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real simple DC motor control circuit no transistors or other active elements

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  • real simple DC motor control circuit no transistors or other active elements

    Please take a look at my little schematic and tell me what you think of it. I didn't design it - I believe it is the way the old Victor track torches were speed-controlled. I was impressed with how simple it is, basically a rheostat, a couple rectifiers, some switches and a fuse. Of course, it depends on the motor having separate field windings, and it doesn't have any feedback. I believe they got away without feedback because the track burner rolls with constant low losses. Also, if you need a little more speed you can just move the rheostat handle a little. In this application precision isn't really required.

    Unless I made a mistake transcribing this circuit, it is time-tested. I don't think it is one of the basic DC motor control schemes I have seen not that I'm some kind of expert. I'm just a tinkerer jack of all trades type of guy.

    Here's the schematic: http://nwnative.us/Grant/images/panelSchematic.pdf

    metalmagpie

  • #2
    It looks like the field coil is powered from a fixed rectified 120 VAC source and the armature is fed a variable voltage from a potentiometer (not actually a rheostat, which is a variable resistor). The symbol looks more like a Variac, or variable transformer, which is much more efficient. The simplest motor controls, usually for universal or series-wound motors, is a series variable resistor (rheostat). Actual speed depends on the load (or "windage" which is air resistance of the spinning armature). Series resistance also limits the torque by regulating current. Some motors accomplish speed control by adjusting field current, where less current results in higher speed to cancel BEMF.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #3
      It looks good.

      What is the purpose of the first bridge rectifier - the one connected to VCI, VC2? Is that a ballast or coupling resistor connected to it at the bottom of the drawing? It's not identified. Just curious.

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      • #4
        Originally posted by GNM109 View Post
        It looks good.

        What is the purpose of the first bridge rectifier - the one connected to VCI, VC2? Is that a ballast or coupling resistor connected to it at the bottom of the drawing? It's not identified. Just curious.
        That's the motor's field winding.

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        • #5
          Originally posted by PStechPaul View Post
          It looks like the field coil is powered from a fixed rectified 120 VAC source and the armature is fed a variable voltage from a potentiometer (not actually a rheostat, which is a variable resistor). The symbol looks more like a Variac, or variable transformer, which is much more efficient. The simplest motor controls, usually for universal or series-wound motors, is a series variable resistor (rheostat). Actual speed depends on the load (or "windage" which is air resistance of the spinning armature). Series resistance also limits the torque by regulating current. Some motors accomplish speed control by adjusting field current, where less current results in higher speed to cancel BEMF.
          Oops. In actuality I think it is a Variac and I screwed up in my nomenclature. My bad ..

          metalmagpie

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          • #6
            The problem with variacs is that they are rather expensive and they are also fragile. There are solid state units now that are variac-based and are inexpensive. The circuit looks fine though.

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            • #7
              Originally posted by GNM109 View Post
              The problem with variacs is that they are rather expensive and they are also fragile.
              And heavy. Smallest ones commonly found are something like 2 amps and 6lbs

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              • #8
                Does it work? Sure it does.

                Is the speed regulation good? It will be between poor and just plain OK, depending on the motor and the load. It probably was OK for dragging a torch along a weld as there was little resistance to it's movement if the rails were kept clean and the cables or hoses kept properly dressed.

                Will it be compact? NO!

                Will it run cool? NO!

                The main problems will be a lot of wasted heat/power and it will be expensive. That variac will have a lot of steel and copper and they cost $$$. Solid state components are only pennies by comparison.

                And variacs will wear out if adjusted over and over. And then they need to be replaced. Been there! Done that! Expensive replacement cost. Solid state switches have no moving parts and if the circuit is designed properly they can last a long, long time.

                I don't think any decent EE would design a circuit in that manner today.
                Paul A.

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

                Comment


                • #9
                  Your circuit would certainly work but the speed regulation would poor if T! was indeed an actual "rheostat: that is a variable resistor.

                  In your drawing legend, you call out T1 in your diagram a "rheostat," a specific type of high wattage variable resistor used in lighting and motor control many years ago.

                  The diagram symbol in your diagram is used to represent a variable transformer - that is a variac or equivalent. If it is a variable transformer, then in my estimation your speed control diagram if transmuted into hardware would work very well and even have pretty good speed regulation. The rectified armature voltage would be little affected by armature current demand and thus be stiff in RPM response to varying mechanical load.

                  The old track burners I've seen had a big knob that adjusted a spring on motor shaft driven centrifugal governor. The speed regulation was excellent: you couldn't hurry it or slow it down by hand without skidding the wheels.

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                  • #10
                    Originally posted by Forrest Addy View Post
                    Your circuit would certainly work but the speed regulation would poor if T! was indeed an actual "rheostat: that is a variable resistor.

                    In your drawing legend, you call out T1 in your diagram a "rheostat," a specific type of high wattage variable resistor used in lighting and motor control many years ago.

                    The diagram symbol in your diagram is used to represent a variable transformer - that is a variac or equivalent. If it is a variable transformer, then in my estimation your speed control diagram if transmuted into hardware would work very well and even have pretty good speed regulation. The rectified armature voltage would be little affected by armature current demand and thus be stiff in RPM response to varying mechanical load.

                    The old track burners I've seen had a big knob that adjusted a spring on motor shaft driven centrifugal governor. The speed regulation was excellent: you couldn't hurry it or slow it down by hand without skidding the wheels.
                    OK. If you revisit the link you will find everything the same except I renamed T1 from 'rheostat' to 'variac'. Senior moment. My bad. Mea culpa. Nuff said?

                    Forrest, I think you are talking about newer old track burners! :-)

                    metalmagpie

                    Comment


                    • #11
                      Speed regulation should be "ok". Shunt motors have decent inherent regulation. If turned down to very low speed, it will be worse, due to resistance vs the low voltage applied.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        Do you have a wound shunt field motor? They are getting pretty rare now, the circuit will work the same for a P.M. field, just delete the field bridge.
                        This is the basic method of the low end KB DC motor controllers, just that they use a couple of SCR's in the bridge in place of the Variac control.
                        Max.

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                        • #13
                          Originally posted by MaxHeadRoom View Post
                          ....
                          This is the basic method of the low end KB DC motor controllers, just that they use a couple of SCR's in the bridge in place of the Variac control.
                          Max.
                          Yes, and many have an option of a low speed boost, to counteract the resistance problem. That improves speed stability vs a no feedback system like a variac. They are probably cheaper than a variac of the same capability, also.

                          As for the field, the permanent magnet (P.M,) field is exactly equivalent to a wound field with constant voltage on it.
                          Last edited by J Tiers; 11-26-2017, 11:55 AM.
                          1601

                          Keep eye on ball.
                          Hashim Khan

                          Comment


                          • #14
                            Originally posted by MattiJ View Post
                            And heavy. Smallest ones commonly found are something like 2 amps and 6lbs
                            Nope.. the miniature panel mount Variacs are small - 2 inches or so across. They were very common up to the 70's.. even later. I still have some.

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                            • #15
                              I have a really old BOC bantam track cutter, so old that thieves didn't even bother to steal it, it's really heavy but when I looked inside it a long time back there was bugger all in it, couple of switches, wire wound pot thing, a bridge that I could see, seems this design got about a bit, speed control is good, I've actually welded with it, welding torch in the cutter clamp, dip dip dip dip...you get the idea, I was toying with sticking my plasma torch on it to see what happens, turn the pot up and it can fairly fast crawl along, going to have to dig it out now (good job I didn't cannibalise the 8' track, there's a rack gear all along it.
                              Mark

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