PSC AC Motor speed controller design

Discussion in 'The Projects Forum' started by ericaustin, Feb 12, 2009.

  1. ericaustin

    Thread Starter New Member

    Jul 22, 2008
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    I'm working with a 1/12 HP Single Phase 60 Hz 120V PSC motor with a 10 mfd CAP permanently installed across the start winding.

    At full power it works great. However, I am interested in adding a speed control.

    I built the this SIMPLE TRIAC MOTOR SPEED CONTROL :

    [​IMG]

    It works Wired in series with the AC +in line. But it does not give repeatable high resolution control.

    If the VR1 is over about 70Kohm the motor is off. As I drop the resistance the current flows.

    Initially the motor goes through a slow start...similar in style to the quick ramp up that a PSC motor does with full power...only now lasting much longer because the speed controller is delivering only part of the power to the system. There is also a very small range of resistance values right around 70Kohm where this low power humming rotation can be sustained indefinitely.

    A few clicks below 70 kohm the motor actually hits a smooth high speed rotation. Now I can continue to vary the VR1 and the smooth high speed rotation gets a little faster and a little slower as it should.

    The thing that confuses me is this:

    I am only able to vary the speed within a few percent of the rated RPM. I estimate that I when I get past the slow start (mentioned above) and get it initially running fast and smooth, that it's running at about 85% of rated speed and I can take it up to what seems like 100% rated speed when VR1=0

    Drawing the speed down from 100% doesn't act quite the same...on the way down I can actually take VR1 down well below 70k ohm...to about 30kohm or so before the motor stalls.

    Any ideas about this system. Is it even plausible to control said motor with said control circuit over a large range? What happens if I replace the 10mfd run cap with a smaller cap?

    Is it possible to control any PSC motor over a wide range?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    AC induction motors cannot be controlled by such means. The only way to vary their speed is with a variable frequency drive (VFD). The applied AC frequency is what causes the motor to rotate. Trying phase control will only prevent the motor from developing full torque.

    A universal type motor with brushes would control very well with your triac circuit.
     
  3. ericaustin

    Thread Starter New Member

    Jul 22, 2008
    6
    0
    Hmmm. Sounds reasonable.

    However, I have read dozens of articles and application notes that suggest Phase angle control for AC induction motors.

    what gives?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    I can't say - can you post links to any of these articles?
     
  5. ericaustin

    Thread Starter New Member

    Jul 22, 2008
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    0
    http://www.st.com/stonline/products/literature/an/3566.pdf

    The application in Figure 9 on page 8 is analogous to the system I built. This pretty much sums up most of what I have read.

    Also, a lot of literature I found indicated that phase angle controllers such as those found in the common lamp dimmer are suitable for speed control of AC induction motors...is it possible that they were more likely referring to universal motors or those with a centrifugal switch to shut off the start cap?

    Cheers.

    EW
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Technically, an induction motor with a squirrel cage rotor is an asynchronous motor. You can always try the controller. I have no experience with phase angle control on AC motors.

    I do notice that the one document does not indicate the motor is capacitor run.
     
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