DC motor inrush current limiter

Discussion in 'General Electronics Chat' started by Manfred Von Steinborn, Jul 23, 2012.

  1. Manfred Von Steinborn

    Thread Starter Active Member

    Feb 3, 2009
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    I'm looking for a schematics of a triac DC motor inrush current limiter.
    Motor Steady state current=5A; inrush current=22A, 12-24V
    Thank you
     
    Last edited: Jul 24, 2012
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    That's a bit of an oxymoron I believe. A triac, when used with DC, will turn on and stay on.
     
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  3. #12

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    Nov 30, 2010
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    That's one reason I didn't answer. Conflicting or not enough information.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    A triac feeding a bridge rectifier can easily run a DC motor. In order to limit the in-rush current, a sense element is needed (usually a low ohm resistor) and the voltage developed is used to limit the drive to the triac. Check out http://www.kbelectronics.com/manuals/kbcc_255_manual.pdf
    for the manual of a KB electronics DC drive (AC input) for an idea of how they do it. R21 near the lower right corner of the schematic is the shunt resistor used to measure armature current. The manual gives a procedure on how to set current limit. Nice little drives and very inexpensive (I don't work for them but have used the product)
     
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  5. Manfred Von Steinborn

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    Feb 3, 2009
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    Last edited: Jul 24, 2012
  6. #12

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    Wow. This is not beginner level!
     
  7. Manfred Von Steinborn

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    Feb 3, 2009
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    Yes, it acts as a (delayed) shunt switch across the limiting resistor.
     
  8. BillB3857

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    What is your voltage source? Batteries or rectifier circuit?
     
  9. Manfred Von Steinborn

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    Feb 3, 2009
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    Source is batteries.
     
  10. BillB3857

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    It sounds like your best bet for speed control would be PWM. The PWM generator would be the controlling element to regulate in-rush current via the current feedback system.
     
  11. Manfred Von Steinborn

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    Feb 3, 2009
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    Hi Bill,
    That is an excellent idea! Could you suggest any place I could get a diagram?
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I have a dumb question about this question. Why limit starting current? Starting current is always higher than running current. The motor will only draw what is needed, correct? If the motor is locked up for some reason and draws excessive current, wouldn't a fuse be a better thing to use?

    A fuse would blow and stop current flow saving the motor windings. A current limiter would limit the current to that level and if not detected smoke the windings. Correct?
     
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  13. BillB3857

    Senior Member

    Feb 28, 2009
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    Sorry! I really don't have a source for a complete PWM circuit with current limit fold back. Others may be able to help. I'm sure somebody makes a commercial unit but just don't know off the top of my head.
     
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