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Evap. Blower

Ideally, the actual capacitance should be within 10% of the capacitor rating.

As far as the rated voltage goes, when replacing a capacitor, the new device can have a higher voltage rating than the old device,


  • Rookie_3
    Rookie_3 Member Posts: 244
    Evap. Blower

    Had a call to a central air system with a block of ice on the evap. coil. I got it melted down and went to take a reading and noticed the evap. blower was running backwards!
    Control board was good. No other problems. I had to replace it and haven't thrown it on the bench to see what is up with it yet. Any ideas why a single phase motor would start running backwards? ............Dan
  • blackoakbob
    blackoakbob Member Posts: 248
    was it a twin......

    I have found that on some twinned furnaces if for some reason they don't get started at the same time one will run backwards. Could there be a serious backdraft thru the supply duct when the unit shuts down? Maybe someone was there before you and replaced it with the incorrect motor, if it was done during the heating season it may not have been noticed. Did you try turning the stator around....take the end bells off and rotate the center and reassemble. Best Regards
  • Rookie_3
    Rookie_3 Member Posts: 244

    Haven't done anything with it yet but I'll give that a try.
    Boss man says no one touched the blower in at least 4 years that he knows of but he did have one guy that use to alter the fans during heating season and you would know which ones he touched when the hot weather returned. This was on a Armstrong split. (Magic Pak)

    Thanks for the input....Dan
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380

    If nobody has worked on the unit in the last four years, it could (I hope) be assumed that the system was working properly. The problem may very well be a defective capacitor. If the capacitor is weak, the motor may start in the wrong direction, especially if there is a backdraft through the air distribution system.

    As mentioned in a previous post, thi sis a relatively common occurrance on twined furnaces (Two furnaces with a common return). In this instance, if one blower starts, air is pulled the wrong way through the other furnace anf the second blower rotates in the wrong direction. When the sedcond blower is energized, it can start in the wrong direction.

    Keep us posted.
  • Rookie_3
    Rookie_3 Member Posts: 244

    Capacitor is 7.5 and the reading was 7.4, it is a dual capacitor jumped to a 30 for the compressor which has a 28.8 actual reading. The new blower is now running off the same cap.. Actually there was nothing else done to correct the problem except replacing the blower. There didn't seem to be any draft causing any movement. I will eventually get a test wire on it and see what it does on the bench.
    While we are on the subject of capacitors what is your rule of thumb for condemning a cap.? Do you use the same stds. as motors? Anything under 90% of the rated amps.on the plate is a bad motor so would it be any reading under 90% on a uF reading is also no good for a cap.?

  • enthalpy
    enthalpy Member Posts: 13
    blower motor running backwards

    Still being a rookie, that was one of the greatest lessons that I could have learned. Have run into that a couple times this cooling season along with motors that will not start.
    Before condeming anything on a single phase I change the capacitor and give it the old college try, too hot in those attics and can you IMAGINE telling a customer that they will be without air because they need an OME motor. It doesn't make for a good day.
    Learned the hard way to this summer that even if the MFD is somewhat below the 10% they have gone and had callbacks, proving this to my boss I change if they are low, don't want callbacks.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Duel Cap?

    You mentioned that it is a duel cap with the compressor on the other side if the cap. Are you talking about the blower,(indoor) or condenser fan motor? Just curious. Must be the indoor @ 7.5 cap.
  • Rodney Summers
    Rodney Summers Member Posts: 748
    check the old motor..

    was this the original motor? check the date code. did somebody mess with this motor previously?

    does it have the leads for hooking it in reverse - cw to ccw? see if the prior guy simply wired it wrong. also, if not original, check the manual / spec for the proper rpm / speeds etc. check for it being a 2 speed motor wired in low speed.

    wouldn't be the first time.

    check to see if the run cap mfd matched the spec on the label usually 4, 5, 7.5mfd @ 370v (higher volt ok). the cap mfd rating must match. if in doubt, throw it out & replace! they cost nothing. caps start to die from the moment they're manufactured - whether used or not. they have a shelf life. for the lousy $2 just replace them before they leak or age - if defective & left in service, they can eventually destroy the fan / compressor that they are assisting.

  • Rookie_3
    Rookie_3 Member Posts: 244

    This was an indoor dual cap. [email protected] two speed. Turns out it was the blower motor. The system has been running problem free for about two weeks now and the motor was the only thing that got changed out basically because everything else tested out OK. H/O says they had no trouble
    all winter. (Split unit, same blower) To be honest it's still sitting on the bench, I haven't had a chance to play around with it to try to see what the problem actually was.
    Wiring to the control was correct unless someone did get inside the motor and then the question would be why?
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