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Motor not turning on but drawing excessive current.

Discussion in 'Sensors and Actuators' started by jackorocko, Aug 22, 2011.

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  1. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I have a table saw that every now and then the motor won't start and it just hums, but the light on the same circuit dims severely when this happens and switch is on. So I know it is drawing excessive current.

    What would cause this sort of thing? The motor is not seized up as I can turn the belt by hand and it all works smoothly. I noticed that if I spin the shaft on the motor and while it's still spinning, flip the switch, and it runs normally. Anyone have an idea on what could be causing this issue?
     
  2. alfa88

    alfa88

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    4
    Dec 1, 2010
    My 1st. Thought

    I'm wondering if there is a starter capacitor on that motor. It may have gone bad.
     
  3. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Motor wont start

    Hi there jackoroko. Whats the type of motor ? if its an induction motor it could be the start run capacitor, the fact you can spin it up then throw the switch surgests this, the start and some times run capacitor gives the motor the phase shift needed to run it up. If its a DC series or shunt wound it might be a commutator/ brush problem, or windings deteriation in the stator or rotor insulation/windings.
     
  4. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Motor wont start

    Dam phone will only give me 512 charecters, to early to fire up the pc. I dont know what motor it is, i guess from what you say and spining it by hand then start and it runs seems to me it might be the start/run capacitor, a lot of induction motors use them to give the phase shift to start up on single phase, i would look at that first. Whats the motor type and power ratings. Dave.
     
  5. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Yes it does have a start/run capacitor. I assume that is what is housed under that big hump on the motor. :)

    That was my first thought and the reason why I thought to spin it and then turn it on while still spinning.
     
  6. duke37

    duke37

    5,340
    753
    Jan 9, 2011
    The hump is a capacitor which may be start or start/run. If the cap is for start only there will be a centrifugal switch to disconnect it when up to speed. These switches often cause trouble.
     
  7. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Is this switch something that is easy to find? Or is it inside the motor? How would the wiring differ if it was a start or start/run capacitor?
     
  8. duke37

    duke37

    5,340
    753
    Jan 9, 2011
    If the capacitor is start/run the capacitor will be permanently wired in series with one winding. If it is start only it will be in series with the switch which will be at the end of the motor. Larger single phase motors can have the capacitor remote from the motor and connected by a timed relay or a manual switch, since the capacitor is mounted on the motor this is not likely in your case. If there are two power wires to the motor, then the switch is in the motor.
    The fault could also be with the winding or a bad connection but these are unlikely.
    You should be able to check that the switch is closed without removing it.
    Be careful of the capacitor, it may bite. The capacitors often have a leakage resistor (about 1M) to discharge them for safety but do not rely on this.
    The two power wires will be connected together through the run winding and the capacitor will be connected to the power wires via the start winding. Thus each side of the capacitor should be connected to the power wires. If this is not so, you should check where the break is. Check that the cap is not short circuit, if it is, it will not give the 90deg phase shift required.
     
  9. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    motor start problem

    Hi again.
    I think the centrifugal switch will be on the rotor assembly, but motors like saw and milling machines don't often have them, other appliances do that need a huge current to start up, i might be wrong, but my bet is on the start run capacitor, these often help get the motor up and running and continue conducting while the motor is running , the supply is not in phase with many induction motors, the capacitor gives the motor the shift in phase it needs to start, and in some apps run, i would start there, if a centrifugal switch is at fault the motor might have to come apart. Dave. :) .
     
  10. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Well the saw was working good tonight, no humming. I did take the cap out and looked at it quickly, it is a 110v 400MFD cap. Motor rating is 1HP at i thought ~4k rpms. I didn't have time to trace the wires to see if there was a switch yet.

    edit: something tells me that this is a start cap with the switch. After looking up the difference between caps the one I pulled out of the motor was in like a cardboard cylindrical thing and didn't weigh very much. I am assuming that the run cap with the metal case and oil inside would weigh more and be of smaller capacity.

    So now my question, How do you determine the size cap needed for your start or run application? How do they figure the value?
     
    Last edited: Aug 23, 2011
  11. duke37

    duke37

    5,340
    753
    Jan 9, 2011
    If the motor started up without assistance, then it is likely that it is an intermittent switch which is causing the trouble. Capacitors or motor windings are not going to repair themselves.
    The size of capacitor is determined by the motor manufacturer, there will be many parameters which will be input to the calculations. Motor power, winding turns, winding inductance, rotor inertia etc, etc.

    You are running on 110V so I assume that you have 60Hz. The motor unloaded will run close to 3600rpm and the speed will drop a little when load is applied.

    If you need to replace the capacitor, replace with an identical component.
     
  12. rootboy

    rootboy

    22
    0
    Aug 26, 2011
    Yeah, it's most likely the switch. I used to do a lot of service work in the milling field, and one of the common problems was sawdust getting into the centrifugal switch and keeping it from making.

    If the OP will *carefully* blow compressed air into the rear of his motor (where the switch is located) the next time it is running, then this should clear it up. Until the next time it happens of course. ;)
     
  13. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Did this the other day when as soon as I realized I had a mechanical switch. Mechanical switches and dust/dirt/wood particles never go together. It sure was dirty as all hell too.
     
  14. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Motor start problem

    Well its all sorted out ?
    It was only ever likely to be two things, the start run capacitor, or the centrifugal switch, any way glad you got it up and running again, nothing bugs me more than a head scratching inconclusive result, induction motors only really have one active part, well two if you count the centrifugal switch, yet the science and physics involved in the mutual inductance between rotor and stator are very bewildering current leading voltage lagging, something like that i think, and all induction motors are wound slightly different depending on the magnetic circuit, any way nice fix. Dave. :)
     
  15. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Well it is working as of right now. Only time will tell if the switch needs replaced. But I am satisfied with it now.

    Good lesson to learn. Now when I look at a motor just by looking at it, I got a better understanding of what type of motor it is and what its use may be for.
     
  16. rootboy

    rootboy

    22
    0
    Aug 26, 2011
    Yup, and it will eventually wear out as well. I have had some success finding replacement switches to get things going again.

    Good to hear that you got it working! :)
     
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