Connect with us

motor speed controller

Discussion in 'Hobby Electronics' started by Paul, Nov 3, 2003.

Scroll to continue with content
  1. Paul

    Paul Guest

    I recently built a motor speed controller from a 20A kit from Oatley. It
    works except for a few minor bugs:

    1. the variation is only over the first half of the 5k pot, after that it's
    at max.

    2. the longer the wire between the controller and the motor, the slower the
    motor goes. When it's connected with 150mm of wires it works at almost the
    full RPM it's capable of, with about 600mm of wire (barely practical for the
    purpose I want it for as a water pump) it works at about half it's
    capability.

    Does anyone know why it runs like this?

    Paul
     
  2. "Paul" wrote

    I recently built a motor speed controller from a 20A kit from Oatley.
    It
    works except for a few minor bugs:

    1. the variation is only over the first half of the 5k pot, after that
    it's
    at max.

    2. the longer the wire between the controller and the motor, the slower
    the
    motor goes. When it's connected with 150mm of wires it works at almost
    the
    full RPM it's capable of, with about 600mm of wire (barely practical for
    the
    purpose I want it for as a water pump) it works at about half it's
    capability.

    Does anyone know why it runs like this?

    ***** To state the bleeding obvious,Oatley Electronics will know!

    Brian Goldsmith.
     
  3. Ken Taylor

    Ken Taylor Guest

    Are you sure you've got the right value components?
    What sort of cable are you using? Is it 240VAC wiring that you're playing
    with??
    Ken
     
  4. Paul

    Paul Guest

    I think so. It came with 2x 4.7k resistors but the circuit diagram and the
    PCB both said it needed 2x 4R7 (4.7 ohm?) resistors. I went and bought 2x
    4.7ohm resistors and used them.

    I've tried it with different types of cable all with the same results. I
    am amased at how much difference there is with the length of the cable. I
    measured some of the cable I was using and it was about 0.4 ohm.

    I just thought it was something to do with the pulse modulation and the
    value of the capacitor on the motor terminals I put on.
     
  5. Unbeliever

    Unbeliever Guest

    Yepp,
    your wire is too high a resistance. If a wire of 150mm gives the speed
    you want, but you need 600mm, then you'll have to increase the cross
    sectional area by a factor of 4. This can be done by using a thicker cable
    (try some car jumper leads to get the maximum practical effect here), or by
    using 4 of the same diameter wire in parallel.

    You have a huge reistance (0.4 ohms is mentioned somewhere) that indicates
    that you are either using very thin (less than 30 gauge) wire, something
    other than copper, or (most likely) a crappy multimeter to measure this.
    Better off measuring the current throug and voltage across the wire and
    using Ohm's law.

    HTH,
    Alf

    www.micromagic.net.au
     
  6. Paul

    Paul Guest

    All my text books are packed up at the moment, so I cant remember what
    aluminium resistance is... I think it's aluminium, it's about 2.5mm
    stranded stuff I had lying around. I will try with thicker stuff.

    Yeah you are right it is a cheap n nasty digital multimeter, wire resistance
    is the only explanation unless it's something to do with the capacitor that
    I put across the terminals of the motor.
     
  7. Phil Allison

    Phil Allison Guest

    "Paul" <



    ** What was it - 10,000 uF??



    ......... Phil
     
  8. Phil Allison

    Phil Allison Guest

    "Paul" <



    ** What was it - 10,000 uF??



    ......... Phil
     
  9. Paul

    Paul Guest

    I fixed it. I forgot a capacitor that goes to one of the terminals to the
    chip. I knew it was a bit strange... The 2 mosfets were getting really hot
    which I thought was strange since they are rated at 10A each, and then I
    realised the 2 transistors driving the mosfets were even hotter. After I
    replaced the capacitor it works at full speed no matter how long the wires
    are, and the mosfets dont even get warm which you would assume since I
    measured the little water pump motor was drawing 550mA... no where near 20A
    that it is rated to.
     
  10. KLR

    KLR Guest

    Usually its "silly mistakes" like this that cause the most frustration
    later ;)
     
  11. Paul

    Paul Guest

    Yep and I've done it so many times I knew it was a silly mistake :)
    Probably what threw me was that I was under the impression that Oatley has
    put in some wrong value components and there was an extra capacitor. But in
    reality I needed the capitor and the 4.7k resistors probably would have
    still worked, since they are used to drop the voltage down going to the
    transistors that drive the mosfets (even after a years study I still have no
    idea what sort of transistors they would be and what the centre terminal is
    called... back to school one day when I can afford it.


    Thanks for your suggestions, but it definately must have been a capacitance
    issue... that much wire (600mm) was dropping it about 6v!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-