I thought fan motor was jammed and needed cleaning; but I was wrong!

Discussion in 'General Electronics Chat' started by PG1995, Aug 30, 2011.

  1. PG1995

    Thread Starter Well-Known Member

    Apr 15, 2011

    I remember once someone told me that a capacitor is used in a motor to give it a 'surge' of power so that it can start revolving by overcoming its inertia, resistive forces such as from grease etc. Electric current alone doesn't have enough strong 'push' to come out of its static position. But once in motion it no longer requires the capacitor as long as it keeps running. I was further told that if you don't have a capacitor in a motor then you can give it a 'push' yourself. Further, if I recall correctly, I was also told that some not-very-large three phase motor can do without a capacitor because three phase electricity has more power per cycle. Do I have it right?

    For last some days the exhaust fan of our store room wasn't working properly. Like when I would turn it on, it wouldn't start. Sometimes it would only make a half revolution and that too very slowly, and then it would stop. So, when this malfunctioning started I was giving it a manual push with a stick, and when pushed, it would start rotating but one of the important thing was that it was only rotating almost at half the normal rotating speed. Therefore, from this I inferred that the inside of its motor needs to be cleaned and greased because it seemed as if the motor was pushing against something; in other words it seemed it was jammed and in my opinion that was the reason for its half-speed.

    Today an electrician came to fix it. I was arguing with him that its motor needed to be cleaned. But he said that it needed a new capacitor (I think he himself wasn't sure of himself that much and part of the reason he wanted to test a new capacitor was that he was finding it too difficult to open the external cover of the fan. But as soon as he changed the capacitor the fan started running! And I was there surprised and partly embarrassed.

    Where did I go wrong with my reasoning? Please help me to figure out. Thank you very much.

  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    No. read this: http://www.allaboutcircuits.com/vol_2/chpt_13/9.html

    In a 3-phase motor you don't need a capacitor because three-phase power is being used directly to generate a rotating magnetic field. Has nothing to do with power.

    He was right. If the capacitor looses capacitance it cannot fulfill it's function, see link above.
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
    You inferred wrong. That's all to it.
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    I think of single phase AC as it relates to single phase motors like a steam train. as in the way it turns linear force into rotational. Single phase pulses directly back and forth. imagine if you stopped a steam train with the push rod extended straight out; without some help ( something pushing on the wheel from a different angle, maybe from the top, or 90 degrees out of phase hint hint), it would never start spinning. once it got spinning (inertia), it wouldn't need any more help. same thing with a single phase motor. The capacitor shifts the phase 90 degrees to get the motor spinning and then once it's up to speed, a centrifugal switch removes the capacitor from the circuit. (that's for a capacitor start motor, there are other types). So it doesn't have as much to do with dirt and grease as is does with physics.

    As far as the fan running half speed with the capacitor faulty, leads me to belive it is one of the other types I mentioned. probably a capacitor run motor, which continues to use the capacitor, as name implies, even while running. The windings are arranged differently for this type.
  5. PackratKing

    Well-Known Member

    Jul 13, 2008
    Strantor.....I like your steam-train analogy...... paints a very clear picture.........

    In essence, all here raise valid points. Rare is the vent fan that uses a PSC motor. most are the little open shaded-pole variety anymore.........some never seem to get additional lubrication........ever ! which eventually dry up, ruins the bushings and binds them up.

    In the summertime, I collect the better quality 20" window fans left in droves on the curb for trash day...........usually a little cleaning and a good soak of the wicking w/ a good quality light oil...........NEVER 3 in 1 ....... resurrects them for 3- 4 years, and yields 10-15 bux each....:D

    same goes w/ dehumidifiers.....only those yield a few more $$ and can be reasonably guaranteed to work for a few more years beyond a one year guarantee given the buyer ........ maybe .1% have come back to bite.
  6. #12


    Nov 30, 2010
    PG, your first paragraph is so full of baloney that I went to the kitchen and started cooking my lunch.

    Some motors can be started with a nudge if the cap goes bad, and that's one way to diagnose a bad capacitor, but don't use your finger if you want to keep it for use tomorrow. There are so many kinds of motors with capacitors that it's hard for me to keep track of them. Just consider yourself schooled and glad you didn't do anything dangeous.