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Monitor mains voltage with transformer

Discussion in 'Electronic Basics' started by fssg, May 22, 2006.

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  1. fssg

    fssg Guest

    Hi,

    I made a microcontroller circuit to control speed of a small fan with TRIAC
    phase control. It works fairly well, but the fan never completely stops,
    among other things, so I am wondering if the TRIAC is being triggered the
    way I expect.

    I don't have a CRO, so I want to sample the output voltage from the TRIAC
    using the microcontroller's ADC.

    I could do this using a simple resistor divider, but then I have to connect
    circuit ground to Neutral, and circuit ground is my computer ground.
    Neutral is supposedly connected to earth, which in theory would make this
    safe (I would test first), but it seems like bad practice to do this...

    I have a transformer, 240V->24V 20VA 50/60Hz. If I put it's primary on the
    TRIAC output and a resistor divider on the secondary, that would probably be
    safe, but will it work? Will I be able to see the sharp edges where the
    TRIAC turns on in a half cycle?


    Another question about whether the transformer will pass the signal: To
    detect zero crossing, I am using an optocoupler, but other circuits show
    using the AC output of the power supply transformer. Because the
    transformer is inductive, wouldn't there be a delay between the actual zero
    crossing and when the zero crossing occurs on the secondary?


    Thanks,
    David
     
  2. Phil Allison

    Phil Allison Guest

  3. Chris

    Chris Guest

    Hi, David. Did you expect a good answer without providing much of any
    pertinent information? Maybe you could start by answering a few
    questions:

    * What type of motor do we have here? Is it an AC induction motor?

    * What's the current rating (or watts rating) on the nameplate?

    * Are you using an optocoupler to drive your triac? (Hope so!) If
    so, which one? Is it a zero-crossing triac trigger? (If it is, throw
    it out and get one that isn't.)

    * Are you timing a delay from the zero crossing of the AC line?

    * Have you provided a snubber (R-C or other) across the triac? Could
    you sketch out your circuit, using an ASCII Circuit program? (possibly
    use Andy's ASCII Circuit v1.28 http://www.tech-chat.de )

    To start with, there are a few things here. First, you should know
    that phase control of AC induction motors is difficult, and sometimes
    it just can't be reliably done. Problems do tend to occur at the low
    end, and can actually be more difficult with smaller motors.

    However, there are several things you can try here.

    * If you haven't already, put an R-C snubber across the triac (start
    with 220 ohms 1/2 watt in series with 0.1uF line service self-healing
    cap).

    * Obviously the biggest problem here is the inductance. You're going
    to have difficulty turning the triac off if the curent is going great
    guns while the voltage crosses zero. One good solution for this is
    using back-to-back SCRs, with separate optocouplers for each SCR.
    Obviously the reverse-biased one won't trigger, and the other one will
    stop when the voltage reverses. There are many solid-state relays
    which are made like this, and all you have to do is provide the 3mA or
    so to drive the opto input (make sure you don't get an SSR with
    zero-crossing triggering). One cheaper solution, which *might* help is
    to get a Teccor Alternistor, which has many of the characteristics of
    back-to-back SCRs. It provides better turn-off behavior than a
    standard triac. I've found I had better luck driving inductive loads
    with these:

    http://www.littelfuse.com/data/en/Data_Sheets/E4AlternistorTriac.pdf

    * If you're desperate, and the motor's small enough, you can start
    trying to change the power factor of the load by adding a power
    resistor in parallel with the motor. Adding a capacitor across the
    motor is a spectacularly bad idea, because you'll cause di/dt failure
    in the thyristor (too fast a change in current causes the triac to
    smoke). Of course, you're just burning power this way, but it can
    help.

    * If worse comes to worst, you can always do it like the dimmer
    switches used for ceiling fan controls -- they reduce the speed as you
    dial down, and then use a switch to turn off. You can always just use
    a relay in addition to your phase control.

    Having said all of this, I'd guess that, especially without a scope,
    you're going to have an easier time just working on solving your
    problem than trying to diagnose it by reading the voltage across the
    thyristor or the load with your uC. You're going down a rabbit hole
    here -- you just want to control the motor load. Take the steps you
    need to do that, and worry about the other stuff later.

    Actually, there are many applications where metering the voltage (or
    more frequently, the current) on an AC load *is* a good idea. It can
    be a good part of the control circuit, and there are many ways to do
    this -- but that's actually another problem. In fact, as long as you
    don't load the transformer secondary, you can get a pretty good idea
    where the zero crossing is, and what's going on with the load voltage.
    And there's no lag to speak of between transformer primary and
    secondary. You are also adding inductance, and changing the circuit
    you're measuring, too. But let's work on the fan first here.

    Feel free to take the time to express your problem a little better --
    the few minutes it might take to actually describe your problem well is
    a lot easier to spend than hours thrashing around in the lab.

    Good luck
    Chris
     
  4. Jasen Betts

    Jasen Betts Guest

    It's likely to trip an ELCB/GFI.
    but if you have an isolating transformer on hand you could run your circuit
    from that and connect its "neutral" to the PC's ground without risk.
    it'll probably make matters worse.
    that depends, it may come early.

    Bye.
    Jasen
     
  5. Jasen Betts

    Jasen Betts Guest

  6. Phil Allison

    Phil Allison Guest

  7. The 24V transformer would be OK to measure the output voltage and there
    would be no "delay". Also, the easiest place to measure the zero crossings
    is on the secondary of the power supply transformer. All it needs is a
    couple of diodes.
    john
     
  8. Phil Allison

    Phil Allison Guest

    "John Jardine."

    ** On the contrary - there can be a quite significant delay ( ie phase
    shift) in the secondary voltage if the magnetising current is a little high.

    Some theatrical dimmer designs have a trim adjustment to allow correction of
    the phase shift in each unit made.

    A toroidal transformer would have negligible phase shift.



    ** Really?


    ........ Phil
     
  9. fssg

    fssg Guest

    I want to make this for a hobby community I am involved in, I don't know
    what fan people might use but it will probably be in the 30-200W range.
    Capacitor run will probably be the most common, but some people use shaded
    pole types.
    I am testing with 3 different fans. One is '240VAC 0.25A 50Hz' shaded pole.
    One is '230V 50Hz 173W' capacitor run. One is a self-oscillating room fan,
    '240VAC 50Hz ONLY 50W', not sure what motor type but it has 3 speeds and
    there are 4 wires going up into the motor.
    I am using a MOC3021, which is nto a zero-crossing type.
    I detect zero crossing with a 4N25 and start a timer in my microcontroller,
    driving the MOC3021 at a variable time after the zero crossing.
    This schematic doesn't show it, but I am using a 1/2W 68 ohm and a 100nF X2
    across MT1 and MT2.

    '------o------o------o--- Active
    | | | |
    470R.-. | | |
    | | | | |
    | | | | |
    MOC3021 360R '-' | | |
    ___ | | | |
    -----' .----|___|-----o | | |
    | _|_ | ' .-.M |
    V -> V_A | _|_ | |O ---
    - / | | V_A | |V --- 100nF
    | '--------------)--- / | '-' |
    ----' | | | |
    | | | |
    --- | | |
    47nF --- | | |
    | | | ___ |
    '------o------o--UUU-o------- To load
    100uH
    T405-600
    or
    BT137
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    I didn't have one when I first started because there appears to be one in
    the schematic above, which I copied from a MOC3021 datasheet appnote.
    Apparently that is something to do with the TRIAC gate so now I have a 68R
    and 100nF extra across MT1 and MT2.
    Is an alternistor what ST would call a 'snubberless' triac? If so, don't I
    need the ability to trigger in the 4th quadrant to do power control?

    I have a very old (Nov 1980 date code) fan speed controller here, which is
    almost identical to the circuit in figure AN1003.15 in
    http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf. This
    controls all 3 fans perfectly, but not to a complete stop. It is triac
    based so I should be able to get away without SCRs. In fact it doesn't even
    have a snubber...
    OK, well since I first posted I have found a few bugs, mostly in my zero
    crossing detection circuit. That part is working properly now and matching
    my calculations.

    I tested my triac firing output from the microcontroller by putting an RC
    circuit on it so that I can get a duty cycle reading. It also matches
    expectations, giving 0-100% over 0-90deg firing angles.

    Unfortunately fan speed control is not so predictable.

    The 173W capacitor run fan:

    Starting with 90deg firing angle (zero power), works as expected, fan
    stopped, no buzzing and doesn't get hot. Speed increases as expected up
    until 45deg when it doesn't get any faster. However, at 16deg, the speed
    starts jumping around, 1s at full, 1s at slightly slower, and so on. At 14
    deg it slows right down, starts buzzing and generally just gets very hot.
    This continues until 2deg, when the fan stops and I cannot hear any buzzing
    at all.

    Starting at 0deg and increasing power, starts silent, then starts buzzing at
    2 deg, buzzes until 19deg, then at 21deg it takes off full speed. Stays
    full speed until about 45deg, when I can detect a bit of a slow down (by
    ear). Can control speed right down to zero.

    The shaded pole 0.25A fan:
    Barely worth writing about. At 90deg, no buzzing as expected, but as power
    is increased, starts to spin but vibrates soflty at what sounds like about
    10Hz. On the way from 90 to 0, fan starts to spin then stops at 3 or 4
    places in this range. Eventually burns out snubber resistor.

    This ^^^ is with 68R and 100nF snubber.

    I have tried 3 different triacs (in brackets, critical rate of rise of off
    state voltage):
    T405-600 (20V/us)
    BT137X-600 (100V/us)
    BT139-600 (250V/us)

    There must be something wrong with my circuit?

    Thanks Chris I really appreciate your help.
     
  10. Phil Allison

    Phil Allison Guest

    "fssg" <

    ** The LED inside the MOC3021 must be driven on and off correctly to avoid
    " half waving" (*)

    Do not pulse the LED - drive it on throughout the time you want the triac
    to be on only turning it off 0.5 mS prior to the next zero crossing so the
    triac can turn off at the next current minima.


    * Half waving is common problem when PULSE fired triacs are used to phase
    control an inductive load. The triac may turn off at an unexpected point in
    the AC cycle since, at that point, current flow has gone through zero value.
    The result is that the output waveform is unsymmetrical and has a large DC
    component. Motors and transformers will make a growling noise and burn out
    if this continues.

    The solution is to continuously fire the triac for the whole time it should
    be conducting.

    BTW

    Get yourself a scope !!!




    ........ Phil
     
  11. fssg

    fssg Guest

    That didn't work :( Maybe because I am using an optotriac? (which only
    turns off when it wants to) Maybe I can use some other type of optocoupler.

    Thanks for your help.
     
  12. Phil Allison

    Phil Allison Guest

    "fssg"

    ** Stop being beinge a PITA.

    Go get a DAMN scope and SEE what the heck is going on.

    Read up on triacs too.

    They turn off automatically when the current drops to near zero.





    ........ Phil
     
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