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  1. #1
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    427

    Motor melting start caps

    I wasn't sure where to post this but since it is used in a commercial kitchen, figured I hit the refrig gurus. I have a problem I have never seen. I have a small Dayton blower motor used in a Giles vent less hood which will melt the start cap.

    Details, small approx 1/4 hp Dayton blower motor, 115/230v wired for 230, data plate 3.7 amps @ 230v, uses a 20uf, 250vac start cap. Last week it melted the original, today I installed a generic Dayton 21-25uf 250vac replacement. (Original oem part made by other than Dayton no longer available at Grainger nor Giles). The generic replacement popped & melted in 5 mins. BUT the motor runs perfectly, no bearing noises, pulls 2.1amps, will start WITHOUT the start cap installed and the unit is working.

    The actual line input volts is 249vac. Little high but I can't see this being the issue. I didn't think to check at the actual volts at the cap terminals at the time. But if this motor is running cool & quiet, what can cause this issue? I'm gone to check the volts at the terminals with the motor running, maybe install a 330vac, 20mf start cap.

    Is this motor defective? Any ideas?

  2. #2
    Join Date
    Jul 2012
    Location
    Richland, WA
    Posts
    248
    How long was the original cap in for or was this a replacement motor and cap already?

    Have you conformed that it is wired correctly?

    I remember the teachers when I was in school, they wired a cap to a motor incorrectly and it would blow up the cap, but would start the motor before it blow up the cap.

    All the hood vents in the 16 restaurants that my work takes care of, all the motors for the hoods are 3/4 to 1/2 hp and don't require caps on the motors.

    Sent from my iPhone using Tapatalk

  3. #3
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    427
    Thanks caheiman, yes I confirmed the wiring is correct. 2x. The motor is original from new in 2009. This is a small hood above a fryer with a direct drive motor blower in the hood..

    I use 3/4hp motors on the roofs for oven hoods & make up air. No start caps either.

  4. #4
    Join Date
    Oct 2012
    Posts
    3,330
    So is there a centrifugal switch in the motor? When the motor gets up to full speed the centrigugal switch takes the capacitor out of the circuit?

    If it blows the cap in 5 minutes, I would put a new cap in and do an amp draw, see if it is pulling amperage for an extended period of time. Maybe motor is slowing down, or centrifugal switch is cutting in and out.

  5. #5
    Join Date
    Dec 2008
    Location
    NW Iowa
    Posts
    121
    If the start switch sticks closed only sometimes, it will blow the cap. in a short time.

  6. #6
    Join Date
    Oct 2009
    Location
    Raleigh, NC
    Posts
    790
    Are you sure this isn't a run cap? It is unusual for motors in low-torque starting applications like a small direct-drive motor to apply a start cap. Also, remember that the voltage rating on the cap is for back EMF purposes and has nothing to do with line voltage.

  7. #7
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,224
    Yeah...depends on the motor set up. Is it in fact a capacitor start induction run motor with the start cap hangin' in too long due to a centrifugal switch not taking it out?

    Since you gave a range of uF ratings I have to assume it is indeed a start cap...and with a very low VAC rating of 250V I would also guess a centrifugal switch. A start relay like a potential relay or a current relay would need much higher BEMF needs to take the start cap out.

    I'm in the wrong neighborhood...but curious to learn the outcome.

  8. #8
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    427
    Quote Originally Posted by MicahWes View Post
    Are you sure this isn't a run cap? It is unusual for motors in low-torque starting applications like a small direct-drive motor to apply a start cap. Also, remember that the voltage rating on the cap is for back EMF purposes and has nothing to do with line voltage.
    Grainger using the blower assy parts listed it as a start cap. But I am not 100% sure it is a start cap other than what the Dayton parts break down states. I did plan on trying to use a run cap too in order see how it reacts. The cost is under $10. BUT, how is this motor running WITHOUT the cap if it is a run cap? remember my first post, the motor currently will run without the blown cap wired up.

    What exactly is EMF? How high can EMF voltage get in a 240vac set up?

  9. #9
    Join Date
    May 2010
    Location
    Brooklyn, NY
    Posts
    99
    i can only think of the start switch is staying on for too long to pop the start capacitor

  10. #10
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,224
    Quote Originally Posted by Capz View Post
    Grainger using the blower assy parts listed it as a start cap. But I am not 100% sure it is a start cap other than what the Dayton parts break down states. I did plan on trying to use a run cap too in order see how it reacts. The cost is under $10. BUT, how is this motor running WITHOUT the cap if it is a run cap? remember my first post, the motor currently will run without the blown cap wired up.

    What exactly is EMF? How high can EMF voltage get in a 240vac set up?
    Back EMF...voltage generated by the motor and, by design in some set ups; used to take the start cap out of the circuit.

    Back electro motive force, if I recall the specific term correctly.

  11. #11
    Join Date
    Jun 2003
    Location
    Tampa Bay, FL.
    Posts
    119
    Correct me if I'm wrong, but isn't a 20uF a run cap not a start cap?

  12. #12
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,224
    Quote Originally Posted by hydra View Post
    Correct me if I'm wrong, but isn't a 20uF a run cap not a start cap?
    It's unclear. The specs for the replacement cap he listed was a range, 21-25uF. This would indicate a start cap as run caps typically have a fixed value.

    Does it have a bleed resistor across the terminals? Not sure if the smaller start caps have one. The larger ones, for a capacitor start, capacitor run compressor all have bleed resistors and will be in the 80 - 108+ uF range...

    The curious thing is such a small, light duty motor with a cap at all. I'm wondering if it needs a cap at all when wired for 230? Maybe the cap is intended for use with 115 only and it just happened to work for a few/four years until it finally melted the cap?

    Thinking this as he stated it runs nice and cool without cap?
    Last edited by hurtinhvac; 04-04-2013 at 10:07 AM.

  13. #13
    Join Date
    Dec 2006
    Location
    DFW Texas
    Posts
    47
    Is this a totally enclosed motor or open? I'm thinking grease + open with centrifugal switch = stuck switch and blowing caps.

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