120VAC motor speed control

Discussion in 'General Electronics Chat' started by zane9000, Oct 9, 2007.

  1. zane9000

    Thread Starter Member

    Mar 16, 2007
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    I have done a lot of searching both on Google and this site for an AC motor control with no luck. Everything that I can find is either talking about small DC motors (under 30V) or just saying how not to control a large AC motor (ie dont just put a lamp dimmer in line with it). I also have a few electronics texts on my bookself that seem to yeild no answers for me.
    Im not looking for a full circuit or anything, just sort of point me in the direction about which way to go...

    Thanks,
    Warren
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I made a similarly fruitless search a couple of years ago. All I learned was that the frequency should be increased when the voltage is decreased. (I never did learn why this should be.)

    I resolved my dilemma by following in Lionel Oliver's footsteps: http://www.backyardmetalcasting.com/castingpulleys1.html
     
  3. zane9000

    Thread Starter Member

    Mar 16, 2007
    19
    0
    Unfortunately, pulleys will not work for this project. Does anyone know if using DC PWM on an AC motor will hurt it?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I don't know if it would hurt the motor or not, but standard PWM DC will not make an AC motor turn.

    You might consider a PWM power inverter. Vary the PWM to emulate a sine wave. This was the path I was looking at just prior to choosing a mechanical solution. Literature on sine-wave inverters will be helpful.
     
  5. zane9000

    Thread Starter Member

    Mar 16, 2007
    19
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    OK, So I think I may have come up with a solution to my problem, tell me what you think.
    All About Circuits - Reply to Topic
    Basically this is an ugly square wave generator.
    The way it works is this:
    A 555 timer sends a signal to a 4017 counter.
    The counter is set up to cycle Q0-3 then reset.
    Q0 will send a rectified 120VAC through an SCR H-Bridge in one direction.
    Q1 will divert the rectified 120VAC through a capacitor and another SCR so the H-Bridge will deactivate.
    Q2 will activate the H-Bridge in the other direction.
    Q3 will deactivate the H-Bridge again.
    Rinse, repeater.
    Speed is controlled by the 555 timer.

    I have a VERY rough schematic of it here.
    Please tell me what you think.

    Thanks!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Instead of using SCR's, take a look at power MOSFETs and IGBT's.
    The problem you'll run into with SCRs is trying to turn them off once they're on. If you open the 2nd one in the "H" before you turn off the 1st one, you'll place a dead short across your power supply. This will run up your power bill considerably, along with the possibility of producing smoke. :eek:

    International Rectifier has lots of info for you to peruse.
    Some examples:
    IRF5NJ6215 - P channel 150v 11a Rds ON 0.29 ohms
    IRF730 - N channel 400v 5.5a RdsON=0.75 Ohms
    IRF3415 - N channel 150v 43a RdsON=0.042 Ohms (this one is quite remarkable)

    Power MOSFETS/HEXFETS have a faster switching time than IBGT's; you can use PWM at a comparatively higher frequency. One really great thing about MOSFETS/HEXFETS is the ease of paralleling; they have a positive temperature coefficient. This makes the power dissipation self-limiting. Just about every other type of power transistor has a negative temp coefficient; the hotter they get, the better they conduct, right up to the point they melt down. :eek:

    One more tidbit from IR's site: below 200V, use MOSFET, above 200V, use IBGTs (general rule of thumb)
     
  7. zane9000

    Thread Starter Member

    Mar 16, 2007
    19
    0
    I may have to try those. The IRF3415 seem like a good way to go. SCRs just seemed easier (and super cheap) at first. Then I remembered I needed a way to turn them off, but for some reason I just stuck with that idea.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Keep in mind you have to ensure that at no time can the high and low sides of the same leg of the "H" be turned on simultaneously. You should take everthing into account; propagation delays, capacitance "Miller charge" of the gate, turn-on & turn-off times, etc. Failure to do so will produce smoke!
     
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