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Universal motor bi-direction speed control

Discussion in 'General Electronics Discussion' started by jackorocko, Sep 12, 2013.

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  1. jackorocko

    jackorocko

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    Apr 4, 2010
    I found a universal motor. I know these can be used on either AC or DC and to reverse the direction all one needs to do is reverse the polarity of the field coils compared to the communicator?

    Now what about speed control, I have read that you can use a triac to act as a speed control for a universal motor, but my question is how would I adjust the rpm dynamically as apposed to statically? I want to be able to turn up and down the rpm of the motor while it is running. Is this possible and if so can anyone give me a quick rundown on how I would need to set this up?
     
  2. duke37

    duke37

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    Jan 9, 2011
    Some drills use a simple triac control with the speed varied with a potentiometer..

    A lamp dimmer is a similar circuit and can be used with a universal motor.

    The dimmer is not complicated, just a potentiometer, capacitor, diac and triac.

    If running off the mains, you will need to make it safe.
     
  3. jackorocko

    jackorocko

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    Apr 4, 2010

    did a little research since you chimed in. A dimmer switch would work in this application as I am only looking at 840 watts. I do however think I will be well under that rating as I don;t plan on running this motor at anything less then 20% of it's rated rpm. Still need to figure that out.

    I did see a video on youtube about speed control for a universal motor and found a guy who used a Cds and a LED to send PWM variable resistance to the control circuit. I wonder if there is something similar I could use but be a little more robust? Any suggestions are welcome. Would be nice not to have to guess what RPM I am at.
     
  4. duke37

    duke37

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    Jan 9, 2011
    You have not said the voltage of the motor, at 840W I assume that it is mains, 110 or 220V.

    To generate a 220V PWM source would be something special.

    A Cds is perhaps a cadmium sulphide (CdS) light sensitive cell which replaces the potentiometer to control the triac. I do not know if these can stand mains voltages. This system will be used to give mains isolation.

    It is possible to get a speed sense from the back emf of a permanent magnet motor. I doubt if the same can be done with a universal motor.
     
  5. jackorocko

    jackorocko

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    Apr 4, 2010
    yes mains voltage of 115 volt so says the name plate. It also says it is DC. I know you can run these on AC or DC. Would it make more sense to rectify mains and use a big MOSFET and PWM to control the speed? Max RPM is listed at 16.5k. I need to be at about 10 - 20% of that speed. But I have a couple options to deal with that. The most obvious is to gear down the jackshaft and increase my motor RPM.

    Right now I am just looking at potential options. I will be trying all these methods and see which one works best for me.

    I also have a variac which I read is another simple method to control the speed on these motors. Does that means it will draw more current proportional to the amount of reduced voltage? Never been good with inductive load theory.
     
  6. jackorocko

    jackorocko

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    Apr 4, 2010
    Well I hooked the motor up to my 12VDC bench supply and it turned the motor at a rather slow rpm. Hooked it up to the 5VDC rail and it turned even slower.

    This gives me plenty of options to power the motor. The simplest being a Variac, I bought one for $10 dollars at the flea last summer.
     
  7. duke37

    duke37

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    Jan 9, 2011
    Your Variac was a bargain. Make sure you do not exceed its current rating.
     
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