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Reversing a DC brushed motor

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  • Reversing a DC brushed motor

    Hello,

    I have 2 axis that use dc motors for movement (24v at 5amps) controlled by a Productivity2000. I purchased a GSD1-48-10C assuming that it had reverse or it just needed to be enabled(field mountable reverse didnt mean what i thought). The instructions showed adding a 3pdt switch which makes sense but i cant seem to find a din mountable one which is making me think i am doing something wrong.

    Is there a more elegant way to get reversing on a motor of this size? The ability to control the speed and direction is important.

    Thank you,
    Grant


  • #2
    I don't see the 3PDT relay control method you mentioned so this may not be more elegant than what you have.

    If the need is 5 amps, the DPDT relay 750R-2C-24D at 10 Amps should work to reverse the armature polarity to the motor. The relay socket 750-2C-SKT is din mountable.

    You would wire the motor armature leads to the two contact commons of the relay.
    DC Controller Armature Positive to Contact 1 NO and Contact 2 NC,
    DC Controller Armature Negative to Contact 1 NC and Contact 2 NO.

    Switching the relay would reverse the motor armature polarity.

    Before switching the relay state, the dc controller must be inhibited by the P2000 and the DC Controller given time to reduce the armature voltage to zero.
    The inhibit switch is P2-5 and P2-4.

    If the relay switches with armature voltage present, there will be a spark at the contacts and they will not last. Damage may also be done to electrical or mechanical components.

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    • #3
      Thats great, thank you.

      Thanks as well for the tip about reducing armature voltage to 0.

      it looks like i just skipped past the part where i had to get a relay socket as well. This looks elgant to me.

      I apreciate your help.

      -Grant

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      • #4
        Since it's a brush motor, reversing the field polarity is a much lower power way.

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        • #5
          Switching the field would require less power and a smaller relay.
          This also would switch under a small load every time as the field is always on.
          The larger relay switching under no load should last longer over time.

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          • #6
            Now days permanent magnet motors are quite common and one can't reverse the field on them, I don't see where the OP said he has a wound field motor, but even if he does one should check that really bad things don't happen if the relay fails and one looses the field while power is applied to the armature.

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            • #7
              As others have mentioned, using a relay to reverse the output voltage will work fine. Just make sure the drive output is at 0 voltage. Instead of using the inhibit input, use the other mentioned described in the manual (cutting one of the poteniometer legs). That will work as well and is more reliable. The inhibit is always backwards for some reason on drives of this type (it should be OPEN to inhibit). You shouldn't have to close a relay contact to make to something stop.

              P.S Make sure you fuse the drive input and armature output.
              Last edited by Adisharr; 05-21-2018, 03:33 PM. Reason: Changed 'relay' to 'close' and other grammar

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              • #8
                Originally posted by Tinker View Post
                Now days permanent magnet motors are quite common and one can't reverse the field on them, I don't see where the OP said he has a wound field motor, but even if he does one should check that really bad things don't happen if the relay fails and one looses the field while power is applied to the armature.
                Oops, senior moment! Reading in a hurry and interpreted "brushed" as "wound field" for some inexplicable reason.

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                • #9
                  Originally posted by Adisharr View Post
                  ...The inhibit is always backwards for some reason on drives of this type (it should be OPEN to inhibit). You shouldn't have to relay a relay contact to make to stop something...
                  That's because it's an Inhibit input, not an Enable. This function was introduced in the late 1970's... it's not about to change anytime soon

                  Breaking the high side of the Pot has always been the standard Enable. Realize, though, that the Inhibit defeats the decel ramp coasting the motor to a stop, but breaking the Pot does not. The drive will ramp to a stop.

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                  • #10
                    Using a Pot, these options each have their own advantages.

                    If an analog output from the P2000 is used, the high leg disable is not available.
                    The PLC would be used to turn the reference to zero to stop if the accel/decel settings needed to apply or cycling the inhibit CR for a fast stop.

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                    • #11
                      Be very careful using a relay to switch the output connections to the motor. Many DC drives do not tolerate switching on the motor or load side of the drive. Check the instructions on your particular DC drive. MOSFET based devices tend to fail when disconnected live. Does you DC drive have forward revers capability? If not, can you replace it with a drive that has that capability? Some require a simple relay selection between forward and reverse reverse inputs, while others require that you swap the analog control voltage between 2 pins for forward and reverse.

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