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Furnace motor drawing high amperage

zepfanzepfan Posts: 266Member
I was doing a routine ac check on Carrier system with a 90 + furnace model 58MXB installed in 2007. The furnace has had the blower motor replaced in it twice. While checking the unit today I noticed the motor that was replaced two years ago was drawing 7.7 with a FLA of 7.5, The service factor on the motor is 1.0. I know that this is barely over FLA, but I normally see these 1/2 hp motors drawing 4-5 amps at maximum. The capacitor is good (10 mfd) and the voltage going to the motor when it is running is 118 volts. Nothing seems out of the ordinary with the ductwork where the motor could be overloaded with oversized ductwork. This is in a fifty year old house with all metal ducts, no flex. I would normally say replace the motor, and re measure, but from what I can tell the motor has been drawing slightly over FLA since it was installed, and this could be a reason why the other two motors failed. After an hour of running the motor was not particularly hot to touch, not like an overloaded motor normally is. Any suggestions would be much appreciated. Thanks to all.

Comments

  • JUGHNEJUGHNE Posts: 5,781Member
    Filter?
    Plugged heat exchanger from lack of filter service?
    Is it the correct capacitor for that motor.
    Is the replacement motor equal to the original?
  • Jamie HallJamie Hall Posts: 10,663Member
    To which I would add -- slightly out of alignment or a bad bearing.
    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
  • LeonardLeonard Posts: 840Member
    edited May 1
    Not much experience with these but:

    Motor's PF (power factor) might be low (normal, from high inductance). That way it could could draw lot of amps, but draw little watts. That can explain why it's not hot. It's the watts VS VoltAmp issue.

    Kill-a-watt makes a nice meter for $25 (home depot) that measures watts, VA, PF, amps RMS, volts RMS of 120VAC loads to 15 amp.

    From my experience with a dead and a good water circulator motor , both drew ~ same amps but dead one had a higher PF ( .63 maybe) and ran very hot while good one had low PF ( ~ .23 maybe) and ran lot cooler. I'm told PF increases as increase load on motor. I suspect my dead motor had shorted windings, have not taken it apart yet to check if start winding switch welded,

    If PF is low a cap (power correction capacitor) in parelle with motor can increase PF closer to 1.0 and reduce line amps. Can calculate needed cap MFD from PF and amps. But as long as everything else is ok why bother, unless these motors typically draw lot less than nameplate amps..
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,919Member
    OEMs pay games with original equipment motors. They sometimes rate their motor with fictisous horsepower ratings. So what sometimes happens is you take out an oem motor with a 1/2 hp rating and replace it with a 1/2 hp motor and no one looks to see that the original motor draws 9 amps and the replacement draws 7 amps. You will see this all the time with roof top units.

    So the question is were the replacements original equipment motors or supply house motors? What I always look for is the replacement must have at least the same amp rating as the original. Don't know if this is part of your issue.

    Other than that can you drop the speed and check the furnace temp rise to see if your ok with that?

    12 year old furnace has had 3 motors in it that one every 4 years..not good

    I think your on to something. What's the definition of insanity? doing the same thing over and over and expecting a different result.

    Your probably the only one that ever checks motor amps. The other techs put a motor in and ran






  • STEVEusaPASTEVEusaPA Posts: 3,080Member
    Was the blower wheel changed with the motor?
    steve
  • HVACNUTHVACNUT Posts: 2,324Member
    edited May 1
    The furnace blower door was closed when you took the reading? Just asking.
    Zone dampers? All open?
  • Leon82Leon82 Posts: 616Member
    A loose wire will draw extra amps
  • pecmsgpecmsg Posts: 836Member
    JUGHNE said:

    Filter?
    Plugged heat exchanger from lack of filter service?
    Is it the correct capacitor for that motor.
    Is the replacement motor equal to the original?

    With a squirrel cage wheel those would lead to lower amperage!

    @zepfan Static Pressure readings will tell you if the blower is working properly.
  • ratioratio Posts: 2,053Member
    Like @pecmsg noted above, check the pressure across the unit & compare to the mfgr's paperwork, that'll tell you how hard the motor's working.
  • zepfanzepfan Posts: 266Member
    Thanks for all the responses, yes the door was on when the readings were taken, the capacitor is the correct size, and from what I can tell the motor is not OEM, it is from a supply house. A plugged heat exchanger, filter, or coil is going to cause a lower amp draw on a squirrel cage motor.
  • Jamie HallJamie Hall Posts: 10,663Member
    I still say to check shaft alignments and all the bearings -- induction motors will try to maintain speed even with a lot of drag, but will draw extra current while they are doing it.
    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
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,919Member
    @zepfan

    Going back to what I said above, like everyone else I learned the hard way.

    When I started we had this old Westinghouse RTU. One of the condenser fan motors went bad (it was the original). Westinghouse was long out of the rooftop business, so off to the local supply house. The original motor was marked .3 horsepower. So at the supply house we bought a 1/3 hp motor. After all .3 is .3 and a 1/3 hp is .33 and bigger is always better. So we put the motor in and got it up and running and left.

    Soon we were called back. new motor tripping overload.

    The Westinghouse rated at .3 was rated 3 amps, the 1/3 hp we put on was rated 2.4A

    I never forgot that and always replace going by amps especially on oem motors.

    I would track down what the original motor is and look into that. or drop the speed and check temp rise.

    Ductwork might be oversized. Just for a test, block a return or two with cardboard and see if the amps drops off
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