Why won't DPDT switch reverse a 2 wire reversible DC motor?

Discussion in 'The Projects Forum' started by DaleG, Feb 11, 2015.

  1. DaleG

    Thread Starter New Member

    Feb 11, 2015
    6
    0
    Have a 24V 5A 2 wire DC motor which, when hooked up to 24v 20A Sola power supply runs great forward and when I reverse wires to power supply terminals runs in reverse just as well.
    Placed a 12v 30A DC DPDT maintained switch with power supply to the middle terminals, cross wired the corner terminals, and connected 2 end terminals to motor.
    When switch is toggled to the left, motor pauses briefly then runs well. Then after toggling to maintained off, push toggle to the right and motor has a 3-4 second pause before it reverses direction or will stall and not turn at all.
    Have a 9-60V 40A PWM ordered, wanting to install a power drive on a cross table. Thought this would be easy straight forward. Any help greatly appreciated.
     
  2. tom_s

    Member

    Jun 27, 2014
    285
    333
    a drawing/pic of how you have wired would be nice.

    this DPDT switch has centre off? (3 position) left / off / right?

    most switches i've found are make-before-break - connect both sets of terminals before full switch deflection. not good in this scenario, you'de be needing a 'centre off' switch.
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,001
    745
    Your switch is rated at 12v, yet you're using 24v, i would use a DPDT relay put the motor to the wipers, and the supply to the crossed terminals.
     
    Last edited: Feb 11, 2015
  4. Reloadron

    Active Member

    Jan 15, 2015
    964
    232
    Either one of these two switch configurations should work:
    Motor reverse A.png Motor Reverse Switch.png

    Since the motor manually runs fine in either direction the power supply is fine. While the switch is 12 volt rated @30Amp I don't see the 24 volts as a problem. The drawings are old and one reflects a 5V motor but matters not. It looks like you have a pause and are not changing motor direction without a pause as some SMPS power supplies really hate that. You do have a pause correct? Since you can manually run the motor fine in either direction less the switch and when the switch is added you have problems things seem to point to the switch and the stall maybe something the power supply dislikes. Can you measure the motor voltage when this happens as well as the supply, upstream from the switch? Measure on ether side of the switch in other words?

    Ron
     
    Last edited: May 18, 2015
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    You might not.. but any safety/certification agency (UL,etc....) would :)
    As Dodgydave suggested a relay (with appropriate rating) would be a better solution.
     
  6. Reloadron

    Active Member

    Jan 15, 2015
    964
    232
    I agree on the relay but my understanding is this is just a temporary test scheme as there is a "9-60V 40A PWM" controller on order which I assume to be an H Bridge. That said and not knowing the switch contact ratings at 24 VDC verse 12 VDC I based what I said on most (and granted not all) DC switch contact ratings are bases on a range of for example 6 to 36 VDC, for example these common toggle switches. The original poster does mention a 24 Volt 5 Amp load which leaves a lot of room to derate the current going from 12 to 24 volts when at 12 volts the switch is rated for 30 Amps. So while I do agree a properly rated relay or H-Bridge is the proper solution for some general quick testing I figured the switch the original poster mentioned would do and should work. Without seeing the data sheet for the switch? No I would never suggest it as a permanent method to switch the motor current. Again, the original poster is waiting on the solution he plans to implement.

    As to switches in general and AC verse DC switch contact ratings I like this read on the subject:
     
    absf, SgtWookie, ErnieM and 1 other person like this.
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Recheck the left drawing.

    ak
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,607
    POTD (Post of the Day)
     
    absf likes this.
  9. Reloadron

    Active Member

    Jan 15, 2015
    964
    232
    I don't believe I did that. :) Looked just fine at the time. Nice catching that and pointing it out, thanks a bunch. That wrong drawing has been sitting in my files and I never noticed it.

    Thanks Again
    Ron
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A single DPDT relay won't work for our OPs application, as the motor would run all the time either forward or reverse. They could use two SPDT relays, or an H-bridge type relay that is essentially two SPDT relays in one package. Don't assume that the PWM supply contains an H-bridge; I'm betting it does not. At this point, only the OP knows for certain.
     
  11. Reloadron

    Active Member

    Jan 15, 2015
    964
    232
    Agree, maybe they will come back and share? :)

    Nice Avatar there Marine.

    Ron
     
  12. DaleG

    Thread Starter New Member

    Feb 11, 2015
    6
    0
    Thanks for taking time to help guys. My experience is limited and while trying self teach what appears to be straight forward obviously has underlying principles I have yet to understand.
    My DPDT switch is 6 terminal (pin), 3 position toggle, on-off-on maintained (not momentary). Terminals are cross wired (1-6) and (3-4). Power supplied to middle terminals (2 , 5) with motor connected to end terminals (1 , 4) as per all of the diagrams I've seen on how to wire these switches. When switch is connected to power supply, terminals to motor (1 , 4) shows +24V with toggle to the left and -24V with toggle to the right. Appears to be reversing polarity as it should. Have not yet attempted to get readings when motor stalls, nervous when it happens because power supply starts making these clicking noises so immediately cut power. If anyone is familiar with power supply, Sola HD SDN 20-24-100C, it has LED diagnostic lights that according to the manual shows 'No DC' because the combination of a green and red light both light up when motor is hesitating or stalls completely. The clicking also coincides with LED,s turning off and on.

    Attempted to buy 24VDC DPDT but only ones available have momentary on-off-on but can't find one with maintained 3 position toggle. That's why I bought the 12 VDC with heighest Amp rating (30A) I could find hoping it would work.
    Have no experience with relays or H-bridges but will certainly learn about. Unsure of how to use relay to manually change rotation.

    My plan was to connect Motor to DPDT to PWM to Power Supply. Unsure of specs for PWM, still waiting for it to arrive. Manufacturer says it is designed to control speed of DC motor and bought what appeared to have ample capacity for my motor. Again, what appears to be simple on paper.....
     
  13. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Important things:
    1) Be sure the motor is at a complete stop before reversing the power.
    2) Make sure you did not melt your switch before you learned about important thing number one.
     
  14. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    [​IMG]
    [​IMG]
    Although the two circuits have an identical function, the circuit on the top can fail catastrophically under certain conditions.
    For high current/voltage conditions with an inductive load, the arcing when the contacts open can cause an arc flashover (with lots of sparks) to the opposite terminal, which is at the opposite voltage, causing a short across the power.
    Thus for any high power application, the bottom circuit is preferred.
     
    Last edited: Feb 12, 2015
  15. Reloadron

    Active Member

    Jan 15, 2015
    964
    232
    Absolutely! I tossed both cartoons out there merely as an example. When we get into DC motors that draw high current there are a number of considerations and rules to follow. What looks good on paper will not necessarily be the best solution. Also, not seen in these drawings is any over current protection, like a properly rated fuse.

    Ron
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
    2,379
    On a DC motor system that does not incorporate some kind of accelerated start, the maximum current can be obtained by measuring the armature resistance, don't bother reading the armature resistance with a Ohm meter, it is not as accurate as a current reading, done by locking the shaft and applying a small DC voltage while measuring the current, confirm the DC voltage and do the calc, move the armature to a couple of spots and take the lower reading.
    Then use the reading together with the system voltage to calculate the max current.
    Max.
     
  17. DaleG

    Thread Starter New Member

    Feb 11, 2015
    6
    0
    Thanks again for your all of your replies. With my limited experience I guess my most pressing question is why motor will run fine ( forward and reverse) when connected directly to power supply but has problems when I use DPDT switch.
    As per my last post, with DPDT connected to power supply, meter shows +24VDC when toggle flipped to right and -24VDC when toggled to the left.
     
  18. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    And, where did you measure? On the wires that go into the motor? If you can get the voltage there, the motor is bad. If not, you simply made a mistake.
     
  19. DaleG

    Thread Starter New Member

    Feb 11, 2015
    6
    0
    With the DPDT switch wired as described in yesterday's post, connected DC power supply +\- to the middle terminals #2 and #5, crossed wired terminals (#1 to #6) and (#3 to #5) placed my red meter probe on terminal #1 and black probe on terminal #4. Without motor connected, pushed toggle from the middle (off ) position to the left and measured +24 VDC. Keeping the meter probes in contact with the same 2 terminals, pushed toggle back to off position and then to the right position meter measured -24 VDC. Does that show switch is reversing polarity thru those 2 terminals which are the 2 terminal which I attach to the motor ?
     
  20. Denesius

    Member

    Feb 5, 2014
    89
    14
    I remember I had an old DC motor that used stator windings instead of permanent magnets. If it was stopped, it would spin in one direction or the other based on the polarity of the power applied. But if it was turning in one direction faster than a few RPM, reversing the polarity didn't cause it to reverse, it just accelerated in whatever direction it was still rotating.
    I wonder if your motor is the same thing. When you switch, are you allowing it to come to a stop? Try that & see if it works. If it does, your motor cannot be reversed while still spinning...
     
Loading...