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Bi-directioning dc motor and back emf

  1. Feb 25, 2010 #1
    Hi Experts,o:)

    I am trying to reverse the direction of a spinning dc motor (around 30watts)
    but not sure how this should be done.

    I have a logic for the switching of the power polarity to the motor but am worried if back emf would be an issue when the motor is spinning and the polarity suddenly reverses. When this happens, the motor would be still spinning in original direction creating back emf and i wonder would the reverse in polarity cause any harm to the motor or controller?

    Must the motor be braked or slowed down to near motionless before it can be switched to rotate in opposing direction?

    So, I am trying to find a simple way to control the motor to spin in alternating directions.

  2. jcsd
  3. Feb 25, 2010 #2
    You should limit the current to the motor to a value that won't damage either the controller or the motor.

    Given current limiting, you don't need to stop the motor before you switch the polarity. In fact, the reversed current will help slow the motor.
  4. Feb 25, 2010 #3
    Thank you very much skeptic2. I feel assured to proceed with the testing now.

    Have a nice day!

  5. Feb 26, 2010 #4


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    You should note too, that some motors will run in the same direction even if you reverse the polarity.

    Series connected motors which have the field coils in series with the armature will rotate in the same direction regardless of polarity. That is why they are able to run on AC as well as DC.

    If the motor has external connections to the field coils, you may be able to reverse the connection to these but not to the armature and get a reversal of rotating direction.

    Motors with permanent magnet fields will reverse direction if you reverse the polarity.
    Last edited: Feb 27, 2010
  6. Mar 2, 2010 #5
    Dear vk6kro,

    I ever tested the motor directly connected to a dc power source and it can rotate in 2 directions with polarity change. This I am sure.

    yes, i forgot to mention my 12v dc motor (rated at 15v), size of a pda about half a kilos in weight, is probably not a brushless but brushed motor as i can hear some light nose probably from the commutators?.

    I enhanced the circuit with diodes connected to the switching points that the relays switch to when the relay coils are powered 'ON' and 'OFF'. Initially, when i troublehooted the circuit, i found that when the circuit is powered 'off', the way i connected the , probably what is known as DPDT type of relay, relays without using any diode caused the switching points of both relays to shortcuit to one another as all the + and - are connected to the same source. With the diodes positioned in the right direction and connected to each of the 2 switching points of each of the 2 relays, the shortcircuiting problem is resolved.

    However, the motor still turns in the same direction. But at times it seem to work bi-directionally but for 1 cycle only. Usually, this happens in the 1st cycle on powering up.

    I can hear the relays switching but almost immediately they concurrently got switched back. I used a multimeter to test if switching was done by hearing for the shortcircuit buzz.

    another observation : I found that the motor terminals are shortcircuited when it is standalone and not connected to power. I used the multimeter to test this. Is this an indication that this is a brushed motor? does it mean a brushelss motor will not shortcircuit?

    Any idea why momentarily it works bi-directionally ?

    Thanks for your advice and reading my problem.

    Best regards
    Last edited: Mar 2, 2010
  7. Mar 2, 2010 #6


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    It sounds like your problem is really in the reversing switch action of your relay.

    http://dl.dropbox.com/u/4222062/Reversing%20switch%203.PNG [Broken]

    In this diagram, the switches at the sides of the motor would be like the contacts of your relay. They are operated by a coil which is not shown.

    There is no need for diodes in this circuit, although one might be needed across the relay coil if you are switching with a transistor.

    The + and - signs at the top and bottom refer to the power supply connections.

    Can you see that this connection reverses the polarity reaching the motor?
    Last edited by a moderator: May 4, 2017
  8. Mar 7, 2010 #7
    Dear vk6kro,

    I would like to appologise to you and the rest for giving the wrong information.
    I actually have been using single pole double throw coil relays, spdt instead of dpdt.
    I just began using relays recently. I have since learnt about the various types available.

    I found a circuit on the net which was essentially the same as what i did except that i connected the transistor to the coils using the emitter and i used 1 transistor only and added diodes to each of the 2 switch points of the relays.

    Here is the diagram found on the net :

    I used a Diode parallel to the coils but the switching is still a problem. I observed that when the signal from 555 is low, like 0 volt, the coil collapses and the switch returns to normally closed position. But this lasts momentarily only , like 0.01 seconds. The coil seems to be energised again even though the 555 output is still low. This patterns of switching goes on and on. I measured the voltage across the coil and when its not energised, the voltage starts to drop but before it has time to drop a few volts lower, its re-energised again thus the motor continues to turn in the same direction.

    Last edited: Mar 7, 2010
  9. Mar 7, 2010 #8


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    Could you try disconnecting the 555 and just applying power to the relay or not applying it?

    If the relay then works normally, maybe there is a stray pulse getting back to the 555 via the power line and this is causing it to retrigger.

    To test for this, you could run the 555 and the relay from different power sources (but a common ground).
  10. Mar 9, 2010 #9
    Could you post your complete circuit?
  11. Mar 9, 2010 #10
    Please allow me to get back to you on this as I am not near the breadboard at the moment.

  12. Mar 9, 2010 #11
  13. Mar 9, 2010 #12
    I wonder if when the transistor turns off and current continues to flow in the relay coil, if it doesn't cause the emitter side of the relay coils to go so negative that it turns on the transistor even though the base is at zero volts.

    Try using the collector to energize the relays instead of the emitter.

    It's still a good idea to isolate the 555 trigger circuit as much as possible from the power line.
  14. Mar 9, 2010 #13


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    I have modified your drawing to show some possible improvements.

    The diodes in series with the motor were not doing anything so I removed them along with one of the diodes across the relay coils. No point in having diodes in parallel.

    The transistor will switch the relay a lot better in common emitter mode, as shown.

    There is some filtering in the power line to the 555. This may need some adjusting. It is there to avoid pulses from the relay coils getting to the 555.

    http://dl.dropbox.com/u/4222062/motor%20drive.JPG [Broken]
    Last edited by a moderator: May 4, 2017
  15. Mar 9, 2010 #14
    Good vk6kro. Yes, exactly like that.
  16. Mar 10, 2010 #15
    Hello vk6kro,

    I just have tried your suggestion. Yes, the relays performed like it should and the motor turns bi-directionally when the coils are energised and de-energised manually.

    I noted that when i connect one end of the coil contact to +12v, the relay will rattle , switching on and off quickly multiple times due to poor contact, unless I make the connection firmly.

    I see that you have made much improvements to the circuit (thanks for saving me so many diodes :) ) . Really appreciate your good advice. Thanks! :) I will give test it out and revert to you asap.

    Have a great day!

    Best regards,
    Last edited: Mar 10, 2010
  17. Mar 13, 2010 #16
    Hi vk6kro, o:)

    I have reconnected the circuit based on the enhanced diagram above. I thought it would be useful to share with you footage of how the relays are switching.

    Unfortunately, i was not able to make the motor spin bi-directionally still.

    Here are 2 videos i have on the switching behavior of the relays :

    The dc motor still spins in 1 direction and either momentarily tries to spin the other direction or stalls for a few seconds before continuing to spin in the same direction.

    The video shows that the relay coils have a tendency to be energised , closing the circuit very quickly.

    Thank you for reading.

  18. Mar 13, 2010 #17


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    I couldn't see much in the video.

    Your earlier comment (now deleted)
    I noted that when i connect one end of the coil contact to +12v, the relay will rattle , switching on and off quickly multiple times due to poor contact, unless I make the connection firmly.

    gives me the impression that these relays are taking a LOT of current to make them work. Maybe the transistor is not able to pull the relay in fully?

    Could you find a single DPDT relay that doesn't need much coil current?
  19. Mar 15, 2010 #18
    dear vk6kro and skeptic2, o:)

    good news! the circuit is working!

    The motor now runs bi-directionally.

    Thanks for your circuit, it works! I just changed the voltage supply for the 555 to a 5v and the motor and relays are powered by 12v.

    Your advice are excellent!


    best regards
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