DC motor voltage

Discussion in 'The Projects Forum' started by ink, Jan 16, 2014.

  1. ink

    Thread Starter New Member

    Nov 28, 2013
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    I had DC motor running 12 volts with pwm for speed control and relays for changing directions, it was working fine until I changed the motor.

    With the new motor I am reading between 0.8 to 2 volt on my meter as I increase speed, with the low volt my relay is not activating.

    My power supply is 12v 2 amp.
    This is the motor : http://m.ebay.com/itm/280657142140?nav=WATCHING&sbk=1
     
    Last edited by a moderator: Jan 17, 2014
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Does the supply collapse when motor is ran?
    Are you testing off or on load?
    Maybe post your schematic.
    Max.
     
  3. ink

    Thread Starter New Member

    Nov 28, 2013
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    Not such what you mean by supply collapse but The power supply looks OK!

    I test on testing off or on load?

    [​IMG]
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    D3 and D4 polarities are such that they short the relay coils.
    Why are you applying the PWM signal to the relay coils?
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    +1

    DC Motors can draw many times their rated running current at start up.
    Did your previous motor have a lower running current specification?

    As Alec_t pointed out, you should connect the left control relay circuit directly to 12VDC. (Unless maybe you want it to drop out at low speed).
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is the 0.8 - 2v across either the motor or supply, IOW does the supply collapse when the motor is on?
    Did you test the motor directly across the supply?
    Max.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Looks to be something amiss with the schematic, I believe the intention would be to take the motor to the M- not the PS common?
    This way the motor would be controlled by the IRF530.
    The way it is drawn the motor would be full on at all times.
    Max.
     
  8. ink

    Thread Starter New Member

    Nov 28, 2013
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    yes it did
     
  9. ink

    Thread Starter New Member

    Nov 28, 2013
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    I made a mistake with the schematic the diode are installed correctly.

    I need the motor to change direction at a certain speed. I am not such this is the best way of doing it. i am open to ideas

    P.S I am a newbie

     
  10. ink

    Thread Starter New Member

    Nov 28, 2013
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    I tested it at the motor. I will try test it at the supply.
    I also tested is at the supply is read 12v.

     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Very bad way of reversing motor when it is under control of a semi conductor device, without bring it to 0rpm first.
    This is know as 'plugging the motor' and causes a very high current until the motor comes up to speed in the opposite direction.
    The way to do it using the method of your circuit, would be a have some kind of detector indicating the motor had actually stopped before applying the reverse power.
    In industry there are special switches called zero speed switches, but it should be possible to mock something up.
    Max.
     
    Last edited: Jan 18, 2014
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  12. inwo

    Well-Known Member

    Nov 7, 2013
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    This is one way if I understand cycle.
    In out stop!

    I'll edit or delete if I have second thoughts.:)

    Start button is held until "home" closes. Latching "F"

    Cycle runs until "in limit" closes. Latching "R".

    Both R and F in until TD off "R" drops out. <1sec for braking.

    When "R" is in and "F" is out, motor runs until "home" opens, stops motor, and resets cycle.

    Both R and F is a valid condition with this type of spdt reversing relays.
    If "F" relay current is small, capacitor on coil may slow it enough.
     
  13. ink

    Thread Starter New Member

    Nov 28, 2013
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    Will one DPDT relay work.

     
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
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    There's an advantage to using 2 SPDT relays as posted by inwo.
    The motor will stop more quickly because the motor is shorted when stopped.
    A small value resistor of sufficient wattage can be used to adjust braking time.

    Google dynamic braking.
     
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  15. inwo

    Well-Known Member

    Nov 7, 2013
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    That's what I'm thinking.

    If one is a dpdt it will do the motor control and the latching.

    The only way it will brake before reversing is if "F" is slow to release.

    Home limit switch looks confusing, as it's a nc. Yet it will be open when in "home" position, waiting to be restarted.

    I'm not sure if this is what OP needs. I couldn't make sense of diagram.

    ps.
    "A small value resistor of sufficient wattage can be used to adjust braking time."
    Need a more complex circuit for that, I believe.
     
    Last edited: Jan 18, 2014
  16. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    It could but instant reverse, and no TD or breaking, is not a good idea. IMO
     
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  17. ink

    Thread Starter New Member

    Nov 28, 2013
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    Do i need to use a diode and are we using the coil in the relay.
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
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    Just a capacitor across the coil might work.
    Or:
    Here thanks to MikeMl.......................
    Is a simple TD off circuit added to the relay coil.
     
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