AC cap start motor low rpm on manual start

Discussion in 'General Electronics Chat' started by martik777, Feb 16, 2013.

  1. martik777

    Thread Starter New Member

    Feb 16, 2013
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    My 110 3/4hp 1725 band saw motor blew it's start capacitor so I disconnected it and temporarily ran the motor by giving the pulley a spin by hand. It never went faster than 1200 rpm and the centrifugal switch would not engage, I assume due to the low rpm. Anyways, I was able to substitute a 220MFD cap I had on hand for the original 150mfd, and it ran normally - 1725 rpm and centrifugal switch engaged.

    What I do not understand is why it only ran at 1200rpm without the cap installed.
     
  2. zacharoni16

    New Member

    Feb 16, 2013
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    My guess is that the start capacitor basically causes phase lag to allow the start windings to allow the armature time to react to the changing field. Without it, the 60HZ is applied instantaneously and the armature can't react fast enough to overcome the excitation field.

    You said you can move it by hand so its not a mechanical problem like bearing, it only ran at a slower RPM because most likely the armature is lagging and being "slowed" down by the excitation field.

    The starter capacitor is removed from the circuit by the centrifugal switch when the motor overcomes the excitation field and gets up to speed. I would check the switch to make sure it's working properly, maybe thats why the cap blew in the first place

    EDIT:

    Usually when the cap doesn't work the motor doesn't move at all and just hums, so 1200RPM is impressive
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The capacitor you replaced was most likely a Start/Run Capacitor, rather than a starting capacitor.

    The run capacitor is required for a phase shift to increase efficiency with 1 phase power.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Without the start capacitor, the motor likely doesn't generate enough torque to get up to speed with a load. If you remove the load, the motor will probably reach full speed if you give it a spin by hand.

    The capacitor generates a phase lead in the start winding to generate a quasi-two-phase rotating field so the motor will start.
     
  5. martik777

    Thread Starter New Member

    Feb 16, 2013
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    I am beginning to think the motor is faulty.....

    It only gets up to 1333 rpm on full load and heats up after a few mins to the point it is uncomfortable to place your hand on the motor (CS (centrifugal switch) does not engage). There is also a slight burning smell. If I remove the blade so the motor is only driving the lower wheel it usually gets up to speed and the CS engages. If I run the motor stand alone the CS always engages and rpm is 1725.

    Could the start cap have blown due to a faulty motor is the first place? It was intermittently not reaching full speed for a few days before the cap blew.

    The CS and contacts are all in good condition. Bearing seem ok, no axial or radial play and it rotates freely.
     
  6. tinkerman

    Member

    Jul 22, 2012
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    The motor has only ONE capacitor right? Usually hard black case. It will be start cap then, in series with the start winding. The starting switch will dis-engage the start circuit at approx 75% full speed. Leaving it energized longer than several seconds (30 seconds ?)will blow the cap usually and it could damage the start winding too.

    I'd open the motor to get a view of the windings. There should be two sets of windings, one with heavier wire which is the run winding. The second will be finer wire. It's the start winding. If the motor hasn't been starting well it could be discolored or even badly burned. If that's the case, unless you're prepared to re-wind, it's probably not worth salvaging.
     
  7. martik777

    Thread Starter New Member

    Feb 16, 2013
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    Windings look ok and yes, just a single cap like this:
    http://www.ebay.com/itm/Industrial-...572?pt=LH_DefaultDomain_0&hash=item4ac0d54284
    Must have been the oil type as there was a tsp of oil on the case from when it blew.

    It will not go over 1333 rpm when loaded but consistently gets up to 1725 unloaded.

    1. Could replacing the original 150mfd cap with a 227 cap cause this?

    2. If I disengage the centrifigual switch and let it run at 1333 rpm will this cause further damage? It seems to overheat at this speed.

    3. Anything else that would cause it to run at the reduced speed? bearings? Shaft seems to rotate freely - turns about 6 revs if I give it a good spin.
     
  8. tinkerman

    Member

    Jul 22, 2012
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    That's a starting capacitor. They sometimes leak a lttle fluid when they fail but not a lot. 227 mf should work OK. It's only in the circuit for very short time normally.

    It will heat at 1333 rpm. It's not building enough reverse emf to limit current.

    I'm wondering if it's dual voltage 120/240 and half the winding is not in service. Dual voltage motors have two main windings. They are paralleled at 120 volt connection and series for 240 volt operation. That would cause it to have less power. Also if connected at 240 volts and operated at 120 volts, probably not the case here if you haven't touched connections, they run fine but have no power.
     
  9. martik777

    Thread Starter New Member

    Feb 16, 2013
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    I checked both windings, one was open, here's why:

    [​IMG]

    Repaired the 3 broken wires and all is well again. No doubt that short caused the cap to blow in the first place.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    It never gave off a burnt smell?
     
  11. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You can talk about it all day on this site, and others, but you can't talk it into working. You have to actually look at it to find the problem.
     
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