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changing direction of dc motor (brushed)

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  • changing direction of dc motor (brushed)

    is there a switch with 4 terminals that does it and what is it called?

    iv usually done it with a double-pole/double-throw but wonder if there is an easier way. also is there any arcing going on during switching?
    Last edited by dian; 07-29-2020, 01:31 AM.

  • #2
    DPDT is the way- and yes there would be some arcing especially if you switch while the motor is running. I don't think there is an easier way, though there may be a more elegant way depending on what you're doing with the motor.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Quad pole switches are available. You can get ganged switches with a huge number of poles. But the typical DPDT with the wires crossed is the standard for reversing a DC motor.

      Dan
      At the end of the project, there is a profound difference between spare parts and extra parts.

      Location: SF East Bay.

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      • #4
        There are specific switches for reversing DC motors. They have a mechanism inside that prevents you from switching directions in one toggle actuation. You can also set things up so the middle position contains a braking resistor which slows it down. Better yet just get a h-bridge motor controller and it handles all that for you, no arcing, no contacts.

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        • #5
          If you tend to switch large DC currents under load, especially inductive loads, a DC-switching relay is used, this has various means of quenching the arc, the most common is arc-blow out, which features a strong magnet fitted next to the contacts.
          This prevents the plasma arc that tends to weld contacts etc.
          Also as mentioned manuf. such as KB supply a DPDT centre off reversing switch that prevents going from FWD to REV etc in one continuous action by forcing a physical stop at centre.
          Max.
          Last edited by MaxHeadRoom; 07-28-2020, 05:14 PM.

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          • #6
            You might mention the size (power requirements/draw) to get more specific answers. A simple DPDT will be fine for a small motor, for a given value of "small". Above a certain size you'll pretty much need to look at something more sophisticated.
            "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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            • #7
              A snubber across the switch (and/or the motor) will help reduce arcing, contact welding, and EMI.

              https://www.jameco.com/Jameco/conten...nd-arcing.html

              https://www.elprocus.com/rc-snubber-circuits/

              https://en.wikipedia.org/wiki/Snubber

              http://pauleschoen.com/pix/PM08_P76_P54.png
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030

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              • #8
                Originally posted by dian View Post

                iv usually done it with a double-pole/double-throw but wonder if there is an easier way. also is there any arcing going on during switching?
                The double pole double throw with centre OFF is preferred.
                Max.

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                • #9
                  Solid state switches do not arc, there are no physical contacts.

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                  • #10
                    You need to reverse a winding to reverse the motor. So there are two contacts to be opened, and two to be closed. DPDT switch (US term) is the article in question. Four contacts, two positions each, so double pole double throw.

                    Originally posted by Bented View Post
                    Solid state switches do not arc, there are no physical contacts.
                    They may not arc, but they still can generate a good spike, which can actually turn the switch back "on". You still may need the snubber, if the SSR does not have one already.

                    Also, for this application, you would need several SSRs for the function. A switch is cheap by comparison
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Originally posted by J Tiers View Post
                      You need to reverse a winding to reverse the motor. So there are two contacts to be opened, and two to be closed. DPDT switch (US term) is the article in question. Four contacts, two positions each, so double pole double throw.



                      They may not arc, but they still can generate a good spike, which can actually turn the switch back "on". You still may need the snubber, if the SSR does not have one already.

                      Also, for this application, you would need several SSRs for the function. A switch is cheap by comparison
                      Cheap being the most important criteria I suspect.

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                      • #12
                        The double (2) pole, double throw switch IS the simplest and easiest way to reverse a DC motor. There are only two DC power lines coming in and only two brush wires going out, so there is absolutely nothing to connect the third and forth poles and their contacts to. There is simply no reason for those extra poles.

                        Perhaps you do not understand the use of the word "poles". In switch terminology a "pole" is a movable arm or other physical conductor that can be moved into two or more positions where it can be in in contact with one or more terminals. A double "pole" switch is one that has two such moving arms.

                        Every switch will have at least TWO positions where the "poles" can be at rest. If there is only one position where a "pole" can be, then it is not a switch: it is just a piece of wire. The positions where there are actual electrical contacts are called "throws". There will be at least one "throw" in any switch as zero "throws" would constitute an open circuit (a disconnected wire that has no way to connect to anything). So a simple, doorbell style push button switch would be described as a Single pole, single throw switch: it has only one moving conductor and that conductor can move to two positions: in and out. In the "in" position it is in contact with a contact and the door bell rings. In the "out" position it is not in contact with any contacts and the door bell is silent.

                        A double pole, double throw (DPDT) switch has six terminals. Two of those terminals are connected to moving poles and the other four terminals are connected to four contacts inside the switch, two for each pole. Each pole is connected to only one of it's contacts at one time (in one of it's two positions). With the external wires to those four contacts connected in an X arrangement, this allows the + and - connections to the brushes in a motor to be reversed.

                        As for arcing, brushes always have it to one degree or another. But that is not the only consideration here. It must be remembered that any motor, when it is running, will be developing a reverse EMF (Voltage). When the Voltage to a DC motor is reversed while it is running, that reverse EMF will still exist in the period while the motor is still rotating in the original direction. So it will ADD with the supply Voltage and produce a combined Voltage which will be about two times the supply Voltage. That will, in turn, double the current. This will produce a strong braking effect which will stop the motor and then reverse it's direction. But it also forces that double current to flow through those switch contacts just as they are trying to come to rest at the new, reverse position. That can lead to more heat and damage to those switch contacts. In addition, the damage caused by arcing at the brushes will also intensify at this switching instant.

                        For small DC motors, many, dare I say most DPDT reversing switches will have contacts that are rated for much greater current and Voltage levels than the DC supply and/or the motor will ever present to those contacts. This is because they simply do not manufacture switches with lesser ratings (note below). So small motors can and are wired with a reversing switch that has no provision for any delay between the two positions (forward and reverse). There is no damage to those very much over-rated contacts. However, for larger motors the size and rating for the switch will be a lot closer to the actual Voltages and currents that the motor will actually be using. So there is little or no reserve capacity for the switch contacts to resist any spikes. It is not good design practice to have a reversing switch that can instantly reverse a larger DC motor. As others have said, these larger reversing switches will have some kind of delay built into their mechanism. Or they may be installed in a manner that discourages using them while power is applied to the motor.

                        Note: Well, they really do manufacture switches with lesser ratings. But they are not generally ones that are used for the control of things like motors. They are used for switching signals like audio and digital and video signals, not for power controls. And they are often more expensive than power type switches so they are not even a temptation for motor/power circuits.



                        Originally posted by dian View Post
                        is there a switch with 4 poles that does it and what is it called?

                        iv usually done it with a double-pole/double-throw but wonder if there is an easier way. also is there any arcing going on during switching?
                        Paul A.
                        SE Texas

                        Make it fit.
                        You can't win and there IS a penalty for trying!

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                        • #13
                          Originally posted by Paul Alciatore View Post
                          It is not good design practice to have a reversing switch that can instantly reverse a larger DC motor. As others have said, these larger reversing switches will have some kind of delay built into their mechanism. Or they may be installed in a manner that discourages using them while power is applied to the motor.
                          I ran into this issue years ago as an end user (not up on the details). I was using a set of small plate rolls (AKA slip rolls) for steel fabrication, and these rolls had been converted from line shaft/steam engine, to a 5HP DC motor. No relays or anything, it all went through a very old drum switch.

                          When the contacts burnt and stuck the maintenance guy stuck some kind of snubber in the switch, he said it was basically a big diode plus resistor. Contact sticking problem went away. Of course the switch was already 50 yrs old anyhow, so it was 50 yrs newer than the rest of the machine...

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                          • #14
                            sorry, my post was a bit misleading as i said "4 poles". what i meant was 4 terminals (with the jumpers inside). i corrected it.

                            this time it was a 12v/3 amps motor, so i dont really expect much problems, but the arcing question was related to larger (next would be 600w) motors.

                            paul, from the first link it seems an r/c is for resistive loads and the diodes would be for a motor? or? are there any tables on how to chose a snubber depending on load?

                            macona: "specific switches for reversing DC motors". can you point me to such a switch?
                            max: can you point me to such a relay?

                            cost: a controller with speed and direction control is around $10. a dp/dt switch is $1-3. i still have to change the switch on the controller but the actual problem is that you have to put it somewhere. btw i needed a small dp/dt switch with a 10 mm thread and neither digikey nor mauser had one (i called). waiting for them to get here from china.

                            edit: even if the motor stops in between, a snubber would be beneficial, right?
                            Last edited by dian; 07-29-2020, 02:05 AM.

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                            • #15
                              A lot depends on the size of the motor for sure. Within say 10 amps or less, you can control a motor using one channel of an audio amp. You use a pot plus some resistors to put about 1 volt plus or minus on the sliding contact of the pot. That voltage feeds a dc input on the amp, and the motor is stopped when the pot is at center. Vary the pot one way and you get one direction of rotation, and some control over the speed of rotation. Rotate the pot the other way and you are reversing the voltage, and the motor. The power you can feed to the motor depends on what you use for a power amp.

                              If you rotate the pot smoothly, the motor responds easily. If you rotate the pot quickly to another position, the motor will very quickly respond as well. The amplifier gives automatic braking, in part because of its high damping factor, and in part because it is trying to correct an error. If the motor is running at say 10 volts, and you instantly dial in 0 volts, the amp will apply what is essentially a short to the spinning motor. You just operate the pot with whatever finesse suits what you're doing with the machine. Just another way to run a motor.

                              A stereo amplifier will run two motors of course.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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