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AC Voltage Conditioner for Appliances on Generator Power?

Discussion in 'Electronic Design' started by [email protected], Sep 7, 2007.

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  1. Guest

    Hello -

    I have company that makes small, low flow, high-pressure pumping

    The systems consist of small digital controller (MSP430 based) with an
    LCD display. The pump motor plugs into the controller and power is
    supplied to the pump motor via a relay inside the controller, i.e.
    there is only one power cord for the unit. The units are designed for
    120V 60 Hz power. When the pump motor is running, the steady-state
    current draw is 7A, when the motor is not running, the controller
    draws 300 mA.

    Recently a number of our systems have been installed on small
    Caribbean islands that have powered supplied by diesel generators. We
    have had an issue with the controllers failing - either via the fuse
    blowing, or the micro getting fried. Since we have near perfect
    reliability with the systems on domestic utility power, my guess is
    that either power spikes or electrical noise from the generator power
    is causing the failures.

    The purchaser of the systems in the Caribbean has told us "everything
    else here works fine so yours should too", while our contract
    electrical engineers are giving us the "we can't be responsible for
    anyting running on generator power". I haven't had much luck Googling
    around on the topic.

    So, my question is: what options would I have as far as some sort of
    reliable voltage conditioning device that could strip out any electric
    noise and/or provide surge suppresion when the units are running on
    the generators? I need something that could be incorporated into the
    unit for < $75.

  2. linnix

    linnix Guest

    You probably need a better voltage regulation circuit. Can you post
    your design?
  3. Guest

    Try this....

    ....I believe this schematic constitutes the voltage regulations/
    transformer circuit (apologies but I'm a ME not a EE!). The rest of
    the board just contains some additional relays for other devices that
    the controller triggers. The LCD and micro are located on a separate
    boad that is piggy-backed onto the power board.
  4. linnix

    linnix Guest

    It can't be driving the micro with 11.6V. What's between the 11.6V
    and micro?
  5. Guest

  6. linnix

    linnix Guest

  7. The "we cannot be responsible etc" is _too_ true. The output of starting
    and stopping generator is a mess of voltages and frequencies till it
    stabilases. So any device connected during this time can fail.
    One of simplest things would be a time delayed relay on line, activated
    by line voltage and connecting the pumps and controllers after ~1 minute.
    When connected beware of stopping generators.


  8. linnix

    linnix Guest

    That's very true. Nevertheless, a properly designed regulator circuit
    should be able to prevent damages. The OP's circuits are wrong and
    very inefficient. It doesn't make sense to have 12V transformer/
    switcher output and wasting 75% of it in the linear regulator. 300mA
    for an LCD controller is ridiculous if not illegal. I am not sure if
    we are still on track to ban such linear regulator by law.
  9. Fred Bloggs

    Fred Bloggs Guest

    You have big problems doing this cheaply because you tap off the
    internal line feed to drive the pump. Might as well figure 1KVA for
    whatever you do.
  10. Joerg

    Joerg Guest

    Very hard to say without seeing the whole unit. It is best to get a
    local consultant to take a look at how it is wired, where the cables are
    running and so on.

    This will be more a shot in the dark but has helped a few times: Get a
    linear regulated power supply of a good brand, such as Condor. Proper
    amperage with lots of reserves, it can get hot in the Caribbean. Should
    be one that has a uA723 chip for regulation (most do). Adjust that down
    from 12V to 11.6V, place a good inline power filter up front. The Corcom
    brand is good. Fuse it all properly.

    A Corcom filter alone right in front of T1 might also work. Use a good
    filter, not just a common mode choke. Right now it seems you have
    nothing like that in front of T1, that lone transzorb isn't going to cut
    it. The motor power should be tapped off before that filter.

    Keep in mind that spikes can also come in via control lines into the
    controller board and then you'd really need a consultant that knows EMI
    stuff well.

    With generators you need to be prepared for lots of brown-out
    conditions. Slow and sputtering starts, running out of diesel, big loads
    being switched where voltage and frequency can go all over the place etc.
  11. ehsjr

    ehsjr Guest

    Feed the input voltage to the MC34063A through a diode installed
    before the R8/C6/pin 6 junction, and add a 100 uf from that
    junction to ground. On the output side, change C15 to 270.

    You can also add the optional output filter to the MC34063 -
    see the datasheet. What is the purpose of R1 and R6?

  12. Jeff L

    Jeff L Guest

    Change the power supply to an offline SMPS, they do not care what
    frequencies they receive, and if done correctly can have a wide input
    voltage. Low cost off the shelf units can easily handle from 90V DC to over
    250V AC with out a glitch. Sounds like your product is low volume, so I
    would not try to roll your own due to regulatory testing. The good news is
    some are available with several voltage outputs, so you may be able to
    eliminate your other power supplies entirely, potentially saving money!
    Efficiency would improve also.
  13. Joerg

    Joerg Guest

    There are also huge "magnetic stabilizer transformers" that were popular
    in the former Eastern block countries because the power grid was often
    so lousy that TV sets would repeatedly use sync or something would fry
    in there. Those monsters must still be around somewhere.
  14. Fried an oscilloscope once with one. The thing didn't get along with its
    load (a few old non-PFCed SMPSes, only 50% of rated stabilizer power).
    Stabilizer overheated, produced weird 100Hz (!) output harmonics, and
    fortunately it was the cheap TDS2xxx scope that went first (got it repaired
    on warranty).

    I didn't really bother to investigate the cause of the weird behavior, I
    just chucked the whole damn stabilizer into the bottom of some cabine where
    I'm sure it still is today. It's a 1.5kVA unit made by Block. Maybe it has
    some future use as a gravity lens.

  15. Joerg

    Joerg Guest

    Some scopes in the TDS series are truly deplorable IMHO. That's why I
    recently bought an Instek instead.

    Watch out for that nasty conducted EMI at around 40/80kHz. It can really
    fool you when looking at analog signals. Probably leakage from the
    backlight inverter or maybe its switcher.

    Well, you aren't supposed to connect switchers and PFC stuff to it. They
    weren't meant for that. In this thread it's a simple air conditioner
    where 99% of the load is motors.
  16. mpm

    mpm Guest

    I didn't have time to dive into all this.
    Let me just say that if the generators are configured to provide
    backup power, (automatic transfer), then you'll sometimes see systems
    that do not bother to synchronize the Utility-provided AC waveform
    with that provided by the backup generator. They just throw the
    switch when normal utility returns.

    So, when the automatic transfer switch goes back to Utility power,
    it's possible to switch the load from the max voltage of one souce, to
    the min voltage of the other. There are also usually slight frequency
    variations between the two sources, but this is likely unrelated to
    your experience.

    If any of this is the case, your customers really need an in-phase
    monitor on their automatic transfer switches. (If manual, it probably
    takes too long to throw the switch?, and this problem would not

    I agree with other posters here that a robust power supply on your end
    could be designed to handle utility transfer switches for generators
    if you feel there is a reasonable design justification for same.
    Good luck.

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