Motor and capacitor question

Discussion in 'General Electronics Chat' started by glynnw, Sep 12, 2012.

  1. glynnw

    Thread Starter New Member

    Sep 12, 2012
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    I am trying to restore an audio record cleaning machine. One of the small motors drives a vacuum pump but needs a small "push" to get started. It is noted elsewhere on the net that this is a common problem with this unit and is why it is no longer made, nor are parts available. Unfortunately the motor is installed in a sealed part of the unit and it is a pain to keep pulling the unit apart to start it. This motor seldom runs longer than 3 minutes before being turned off. This is a simple 2 wire motor (and ground wire) that has no capacitors. I experimented with different values and ended by placing a 20 mfd cap in series in a wire going to the motor. Now it starts and runs fine. This is a 440V cap that says it is a "start and run" cap. Here is my question - can I simply leave this in the circuit or do I need to install a switch to remove it from the circuit once the motor starts?
    Thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Run it for a while (5min). Feel for heat. If nothing burns your hand, you are good to go...
     
  3. cork_ie

    Member

    Oct 8, 2011
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    It really doesn't make a lot of sense to me using a capacitor in this situation, if the motor is simple 2 wire and was never designed to have one. However the proof of the pudding is in the eating and if it works that is proof enough for me.What prompted you to use a capacitor to sort your stalling problem?
     
  4. glynnw

    Thread Starter New Member

    Sep 12, 2012
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    I tried the cap as a result of my audio hobby, which can be a bit eccentric at times. Many audiophiles are always searching for caps to use in their electronics and speakers (no arguments about this please - some caps of equal value but different construction do sound different) and a popular phrase is "well, I took this motor run capacitor and placed it in my amp" so I knew of their existence from my hobby.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    I have 120V electricity but I found a 240V fan. it does not run on 120V.
    I connected a capacitor in series and it ran fine on 120V because the capacitor tuned the inductance of the motor to my mains frequency which boosted its voltage.
     
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  6. cork_ie

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    Oct 8, 2011
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    I would be really interested in looking at the mathematics of this and the theory behind it if anyone knows it. I assume it is similar to some sort of voltage doubler circuit.
     
  7. J-Ri

    New Member

    Mar 14, 2012
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    No argument here. The reason for this is the ESR, or Equivalent Series Resistance (and possibly other factors). Different materials have different ESRs. The higher the ESR, the longer they take to fully charge and discharge.

    x2!
     
  8. Audioguru

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    Dec 20, 2007
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    The inductance of an AC motor reduces its current. But when its inductance is tuned with a series capacitor the current reduction is eliminated and the higher current allows the motor to run when the input voltage is too low.
    The voltage across the inductance (motor) exceeds the supply voltage.
     
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  9. glynnw

    Thread Starter New Member

    Sep 12, 2012
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    Great info. According to the manufacturer of this record cleaner, it's main failure as a product was in England (where it was made) where the electrical grid around 1980 was still iffy at times of heavy use. When people got home after work in the evening and everyone turned on their electrical appliances, these motors would fail to operate.
     
  10. cork_ie

    Member

    Oct 8, 2011
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    I understand how inductance will cause the current to lag behind the voltage and capacitance the opposite. However what I don't really understand is how the capacitance suitable for a (relatively) high starting current will be also suitable when the motor is in run mode.
    Do you have the mathematical formula for calculation of the start capacitance required for a motor of given inductance.
     
  11. XerxesX24

    New Member

    Sep 13, 2012
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    Would there ever have been a capacitor for the motor?

    Any solder lumps or wires that go nowhere?

    Just a thought: DC commutated motors for model trains/cars etc often had a capacitor in parallel with the contacts. That can be said for many different kinds (and sizes) of motor, obviously the values and capacitance parameters would be different.

    Perhaps there was one fitted and it's missing?

    That might expain why it works with and won't self start.

    Anybody else thinking along the same lines, or should I put my spade away? ;)
     
  12. cork_ie

    Member

    Oct 8, 2011
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    I think he is talking about an induction motor in this particular case.

    Capacitors in brush motors perform a different function, to reduce arcing between the commutator and the brushes.

    As the segments swing away from the brushes the current induced in the particular winding wants to continue flowing in the ionised gap and will try to continue flowing until the gap gets too great. The capacitor creates an alternative path and the arcing (and consequent EMI) is reduced.
     
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  13. XerxesX24

    New Member

    Sep 13, 2012
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    Good to know, thanks very much!
     
  14. glynnw

    Thread Starter New Member

    Sep 12, 2012
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    I finally got the cap fully wired in and let it run for a few minutes. The cap was cool to the touch so I thought it was good to go. Then I touched the motor. Yeow! Hotter than you know what. So it looks like I'll put a switch in. Thanks for all the advice - probably kept me from burning something (say, a house) down.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    I had a two-speed blower motor for furnace. It was 1/2hp.
    It got VERY hot when running on its slow speed.
    A switch selected the fast wire or the slow wire. Both wires were not shorted together.

    My new high efficiency furnace has a DC variable speed motor that is always cool.
    It has much more power than my old AC motor.
     
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