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Single phase 120VAC speed control

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  • Single phase 120VAC speed control

    Is it possible to control a 120VAC motor such as a router? The input is 120VAC also. I found this device that does it but I'd love to be able to do the same thing from a PLC or by using a readily available device from AD.


  • #2
    rrc1962, yes, you certainly can do this - control your router from a PLC. But you will need some parts not available from AD - not yet anyway.

    Using a PLC will be more expensive, but more fun.

    The hand-held tools (like hand drills) use small universal motors. The variable speed hand tools I've seen control speed by chopping off portion of the 120 VAC sine wave (both positive & negative symmetrically).

    The "SuperPID" device you show the link for says it works with universal motors, and probably controls speed the same way - chopping the sine wave.

    So you could use a "phase-angle-fired" solid state relay with an analog control signal as your control device. An analog output PLC module then could control your motor speed. Try:

    The "SuperPID" device uses "a reflective optical infra-red sensor that is simply pointed at the router output shaft and detects a spot of white paint on the shaft. This enables closed-loop speed control." (Quoted from your web site link.)

    So you will need a high-speed input module (CTRIO) for your speed input signal and a photo sensor (or other sensor) to monitor speed. AD sells lots of sensors.

    And you will need a C-more to adjust setpoint and monitor/tune PID loop parameters.

    This should do it for the basics, but you will probably need additional safety, overload, and other controls. Also, check all your design specs so you get components to handle your speeds and loads, and an idea of the control response you need: speed, accuracy, over-shoot, etc.

    Sounds like a fun project.
    Last edited by KPrice; 01-02-2011, 05:11 PM.


    • #3
      Thanks...The phase angle fired relay approach is what I was looking for. I'm thinking the on-board HSIO should be sufficient to generate an analog control voltage. Could I just use a PWM signal to generate that voltage or would I need an analog output module?

      I was also thinking that the on-board HSIO might be OK for reading the photo sensor, although I'd like to come up with a better solution than that.

      Since this really doesn't require many inputs or outputs, I'd like to use a DL05 and no expansion modules, although if I understand correctly, you can only configure the on-board HSIO for one purpose. IE: Can't have HS input and HS output at the same time. I would need HS input for the tach and HS output for the PWM.

      I suppose the cheapest way around that would be to use an analog output module with the ob-board HSIO considering the cost of CTRIO module.


      • #4
        rrc1962, you would have to check the specs of your tool, but yes, I would guess the on-board HS input would handle the RPM measurement. The on-board HS counter is rated at 5kHz.

        In the idea I suggested, you would not need HS outputs at all. The units I was looking at take an analog signal (0-5VDC, or 0-10VDC, or 4-20ma) directly - and then automatically fire the solid-state relay proportionally. There are units that proportion the output over a time-base, 20 AC cycles for example. Other units chop each cycle (twice) in phase-angle firing.

        I was not thinking of this, but yes, you could use the DC output to drive a solid-state relay directly. We have done this on a number of temperature control loops using time-proportioning with various time bases, 10 sec for example. But I was thinking of the phase-angle fired units because that is the way hand tools work, and I believe you would get better control.
        Last edited by KPrice; 01-02-2011, 10:15 PM.


        • #5
          At 20,000 RPM you would have 333RPS or 333 counter pulses per second, which should be well within the operating range of the on-board inputs. I'd have to check the specs, but a standard input might even work.

          Unless I'm not looking in the right place, those relays are pretty expensive. Looks like the models that provide proportional control with a 0-5 or 0-10V control voltage are around $250. Total cost is approaching that of an import VFD spindle. I don't think pulsing a standard solid state relay would work. This wouldn't properly chop the AC cycle. I Came up with this from looking at a simple drill motor speed control.

          Take a triac, a capacitor and a variable resistor. Feed the gate of the triac through the capacitor and variable resistor. As the resistance increases, the charge time on the cap increases. The cap controls when in the AC cycle the triac fires.

          In order to make this work with a PLC, you'd have to replace the variable resistor with something that the PLC could directly control. You could take a photo resistor and an LED. You could control the brightness of the LED via PWM from the PLC. As the LED changes, so does the resistance of the photoresistor.

          A more direct and "industrial" method would be preferred, but this works.


          • #6
            rrc1962, yes , these things can be expensive depending on load and application. And I agree that chopping the sine wave will give better operation.

            Have you looked at Newark products? They carry Crydom, and many other manufacturers of controllers. Below is an example. It takes 2-10 VDC analog input for $93.


            Your explanation of how the pot-controlled triac works is interesting. Crydom makes pot-controlled controllers also and this is probably the way they work.

            As far as the PLC providing the control, you could control a stepper motor controller, and use the stepper motor to turn a pot.

            Your idea about making a controller with a LED is interesting.

            But, you were looking to "do the same thing" as the "SuperPID". The "SuperPID" is looking pretty good at $139.
            Will this do all you want? or do you also need additional PLC control?


            • #7
              The SuperPID is starting to look good. I was just trying to do it using a PLC. I don't like buying one-off gadgets with only one supply source or line of support. I'll keep poking around. No real big hurry, I have a couple other PLC based projects keeping me busy right now anyway.