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Thread: HF DC - Motor or Capacitor problem?

  1. #1
    Join Date
    Sep 2003
    Location
    Winnsboro, SC
    Posts
    81

    HF DC - Motor or Capacitor problem?

    When I turned on my 4 years old HF DC Monday it took at least 3 seconds to come-up to speed and it drew so much electric that it dimmed the lights in the shop to almost nothing.
    The DC is on a dedicated 20 amp circuit. I plugged it into another 20 amp circuit and the same thing happened. It did not trap the 20 amp breaker nor the 100 amp feeding the shop. I opened the receptacles and panel looking for melted wires or fried breaker and there were none. I checked the DC switch and capacitor and they looked good. I then turned off and on the breakers just to reset them. Then tried the DC again. This time it drew so much power the HDTV in the house lost power and had to be reset. There is no blockage in the ducts.

    So, motor or start/run capacitor?
    Any other suggestions?

    I have a month to buy a new HF DC for $140.

    I want to thank everyone that took the time to read this and I hope explained the problem clearly.
    Bob

  2. It sounds like it could be a bad capacitor. There should be a cover over the capacitor. Take that off to find out what type it is. A replacement might not be too expensive. Be careful though the capacitor could still hold a charge. I would use insulated pliers when you cut it out. The other possibility is bad bearings in the motor...

  3. #3
    I would recommend checking the motor for the ability to spin freely.If it is able to spin freely,next you need to check out the mechanism which cuts out the start capacitor when it gets up to speed..it seems that the start cap is not electrically disengageing.It could simply be caked with dust.
    To explain it a little further ,i am reminded of the expression "eli the ice man"
    e is the expression for voltage(emf) ,L is the expression for inductance,i is the expression for current.
    What does this have to do with the topic at hand?????
    An inductive motor is inductive. When you first apply the EMF)(voltage) to an inductive circuit the voltage leads the current by 90 degrees (more or less)
    The makes for lower overall power applied to a device,because the instantaneous current is LAGGING.
    This is corrected (somewhat) by applying a CAPACITOR to the circuit correcting the phase Lag..
    as the motor gets up to speed...it becomes less inductive and the current /voltage phase correction is no longer needed or desired.A stuck on capacitor circuit will present itself as drag to the motor.
    Note: some devices uses mechanical means to disengage the startup circuit (on induction motors) , especially if there is significant "flywheel effect"

    The second part of the expression ICE basically says that in a capacitive circuit the current FLOWS 90 degrees sooner than the voltage rises (the opposite of eli)
    hove this helps you to understand what may be going on....
    There will never be a shortage of folks telling you why you can't or shouldn't do something...even though much has been accomplished that hasn't been done before !

  4. #4
    A single phase inductive motor can be started without a capacitor by giving it a spin before applying voltage to it. Once it starts to spin, a positive feedback occurs which causes the motor to get to speed rather quickly. So what does this have to do with your question?

    As long as the motor starts to spin when turned on, the capacitor is doing its job. There are two windings on a single phase motor, one for regular running and the other for starting. The capacitor is placed in series with the starting winding which, combined with the running winding, causes the field to have a favored rotation (it actually rotates in both directions but slightly stronger in the favored direction). The rotor follows that favored field rotation.

    Anyway... I would suggest the same thing as Steven, above. Check the motor for free rotation. If the bearings are badly worn, the rotor can be rubbing on the field which might not show up when you just spin it by hand, so check for movement in the shaft. Pull it up and down and all around. If you have any questions at all, change the bearings. Check anything connected to the motor for rubbing or bad bearings.

    When the motor is running at significantly less than its rated RPM, it will draw excessive current, which may be why you are experiencing the excessive current draw.

    There could be other causes, but I'd check the above first.

    Mike
    Last edited by Mike Henderson; 01-10-2011 at 11:39 AM.
    Go into the world and do well. But more importantly, go into the world and do good.

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