18 VAC Motor Control

Discussion in 'The Projects Forum' started by toppytop, Aug 1, 2009.

  1. toppytop

    toppytop Thread Starter Member

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    I would greatly appreciate someone to help me out with this design. What I am doing is controlling a Lionel train with a simple triac/diac (actually a Quadrac) type control for a fan motor at 120 VAC. I can wire the control on the primary side of my 120 VAC to 18 VAC transformer and it works fine to control the train. My problem is I don't want it on the primary side I want it on the secondary 18 VAC side. I can run multiple trains from this transformer and just need individual controllers. The breakover voltage of the diac is too high for this lower voltage. I must start from scratch with the control and don't know if I need zero crossing detection or not or if I will need a opto-isolator or not. In my small mind this shouldn't take much of a circuit but my background isn't electronics (have alittle knowledge), it's in control wiring. Could someone post a schematic or two on how to accomplish this control. Thank you.
  2. yourownfree

    yourownfree Active Member

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    I have a question for you. the transformer is 18 vac? I dont know much about trains, so how do they go in reverse? does it get converted to dc somewhere? Are you trying to build a speed controller? more explanation please.
  3. rjenkins

    rjenkins AAC Fanatic!

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    Hi,
    a very simple train speed controller:

    This assumes you have a transformer with a full-wave rectifier giving somewhere around 12 - 18V.

    Code:
    + -----*-------------------------
    IN     |                        |
          ---                       |
          \ / 1N4007                |
          ---                      ---                 
           |       R1              \ / TH1
           *------/\/\---          ---
           | +          |       -- /|
         ----- 4700uf   \      |    |
         ----- 25V      /      |    |
           |            \<-----|    -----> + Out
           |            / VR1 --- 10uf
    IN     |            |     --- 25V
    - -----*------------*------*---------> - Out
    
    The input is unsmoothed AC straight from the bridge rectifier.
    (Or via a fuse/thermal overload, as required).

    The Diode and Capacitor at the left create a smoothed DC supply to feed the pot VR1
    The resistor R1 limits the maximum output / speed.

    The Thyristor TH1 needs to be a sensitive gate type with a high enough current rating to run the train motor.

    To start with try 1K for the pot and 1K for R1.

    Operation is very simple; it's a low-voltage equivalent of a DIY 'drill speed controller, by the way..

    Any time the Back EMF from the train motor is less than the voltage on the cathode of the thyristor, as set by the pot (-0.7V for the junction voltage drop), current is grawn through the gate, the thyristor fires and current is supplied to the motor for a single half-cycle.

    It is at least 25 years since I built some of these and the details are slightly fuzzy, I do remember that you could get perfect speed control from full down to an inch or so per minute..

    The diode is not critical, I use 4007s for everything but any small rectifier would work. There will be a bit of a dead zone at the low speed end of the pot, you could add a preset resistor in the negative pot connection to adjust this.

    If you interested in scale speeds and want the slow speeds easy to control, you can use a double-gang pot and connect the slider of the first half to the 'top' of the second half. The bottom (ccw) of both go to negative and the output is from the slider of the second stage.

    This give 'square law' control, where 50% on the pot is quarter speed, 25% is 1/16 speed and so on. Fine adjustments are easy.
  4. toppytop

    toppytop Thread Starter Member

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    Thank you so much for your responses. Sorry about my lack of detail. Rjenkins: thanks for your thoughts and effort. The train does in fact operate at 18VAC max. The train "engine" has a "reverser" built into it. If it is in forward, the next time the voltage hits zero it shifts to neutral. Make it hit zero again and it shifts to reverse and then back to neutral and then back to forward ect. Most Lionel trains operate this way. Maybe rjenkins method could still work by replacing the thyristor with a triac? Thanks again.
  5. yourownfree

    yourownfree Active Member

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    Oh ok. I built a PWM a pulse width modulator that should the trick. This is my own design. Where it says to cell, hook up a transformer to that. Power it with about 20 vdc to the primary side of the transformer. That said you will have to modify the circuit slightly. the mosfet can be directly hooked to 20 vdc while add a 12 volt regulator chip to the control circuitry. As for the transformer use a 1:1 isolation transformer. and the other side of the transformer goes to your train. Never let the pwm go below ?? Hz or it could shift your train back and forth if it gets down to to low in frequency. Than can be set with a pot. I am not sure how much of a pause it takes to shift the train. But I am thinking that you should be able to allow the frequency to go down enough it will just chug along without shifting. At that point just have a switch or something that says"shift" that kills the power (secondary side), release the button and away you go again. The PWM will adjust the output voltage with a pot.
  6. toppytop

    toppytop Thread Starter Member

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    Thanks for the reply, unless I misunderstand, it sounds like your idea is to control the primary side of the transformer. I want to control the secondary side only since this transformer is big enough to power more than one train. I do like your PWM, I can think of a couple uses for that.
  7. yourownfree

    yourownfree Active Member

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    Question for you. what is the output current of your transformer and how much current does each train take? It might be possible just to use a power pot to control each one. Will have to get the figures to calculate the size needed. That would be the simplest. The idea I had with the pwm , would need three small toroids at 1:1 ratio and the use of your 18 vac as the power source to power three pwms each with the added 1:1's.
  8. yourownfree

    yourownfree Active Member

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    Question for you. what is the output current of your transformer and how much current does each train take? It might be possible just to use a power pot to control each one. Will have to get the figures to calculate the size needed. That would be the simplest. The idea I had with the pwm , would need three small toroids at 1:1 ratio and the use of your 18 vac as the power source to power three pwms each with the added 1:1's.
  9. rjenkins

    rjenkins AAC Fanatic!

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    Hi,
    OK, my model train experience is rather limited, I've only seen DC motor type.

    Have a look at phase control using a Programmable Unijunction transistor (PUJT or PUT - 2N6027 is a common type). With one of these, a Triac and a few other components you can build a 'light dimmer' style phase control circuit that will happily work on low voltage AC.

    Y-O-F - your circuit appears to be a variable frequency pulse generator?

    PWM varies the duty cycle of a *fixed-frequency* carrier, usually with the carrier frequency set too high for the load to respond directly. PWM with very low frequencies also exists and is often called Burst Fire control; typically used for heating elements etc.

    Edit:
    A thought: Do they actually NEED AC, or is the first item connected to the power pickup a bridge rectifier?
    If there is nothing before the bridge, you should just be able to use DC with my circuit.
    It may possibly need a resistor adding from the + input to the + output to give a small bias current to the direction circuit in the train - try 1K or 470R or 220R to see what works best.
    You will also need to switch the input (or output) to allow the direction circuit to reset.
    Last edited: Aug 2, 2009
  10. toppytop

    toppytop Thread Starter Member

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    Five amps max is what will be required. I have attached the schematic of the fan control I used on the primary side of the transformer. The breakover voltage of the internal diac is 33 volts min. Maybe this control can be modified to work on the secondary low voltage side. This is the type circuit I would like. Thanks

    Attached Files:

  11. yourownfree

    yourownfree Active Member

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    just for the record my pwm is not fixed frequency. I have seen the ones that are fixed. All they do with the pot is set the level to the last stage how much of the portion of the signal it will allow through, basically setting a voltage tripping point. Mine is truly adjustable. adjusting it changes the duty cycle. as the frequency get higher the pulse width is shorter. but by adding a flip flop you could get an exact 50/50 if needed while still adjusting the frequency. I only mentioned adding the output transformers to be sure it would run an induction motor, and to keep it ac.
    The circuit just presented might work just fine for your needs. Kinda have to just give a circuit a test run and see what happens. Simpler is good, I wont argue about that. I am always looking for simpler ways. Great job coming up with a plan.
  12. fanie

    fanie Active Member

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    It is always easier to control DC than AC, so if you have to control the AC supply you can put a diode bridge in series in the AC supply. The ~ and ~ is where the AC does in and out. You control using the + and - on the bridge only.
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