single phase motor won't start

Discussion in 'General Electronics Chat' started by JoeR, Sep 19, 2013.

  1. JoeR

    Thread Starter New Member

    Sep 19, 2013
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    1HP single phase reversible motor on lathe. After many cycles "forward and reverse"... motor will not start...only humms. Centrifugal switch seems ok. capacitor looks good. Took motor to a shop and they tested it as very good. Brought back home and used it for a month then same problem. Any ideas out there?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you give it a good spin and apply the power does it run?
    If so, points to the start circuit somewhere?
    Max.
     
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  3. #12

    Expert

    Nov 30, 2010
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    If it isn't in the start circuit, the control system has a bad contact.
     
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  4. JoeR

    Thread Starter New Member

    Sep 19, 2013
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    Thanks for replying guys. If I give it a spin it will rotate but very little heart.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Could be a winding short, possible start winding?
    Max.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If the "forward and reverse cycles" are back to back without the motor coming to a complete stop, your motor may just be overheating. A lot of the tool motors have a thermal shut off switch built in them. When the thermal switch gets above a certain temperature it opens the stator windings, until it gets below another certain temp and resets.

    Only the run windings open, not the start windings. The start windings are probably what you hear humming. The older motors had a red reset button on the non-drive end bell that you had to manually reset. Now they reset automatic at the cooled down temp.


    All of this is assuming that if you let the motor cool down or set for a while, it will run OK again.

    A single phase motor, or brushed DC motor, should come to a complete stop before reversing direction. A three phase motor is more forgiving about reversing.
     
  7. tinkerman

    Member

    Jul 22, 2012
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    In most cases single phase motors must come to near stop condition in order to reverse direction of rotation because of the starting switch. Failure for the switch to close will result in the motor continuing in the original direction of rotation.

    I'm not aware that DC motors have that problem and certainly 3 phase do not.

    What voltage is this lathe motor operating at?
     
    Last edited: Sep 19, 2013
  8. JoeR

    Thread Starter New Member

    Sep 19, 2013
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    The problem remained days after cool down.

    I think my son was over zealous with the "start - to - stop" swing and didn't let it come to a complete stop.

    The voltage can be 110 or 220. I've got it wired for 110 right now.

    I even spun the motor with drill and then turned it on and it would turn very slowly and sounded like it was drawing a lot of current.

    I know very little about electric motors... seems by getting up close to 800 rpm with the drill would have eliminated the start windings question.
    So far I haven't found anyone who can say for sure.
    This is why electricity is so magical to me. I'm a miner...rocks are predictable!
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It sounds typical of a start winding failure/short circuit to me.
    Max.
     
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  10. tinkerman

    Member

    Jul 22, 2012
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    Usually start switches are designed to operate at about 75 % of full RPM. That would be over 1300 rpm for a 1750 rpm motor. At 800 rpm the switch would still be closed. I haven't got a 120 reversing drawing in front of me but I think another poster is correct in stating that the run winding will have the overload in the circuit so it is possible the run winding is not being powered. The start winding is outside the overload and still would get power. (in a 230 volt connection this situation can be avoided. The overload feeds both windings). If the start winding only is energized the motor will draw considerable current and growl. As the other poster mentioned this might be what's happening.
     
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  11. tinkerman

    Member

    Jul 22, 2012
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    I found this link for a drum forward/reverse switch, center off:
    http://www.practicalmachinist.com/v...phase-motor-reversing-switch-my-lathe-264163/

    Split phase top right probably applies in your case.

    Wires attached to 2, 4 are line.

    Winding attached to 1,5 would be run winding and have the overload (not shown) in series.

    Winding connected to 3,4 would be start winding and has no overload protection. The start switch would be internal of the motor and not shown here.

    If the overload is open or something is wrong with the run circuit causing an open (bad contact perhaps) only the start winding gets power and would cause the noisy operation and failure to achieve speed.

    You could change to single phase three leads but that leaves the motor attached to one leg of the source at all times and the overload would feed both windings. As long as the common is attached to the neutral not too bad. Not so good it it's the hot leg.
     
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  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    In a DC motor, it will instantaneously reverse, but there will be sparks at the brushes. Depending on the amperage of the motor it can even blow out the commutator sectors.
     
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