need help with 120v motor

Discussion in 'General Electronics Chat' started by deadbunny, Oct 15, 2009.

  1. deadbunny

    Thread Starter New Member

    May 1, 2009
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    Im trying to get a a three pin 120v motor to run on my 1000/2000w 12v inverter. the motor is a 1/4hp 4 amp motor from a dryer. it has a start up that is cut off by a SPDT centrifugal switch. What Im guessing is that the the motor uses more than 2000w to start thereby throwing to inverter into overload. What I suppose I need is a large start up capacitor. However I have no idea how to hook that up to an AC circuit or even what kind I need.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A capacitor can't be used to store energy in an AC circuit for a boost to a motor.

    Instead, you need to limit startup current, which is very high with a non-sinewave source. Something like an inrush current limiter might work - http://www.ametherm.com/inrush-current/selecting-an-inrush-current-limiter.html

    For proper sizing, you could find the startup draw with line voltage, and get a limiter to hold at that value. Or simply pick one that doesn't cause the inverter to choke.
     
  3. deadbunny

    Thread Starter New Member

    May 1, 2009
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    but wouldn't limiting the current prevent the motor from starting? couldn't I use a bridge to provide a DC to a capacitor with some kind of voltage controlled breakover switch to power the start up circuit?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Limiting the startup current to something like 15 amps stays under the limit of the inverter. The only thing that might happen is that the motor will take a bit longer before reaching full speed.

    How could you apply that charged capacitor to the motor to help it start? It's an AC motor. Here is a link to our Ebook that discusses single phase capacitor start motors - http://www.allaboutcircuits.com/vol_2/chpt_13/9.html
     
  5. deadbunny

    Thread Starter New Member

    May 1, 2009
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    Ya I realized that I would have to apply a DC signal with a capacitor after a clicked the post button. Thanks for the link BTW :tup:
     
  6. deadbunny

    Thread Starter New Member

    May 1, 2009
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    Ok. change of plans. I just found a 1/4hp DC motor for $20 Im thinking about getting. trouble is its a 24v motor and my truck is a 12v system. What kind of options do I have for a high current step up transformer.

    Sorry about the silly questions. Iv been out of that electronics game for 10 years now and don't know where to begin researching.
     
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    if your AC motor doesn't have a capacitor, it's likely a resistance start winding. You can confirm this if one of the windings (start) has a considerably higher resistance than your run winding. This type will have a higher draw as the start winding's pf isn't as good as that of a capacitor start winding. It will have a lower starting torque as well.

    You can play around with it, but the simplest 'limiter' is additional series resistance in the start winding. You will save on start current, but your starting torque will fall accordingly.
     
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