of it was like charcoal. He was surprised the reset on the motor hadn't tripped. I could have checked the capacitor myself I guess, but I wouldn't have been confident I knew why the capacitor failed or whether the motor would work if I replaced it.


    Also, do you know if it is an ACTUAL 7HP motor or if it is a "compressor duty" 7HP. Cause there is a big difference in size and cost. A TRUE 7HP is about twice the size and twice the price. What does the nameplate say for FLA (Amps)??
    It's 7HP "peak", which is apparantly some marketing ploy to impress people like me who don't know better. The guy at repair shop said it's about 3hp. The nameplate says 15A.
    Kubota B2710, New Holland CM274 front mower, Toro Zmaster ZTR, Ford 908 bush hog, New Idea manure spreader, Swisher trail mower

  7. #7
    Super Star Member LD1's Avatar
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    Default Re: Motor on air compressor

    Quote Originally Posted by BeezFun View Post

    It's 7HP "peak", which is apparantly some marketing ploy to impress people like me who don't know better. The guy at repair shop said it's about 3hp. The nameplate says 15A.
    Yep. Thats marketing for you. If it is 15A then it is equal to a 3HP as he said. I was asking because I know there is no way you found an actual 7HP motor online for the price you mentioned.

    Congrats on being resourceful and getting a cheap repair It pays to do your homework.
    ".........there is only one way to find out."
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  8. #8
    Silver Member heviduty's Avatar
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    Default Re: Motor on air compressor

    Quote Originally Posted by BeezFun View Post
    It was the starting capacitor. The guy at rebuild shop didn't have the right one to replace it, but he had one that was a different form factor so he connected it and tested the motor, it ran fine and starting torque was good. Charged me $20 for diagnostics. Then it gets interesting. Went to an online site that sells replacement parts for Sanborn compressors, they want $130 for the starting capacitor, that sounds nuts. So I go online at Graingers and they sell an equivalent one for $13.50, so I ordered it. Mission accomplished. thanks for help and suggestions.
    Good for you!
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  9. #9
    Gold Member flyerdan's Avatar
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    Default Re: Motor on air compressor

    For some reason air compressor manufacturers seem to think it's okay to play fast and loose with hp ratings, when in reality you can only suck so much power out of a circuit. If I recall correctly, the biggest real motor that can run on a 15A 117VAC line is 1/2hp. I know that Campbell Hausfeld had a class action suit for misleading power ratings. Since I have one of the models involved I filled out all the paperwork they sent and got a free jitterbug sander. Of course I knew that the "6.5 hp" motor couldn't be that given the current draw and the fact that is was no bigger than a furnace blower motor in size, but a free sander is a free sander.
    Anxiously awaiting Dr. Savage's return to the airwaves.

  10. #10
    Super Star Member LD1's Avatar
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    Default Re: Motor on air compressor

    Actually you can go to about a 1-1/2 HP motor on a 15A circuit.

    General rule of thumb......10A per HP on 120v
    .........................................5A per HP on 240v
    ".........there is only one way to find out."
    "Ok, hold my beer and watch this.........."


    Ford 5500 Backhoe
    Kubota L3400GST W/LA463 FEL
    2005 Dodge 3500 4x4 Diesel
    8N Rebuilt and restored
    Bushhog 306
    3 Homemade wood hauling trailers
    Dolmar 6400 84cc ported
    Sachs-Dolmar 120SI Ported
    (4) Sachs-Dolmar 116SI Ported
    Dolmar PS540
    Sachs-Dolmar 115i
    Sachs-Dolmar 117
    Sachs-Dolmar 112

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