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Need advice on what motor flyback diode to use.

Discussion in 'Electronic Basics' started by [email protected], Apr 17, 2005.

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  1. Guest

    I will soon wire up a 36 Volt 80 Amp permanent magnet DC motor to be
    run at 48 VDC. It will be used in a homebuilt trike.

    I am looking for a flyback diode that will take the full current during
    slowdown when the power is turned off by a contactor (no PWM).

    I would appreciate knowing whether to use a single diode, series
    diodes, a Schottky, a series resistor, etc.

    What is the most cost effective solution?

  2. If you include the time cost of connecting big wires to the diode and
    heat sinking it, you may be ahead by selecting something easy to
    mount. A DO-5 stud package might be good, but I think a Half-Pak is
    better. A 120 amp 100 volt Schottky costs about $17 plus shipping
    from Digikey:
  3. Guest

    Thank you John,

    I appreciate your response.

  4. Rich Grise

    Rich Grise Guest

    Let us know how long this 36 volt motor lasts at 48 volts with
    bang-bang control.

    Good Luck!
  5. Chris Dugan

    Chris Dugan Guest

    Let us know how long this 36 volt motor lasts at 48 volts with
    Dunno about you guy's but I'd feel safer riding that thing with some kind of
    intermediate speed to lessen the strain on the motor/battery/transmission,
    or something is likeley to go bang from the startup loads.

  6. Guest

    from an 1979 article

    With the price of gasoline already out of sight, just about everyone is
    scrambling for a way to squeeze the last possible drop of energy from
    each precious gallon. However, David Arthurs-of Springdale, Arkan
    sas-probably couldn't care less ... because he has designed and built a
    car that can travel 75 miles or more on just four quarts of the
    expensive liquid!
    What's his secret? Well, Dave's Opel CT is a hybrid electric vehicle.
    That is, the car is driven by an electric motor . . . but that
    powerplant's "juice" is generated with the help of an ordinary,
    fuel-stingy lawn-mower engine! Now the fact that the system works isn't
    really surprising. What's amazing is that the crossbreed hookup
    performs so well! According to David, the Opel has not only a virtually
    unlimited range (when driven prudently), but also a top speed of 90
    miles per hour . . . and emits a minimum of pollutants as it tools
    along the highway. Better yet, the car can-if need be-run on its
    batteries alone for short in-town hops . . . and will never be
    "stranded" as long as there's fuel in the "on board" generator!
    Mr. Arthurs is the first to admit that there's nothing "new" to the
    system he's developed . . . in fact, all the technology incorporated
    into his design has been available for about 35 years, just waiting for
    someone to put two and two together and make the whole thing work. "I
    began researching the idea for a hybrid electric auto about a year ago.
    There wasn't much information to be found on the subject, so I designed
    a system from scratch. In about a month's evening-and-weekend time, I
    had the car finished and running."
    Surprisingly enough, the project didn't cost a fortune, either. Because
    the vehicle's components are either standard "off the shelf" hardware
    or available as reasonably priced military surplus, the conversion to
    "hybrid drive" only set Dave back about $1,500. By the same token, any
    necessary replacement parts are easily obtainable . . . and a good deal
    of the equipment can be "scrounged" rather than purchased new.
    In essence, David has utilized a small gas lawn-mower engine to drive a
    generator, which-in turn-supplies the vehicle's drive motor with
    electricity. To do so, he first removed the Opel's original power-plant
    and installed a 400-amp DC motor/ generator (actually a jet engine's
    starting motor) in its place. (Since there's no need for a clutch in
    Dave's setup, the stock unit was pulled out and the main shaft of the
    drive motor was fastened directly to the input shaft of the car's
    transmission.) Then, to provide a consistent source of power for this
    motor (and to supply an energy storage bank), the engineer installed
    four 12-volt, heavy-duty automobile batteries-in series-which are "fed"
    by a 100-amp generator that's run off a 5-horsepower lawn-mower engine.
    Of course, other components (such as relays, charging diodes,
    rectifiers, and an additional motor speed regulation circuit) are
    necessary to keep input and output power within optimum limits-and to
    allow full control of the vehicle at all speeds-but these are standard
    electrical parts which have been available for years.
    The engine-driven generator can handle the demand from the main motor
    up to speeds of about 50 miles per hour. The "stored" energy in the
    batteries comes into play at higher velocities, giving extra kick for
    passing and climbing hills. To guarantee that the charging system isn't
    overworked . . . Dave has rigged up a regenerative braking circuit
    which- in effect-turns the drive motor into a generator, to feed the
    batteries when the vehicle is decelerating. (This not only takes
    advantage of normally wasted energy, but also saves wear and tear on
    the car's conventional braking system.) Since the Opel's "stock"
    cooling apparatus has been removed, two small thermostatically
    controlled electric fans provide ventilation to the motor and generator
    as required ... while the gasoline engine is, of course, air-cooled by
    Any project fresh off the drawing board has its share of problems, and
    the Opel hybrid was no exception. When David pressed the accelerator
    for the first time, he got a 300-amp surge which melted his relays. So
    he searched his graduate texts for the answer ... and finally found it
    in-of all places-an old high school physics book: A pulser was
    necessary to "chop" the current flow and prevent a heavy initial draw
    to the drive motor.
    As Dave explains it, "The motor will always have full voltage and full
    current, but the pulser makes it 'think' the voltage and amperage are
    cut down to about 1/4 of what's actually available. With this
    gadget-which is simply a combination of a reworked car generator and an
    old fan motor-I can keep the draw within limits and effectively control
    the car's acceleration . . . without sacrificing the maximum current or
    voltage that's necessary for high-speed driving. I could have achieved
    the same results with a commercially available FCR control ... but one
    of those units would have cut my power slightly, and cost in the
    neighborhood of $800! I can build my own device for about $25, and I
    can fix it myself if it breaks!"

  7. Lord Garth

    Lord Garth Guest

    Well, motors for A/C, power steering and hydraulic brakes come to mind
    rather rapidly.

    5 horsepower is 3730 watts at the gasoline engine, 48 volts x100 Amps= 4800
    Watts demanded by
    the generator. Assuming the 3730 Watts are delivered to the generator,
    3730W/48V= 77.71 Amps
    at best from the generator. Not all can go toward propulsion.

    How efficient is a jet engine starter motor? Aren't these usually mounted
    on a truck? What sort of
    generator runs the starter when used in its intended role? Such a motor
    isn't designed for continuous

    This sound like a fanciful tale.
  8. Guest

    I suggest that you read the article. It is about 25 years old. I
    believe that over 15,000 sets of plans have been sold. There was also
    a follow up article a few years later.

    My only point in this forum is that a mechanical PWM can soft start a
    DC motor.

    If you think about a brushed DC motor, there is no reason to doubt that
    a brushed 'controller' could handle any current the motor required. It
    might even be possible to vary the duty cycle for different speed.

  9. Lord Garth

    Lord Garth Guest

    I don't doubt that plans have been sold, more power to the guy for his
    marketing prose. I just think the figures need to be revisited. Today,
    a solid state PWM would be the speed control choice.

    What is needed is an up-to-date rebuild and perhaps a chart plotting the
    acceleration. For highway, we need 60 in town and 70 on the intercity
    runs & 75 on a few tollways with headroom to accelerate, admittedly a
    tall order. Range should not be less than 350 miles though more is nice.
  10. Guest

    "Today, a solid state PWM would be the speed control choice."

    I totally agree.

    Here's a thought provoking problem:

    You are in a "Junkyard Wars" episode and need to control a 500 Amp 48
    VDC motor. You are given a $30 and free rein of the junkyard. What
    would you do to solve this problem?

  11. Lord Garth

    Lord Garth Guest

    Umm, build a wafer fab?

    I guess that's why I find shows like this fascinating!
  12. Rich Grise

    Rich Grise Guest

    A lot will depend on what "kind" of junk. Military surplus avionics,
    or scrap cars? Or something in between, or broader-spectrum? Lawn
    furniture? The kitchen sink?

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