Variable speed control for an inductive load.

Discussion in 'The Projects Forum' started by Max Kreeger, Aug 22, 2015.

  1. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    Hey guys,

    I have a brushed DC motor which I have hooked up to a PWM speed controller. However I noticed in its specs sheet it said "For inductive loads such as DC motors it is recommended to place a 400V 3A diode in parallel with the load"

    Why is that?

    Thank you in advance,

    Max
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    to suppress back emf spikes, these will damage the transistor driving the motor...
     
  3. crutschow

    Expert

    Mar 14, 2008
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    As Dd said.
    An inductive load tries to keep the current moving through it when the controller shuts off (rather like the inertia of a mechanical load).
    If this current has no place to go, it will generate a large negative voltage spike at the controller output, which can damage the controller.
    The diode provides a low impedance path for this current, thus preventing the damage.
    It's common practice to add a diode across any switched inductive load, such as motors and relay coils, to prevent inductive spikes.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Does the motor connect between controller and the most negative voltage on the power supply, or between controller and the most positive voltage on the power supply?
     
  5. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    Thanks for the reply guys. Mike I wasn't too sure what you meant so I've included a picture of how its all wired 11913559_10206822633978426_1145600383_n.jpg
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is the motor reversible, or does it always turn one direction?

    If it always runs only one direction, put the cathode of the diode to the + (red) lead, and the anode to - (blk) lead.
     
  7. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    So should I still add the diode in like it said in its manual? If so, would adding it directly in parallel with the motor contacts be ok?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    As long as there is no reversing function, and as long as you wire it as I said in post #6
     
  9. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    No, the motor is reversible.

    EDIT: So what would happen if I added a diode in?
    By reversible I mean if I change the polarity.
     
    Last edited: Aug 23, 2015
  10. Glenn Holland

    Member

    Dec 26, 2014
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    If the motor is reversible (it looks like a plain vanilla permanent magnet motor), the polarity reversing contacts (an "H" Bridge configuration) would be at the input connections to the motor and the diode would be at the output of the PM controller.
     
  11. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    ..now I'm getting confused haha..so I DO add a diode in parallel with the motor? even if it is reversible.
     
  12. Glenn Holland

    Member

    Dec 26, 2014
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    The diode is in parallel with the inductive load (the motor) so it provides a path for the induced voltage (and current) when current to the load suddenly decreases or shuts off completely.

    If the diode is next to the PWM controller, it is in parallel with the motor and it will always have the same polarity regardless of the state of the motor reversing contacts.
     
    Max Kreeger likes this.
  13. DickCappels

    Moderator

    Aug 21, 2008
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    Yes, put it in according to the instructions that came with the speed control. If you put it in backwards it will look a lot like a short circuit to the speed control and if it is not protected against a short circuit, something could be damaged.
     
  14. Max Kreeger

    Thread Starter Member

    Oct 1, 2013
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    So just to confirm, this is correct? 11908245_10206824652828896_610284311_n.jpg
     
  15. DickCappels

    Moderator

    Aug 21, 2008
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    Yes. The connection of the diode appears to be correct.
     
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