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24v 300w power supply

Discussion in 'Electronic Design' started by E. Lee Dickinson, Jun 1, 2004.

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  1. Hello, All -

    I am curently working on a project (automated scenery, to explain the
    crosspost) for which I need a 300 watt motor. I'm looking at a 300w 24v DC
    motor, since I am pretty comfortable with DC motor speed and direction
    control.

    My problem is in coming up with the power source for that. I prefer to buy
    a readymade power supply, but everything I'm seeing is either $1000 or "not
    for variable loads." I'm not adverse to building a supply, but I know
    little beyond the fact that I need a transformer, rectifier, and regulator.

    Any suggestions? If the best answer is, "Learn how to control AC motors,"
    I'm open to that, but I've been told that AC motors don't reverse well and
    don't operate well at low speeds. This project has to change speed and
    direction fairly rapidly.

    Thanks in advance.

    :Lee
     
  2. Chris Warner

    Chris Warner Guest

    either use an industrial servo drive, or get a 300w 24V transfomer and a
    rectifier. The transfer will probably run you about 30 or 40 bucks, teh
    rectifier will probably cost you less than 5 bucks.

    ..02 from
    Chris Warner
     
  3. How automated is this scenery? If you need a lot of torque, maybe you
    could research an alternative using hydraulics?

    I work at UPS stateside, and there are several miles of belts. Some of
    them can go in reverse from the flick of a switch. All run off AC and
    have some transmission things that turn the rpms into an unbelievable
    amount of torque. None of them change speed though. I know some high
    pressure fountain (the water kind) displays like the one at the Dorothy
    Chandler Pavillion in Los Angeles use some kind of electronic 'thing' to
    drop the frequency from 60 hz to something variable between 10-30 hz,
    keeping the voltage the same.

    Torrance
     
  4. budgie

    budgie Guest

    There's a bloke in Adelaide (that's South Australia to you furriners) selling
    some brand new Lucent units that would appear to more than meet your needs. He
    seems to have a steady availability so might be worth contacting him.

    http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&category=296&item=5702486892
     
  5. Hello Lee,
    I am a cheapskate, so I would be scrounging a couple of
    car batteries that can no longer start a car engine but
    still have some life left in them. You can get them for free.

    Battery chargers. You can find them everywhere. Bludge one
    from your mates/aquaintances. Float charge your batteries.
    On the night when the batteries are used often, stick the
    charger on boost.

    Here is a forward reverse stop motor controller already made up.
    http://www.tecel.com/d200/
    http://www.tecel.com/d100/
    Price looks to be very fair.

    Speed control circuit, here is one that you could adapt to the
    the above motor controller since it has to be PWM under I KHz.
    http://www.solorb.com/elect/solarcirc/pwm1/
    That one operates at 400 Hz.
    You don't need the power Fet.
    Reduce the PWM output signal to 5 volt maximum
    to suit the d200 input.

    You don't have to use the tecel H bridge.
    Just reverse the motor using a heavy duty switch, or
    better still, a heavy duty relay. (automotive from wreckers).
    Use the speed controller as it is shown.

    Some food for thought. Sounds like an intersting project.
    Have fun.

    Regards,
    John Crighton
    Sydney
     
  6. John Nagle

    John Nagle Guest

    These Vicor power supplies will work just fine with
    variable loads. But expect to pay about a dollar a watt.

    http://www.vicr.com/products/dc-dc/power_supplies/vipac/

    We use these in our DARPA Grand Challenge vehicle.

    But really, you're better off finding an industrial
    motor controller that runs from 110VAC or 220VAC.

    Incidentally, it's worth noting that "brushless DC"
    motors and "AC synchronous motors" are the same thing.
    Large servomotors (above 1HP) are usually referred to
    as "AC synchronous", while small motors are referred to
    as "brushless DC". So if you're having trouble finding
    larger servomotors and controllers, that may be the problem.

    Synchronous AC servomotors are available all the way up
    to locomotive size.

    The phrase to search for is "industrial drives".

    John Nagle
    Team Overbot
     
  7. DuncanWood

    DuncanWood Guest

    The phrase "as well" is more like it, AC motor controllers have improved
    dramatically in the last 15 years. & just build a slightly oversized PSU &
    you shouldn't need a regulator for most DC controllers.
     
  8. Dan Mauch

    Dan Mauch Guest

  9. James Lerch

    James Lerch Guest

    Perhaps an ill formed idea, but how about buying a hefty 12vdc battery
    charger (say 50 AMP model). Modify to accept 220vac input and 24vdc
    output.

    I just picked up an el-cheapo 50amp battery charger from Wal-Mart, and
    modified similarly. Seems to work just fine..


    Take Care,
    James Lerch
    http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

    Press on: nothing in the world can take the place of perseverance.
    Talent will not; nothing is more common than unsuccessful men with talent.
    Genius will not; unrewarded genius is almost a proverb.
    Education will not; the world is full of educated derelicts.
    Persistence and determination alone are omnipotent.
    Calvin Coolidge
     
  10. Dan Mills

    Dan Mills Guest

    Mainline surplus in the UK have 400W 28V switchers for 30 quid each and they
    are NICE! I used a few to build a battery charger for a boat.

    http://www.mainlinegroup.co.uk/mless/main.php?act=Card&id_element=37

    Should do what you need.

    Regards, Dan.
     
  11. Terry Given

    Terry Given Guest

    field-orientated control essentially decomposes an AC motor into an
    "equivalent" separately excited DC machine, thereby decoupling flux and
    torque. This allows AC motors to have dynamic response pretty much as good
    as DC motors.

    With a large enough field-oriented AC drive it is quite possible to demand
    such an abrupt reversal that load inertia can twist the shaft right off the
    AC motor!

    But forget trying to roll your own - you will spend more money on dead
    transistors than a whole working drive will cost. And thats before you make
    your software work properly.


    Cheers
    Terry
     
  12. Daniel Rudy

    Daniel Rudy Guest

    And somewhere around the time of 05/31/2004 19:16, the world stopped and
    listened as E. Lee Dickinson contributed the following to humanity:

    300W / 24V = 12.5 amps.

    You could get away with a 24v 20AMP transformer (Extra capacity for the
    surge current to start the motor, which could also be handled by large
    filter caps). You can pick up a 50PIV 25A stud mounted bridge rectifier
    at Radio Shack for under 5 bucks. You will need to wind your own
    inductor and find the caps needed in the filter, but as for the
    regulator, a PWM controller using power mosfets in a h-bridge will work
    just fine. You can even use digitial logic to drive the fets if you
    want as they approach the ideal transistor.

    I have seen fets that are able to handle 80+AMPS of current at various
    voltages, case limitation, not die limitation! They are immune to
    thermal runaway, and have good tolerance for surges, transiants, spikes,
    etc. which is good for a motor controller environment.
     
  13. Terry Given

    Terry Given Guest

    "Daniel Rudy"
    Semikron make some monster FETs - I just designed a pair of SK115MAA10 dual
    95A 100V FETs into a product (all 4 FETs in parallel :). They work well
    precisely because of their careful thermal engineering. ordinary packages
    are crap for real power.

    btw *ALL* fets suffer from thermal runaway (even though books tell you they
    dont :). This is because Rdson = f(temperature) - as T increases, Rdson
    increases (often its almost proportional). FETs usually operate with
    constant(ish) current - ie Rdson doesnt set Ifet. In which case as it heats
    up, Rdson increases, so Pfet increases, so Tj increases.....voila, positive
    feedback. If your overall junction to ambient thermal resistance is
    sufficiently high (Rja_crit), thermal runaway occurs and the FET fries -
    this is sometimes the case. OTOH if Rja is much below Rja_crit, the effect
    is negligible - this is hardly ever the case. If Rja is somewhere in-between
    these two extremes (as it usually is :) then Tj is going to be noticeably
    higher than you might expect, and moderate changes in Rja can produce quite
    large changes in Tj

    Cheers
    Terry
     
  14. Terry Given wrote...
    Their 0.7K/W spec isn't very impressive, even tho it is insulated.

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  15. Chris Warner

    Chris Warner Guest

    I have personally see Insulated Gate BiPolar Transistorst that would do 200+
    amps.... A simple microchip PIC microcontoller, would cost ya about 5-10
    bucks, you can find the circuit to roll your own programmer. But here if we
    have a transformer, and a recitifer bridge why not use a dimmer? Better
    yet, could hard limit a dimmer at 24v AC, or just a little bit more, then
    you can do everything you want with it.

    Just a thought, Chris.
     
  16. Tam/WB2TT

    Tam/WB2TT Guest

    "Daniel Rudy"
    You didn't tell him how much the transformer costs. The last time I looked,
    a new one was $149.

    Tam
     
  17. Terry Given

    Terry Given Guest

    Its not too bad though. The large surface are is helpful, as is the low
    internal inductance. Isolation is a huge issue - if required(it usually is),
    it inevitably increases the overall thermal resistance dramatically. Semitop
    3 packages have a thermal resistance of 0.45K/W. CF a large IGBT module
    having 0.22K/W.

    device mounting is a major issue at high power. no way would I design a high
    power converter with discrete switches!

    Cheers
    Terry
     
  18. Rich Grise

    Rich Grise Guest

    I saw a 24V 15A switcher at Fry's the other day for about 30 or 50 bucks.

    What's 24 X 15? lessee, 24 * 10 = 240, + ((1/2) * 24 * 10) = 252 watts,
    but I didn't really look at the next size up - I think they went up into
    the hundred-dollar range.

    Good Luck!
    Rich
     
  19. Dan Mills

    Dan Mills Guest


    Ya might want to check the math on that, I make it 360W....


    Regards, Dan.
     
  20. And the lack of an Rth(j-a) value makes it impossible to guess what they
    would do without a sink, but I guess they assume you will always use a heat
    sink on them.

    --Chuck
     
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