Motors and start/run capacitors

Discussion in 'General Electronics Chat' started by ajspinner, Apr 19, 2011.

  1. ajspinner

    Thread Starter New Member

    Apr 19, 2011

    I have a problem with a motor that has a capcitor mounted to it. Recently the motor has begun not come up to full speed, rotation does begin but it appears to be running about half speed and there are no controls for speed adjustment. The motor is part of a direct vent system for a pool heater..about a week ago the heater stopped working and the indicator light say air-flow. I am familar with the piece of equipment but o not want to replace the blower motor for $600 if either the motor or capacitor is bad. I know the blower is not up to speend from the "noise" airflow and the fact that the vacuum switch (to sence draft) is not being made...putting an external vacuum on the switch rectifies the fault.

    Can I be having a capacitor issue? How can I tell? This is one area I am not familar with and need guidance on theory and diagnostics.

  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    would be the first place to look. How much of what variety of lint or other detritus might be in the motor/ ducting/ fan blade -- impeding either switch action. Similar to a table-saw, it maybe fouled by the sawdust it creates. crud could also be affecting the onboard vacuum switch, since you stated an external source helps..........

    Your capacitor, is likely a start cap for the motor. At rest, the switch, places the cap is series with the start windings. Powering the motor, it will accelerate to about 3/4 of its rated rpm, then a centripetal device kicks the start cap and start windings out of the equation, and the motor takes off and runs as it should. If this switch is not being allowed to contact / disconnect - or if the cap is indeed bad, the motor will do as you said, just hum, and not come up to speed, overheat / burn out / smoke in short order.

    Alternately, it is a PSC motor -- permanent split capacitor -- and there is no start switch involved, capacitor will affect starting torque and running amp draw.

    Either style of motor requires a regulated airflow -- baffles in the ducting. erstwhile if the airflow is unrestricted, the motor will draw far over amperage, trip its overload, or start smoking / burn out quite rapidly.

    In either case, remove the capacitor from the circuit - polarity won't matter, have it tested, and replace if necessary with an identical unit. The vacuum switch, when closed - no power no load , should read one ohm or less on the contacts.

    If replacing the capacitor or the vacuum switch doesn't do the deed, you might be looking at that $600 motor...............hopefully a competent shop should be able to help you IF the motor is indeed repairable.
    It's all a process of elimination.
  3. ajspinner

    Thread Starter New Member

    Apr 19, 2011
    Is there a way to do a quick test on the capacitor to check if it is bad? Can I put a meter on it to check it? If I disconnect the capacitor fromthe motor...and indeed it is bad...should I see the same results...not coming up to full RPM.

    The blower wheel is in good condition, the vacuum line and switch are clear and work I know I am not getting the rpm I need to draw enough airflow and create the negative pressure I need to close the switch
  4. SgtWookie


    Jul 17, 2007
    There may be other things wrong with it than a cap going bad.

    It may be that the rotor is dragging on the stator, and/or the motor bearings are worn.

    You should consider taking it to a motor repair shop for evaluation. They have tools such as meggers and growlers that most people would not have. If you keep trying to operate the motor when it is running at a reduced speed, you may severely overheat it, which will damage it.
    ajspinner likes this.
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    I had the same problem with the furnace in my garage last fall, a 90% efficient gas furnace. the exhaust blower would not come up to speed. Took the blower motor out and SURPRISE, a mouse decided that the squirrel cage blower made a great condo! Before going too far unbolt the motor and housing and check for a critter nest :)