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Am radio for motor speed controler

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  • Am radio for motor speed controler

    I have 3 different machines that I built. They use cheap 1500 watt high power drill motors from 3/4 electric drills. These are brush type universal motors. I vary the speeds using a variable transformer or a router speed controller. These don't work well. They slow the motor down but when you load the motor they loose more speed.Any controller that has closed loop feedback needs to have a separate senser on the motor. I am looking for a universal plug and play controller that REALLY MAINTAINS THE SPEED, no matter what the add says they don't exist. SOOO I noticed when I have my AM RADIO tuned between stations and use my machines,I CAN HEAR THE RPM OF THE MOTOR.I think what I am hearing is the arcing of the brushes. I am quite an accomplished electro-,mechanical wizard,but all old school . I would like to build a controller using this radio interference for feedback.I have almost no experience with solid state electronics.I am 74 years old and it is time to learn. So maybe some of the wizards on here could point me in the right direction or shoot this idea down. Thank you ,Edwin Dirnbeck
    Last edited by Edwin Dirnbeck; 04-21-2016, 10:36 AM.

  • #2
    I think there are some closed loop (or I guess it would be pseudo-closed loop) systems that monitor the load (current draw?) itself to compensate and give a more consistent speed. In fact, I think that the closed loop systems in some of the VFDs work that way. Speaking of VFDs, it's overkill, but you could use an inexpensive one for this purpose. You can get them for about $100 and I can tell you firsthand they work great for 3 phase motors but I haven't tried them on small universal motors. If you don't need to run more than one machine at a time, you could switch the VFD output to use a single VFD unit for all 3.

    Anyway, getting back to compensating for motor load without needing a separate sensor, I came across this that might be of use:

    I have to imagine some of the readily available speed controllers out there make use of that idea, but I don't know how you'd identify one that had load sensing compensation versus one that didn't.

    I'm curious about this topic as well, because I'd really like variable speed control on my little watchmakers lathe. I have a nice old universal motor that I could use, or the little DC motor I'm currently using.

    EDIT: If accurate speed control is required, compensation based on load probably isn't what you want... in that case a true closed loop system using an external sensor is probably the only way to go.
    Last edited by mars-red; 04-21-2016, 11:22 AM.


    • #3
      So you plan to replace a hall sensor with a rf receiver, sounds complicated with the added possibility of the receiver picking up other signals, if that makes sense?
      However my electricery is limited in respect, mind I just watched an Eric dollard vidio, I don't think I'll ever recover!
      In answer, try!, PWM occurs to me


      • #4
        There is SuperPID for closed feedback control of a Universal motor.


        • #5
          Using commutator hash as a feedback reference signal. Hm. It probably has a frequency component based on the commutator bars count times RPM but how to separate a usable signal from the random electrical noise? Some combination of filters and freq to voltage ... Hell, I'm not smart enough to work out the details. The people who design shop tools are well aware of the competitive advantage of series wound motors with good speed regulation. You can bet in the last 40 years of solid state electronic motor controls they looked at all the possibilities. I can't imagine they would miss comm noise if there was any potential.

          Couldn't hurt, try it.

          There is an existing solution and that is to use a Dart or KB SRC motor control. They sense the back EMF of the motor to compensate for speed regulation and are adjustable over a wide RPM range. It's not perfect. In fact the speed reg is better in some parts of the range than others but it works quite well for most purposes. Some SCR controls have an input for governor generator feed back. This governor generator can be quite simple such as coupling a small Mibuchi style DC motor to the load motor shaft but that violates your initial requirements.

          Anyway, there are no really cheap solutions that I know of. But there are several low cost ones:

          Don't forget to look at Dart controls. There are others (Dayton and many automation controls hoses have private lable controls) but these two offer a wide range of reliable products and have a large installed base and good word of mouth.
          Last edited by Forrest Addy; 04-21-2016, 01:07 PM.


          • #6
            A series AC/DC motor, such as the "universal" motor in a drill, is INHERENTLY very poor at speed regulation. It's baked-into the design, and there is no way to avoid it*.

            So, you HAVE TO have an external speed regulator to keep it on-speed. Otherwise it may vary speed over a 2:1 (or more) ratio.

            Now, I do not know what sort of accuracy you think you want, nor why you think you want it. But do not be fooled, a standard induction motor does not maintain speed either. About the best an induction motor does is 5% loss of speed from no load to full power, and 10% from no load to the maximum overload point. And nobody has a problem with that in normal operation.

            So, you really don't need absolute accuracy of speed control.

            BUT, that said, the RF method is not particularly good. It's capable of being accurate in the sense of it ultimately being based on counting the number of commutator segments per unit time, which would give speed, but its really the long way around the barn when you come to actually DOING it.

            You would be far better off to count fan blades, etc, optically, or with a hall effect sensor, etc.

            You can then use a frequency to voltage converter, such as LM2907



            With that you get a voltage output related to speed, which you can compare against your speed pot voltage, and control the PWM or phase controller with the difference (error signal) between actual and setting.

            * The basic issue is that the "normal" or "design intended" speed (generally called the "base speed") of a series DC or AC/DC motor changes with motor current. The more current, the lower the base speed. This is PERFECT for a drill, or a locomotive, but a bit disturbing or even disruptive for a lathe.

            The problem with sensing current is that you cannot be sure how current relate to speed with a series motor. Current relates to TORQUE, but not to speed, and the same speed may require a different torque under dfferent conditions.

            Even sensing POWER does not relate to speed, since power is related to speed x torque, and you only know one of those. If you have to sense speed, you may as well use it directly.

            Keep eye on ball.
            Hashim Khan


            • #7
              The advantage of something like the SuperPID it uses AC switching (Triac) if using PWM then a suitable large DC power supply is needed.
              The unit above uses a simple retro reflective sensor with very accurate rpm control at all loads
              BTW, I do not have any vested interest in the site, but have used one of these controllers in the past.
              This is the site of the guy that developed it,
              Last edited by MaxHeadRoom; 04-21-2016, 01:31 PM.


              • #8
                The simple method of speed control for DC or universal motors is to compensate for the copper losses (which is basically emf control but then from the other end). A circuit measures the current and then adds voltage at a fixed ratio to compensate for the losses. This always leaves a fixed amount of voltage for the back emf at the required speed.
                All walkmans used to work this way for their speed control and were pretty good at it.
                It's simple for tiny motors with an opamp, but you could probably hack something with a class D audio amplifier chip for larger motors when so inclined.


                • #9
                  Universal motors will run on DC. Just get a DC motor controller and adjust the IR compensation for reasonably good speed control. If you want better get one that will take a digital encoder. Dart sells the drive and a little blue hall effect encoder that you can install on the end of the motor shaft by tapping it for the 10-32 screw it comes with. It has a built in bearing and the cord restrains it from rotating. I've used them several times and they work great.


                  • #10
                    1. Using AM radio to receive the hash/noise from the brushes of a motor is a very poor way of monitoring the motor's speed. First, the radio receiver will pick up everything in the area and many things that are more distant. This unwanted "noise" will vary from one place to another so it will be almost impossible to separate the wanted noise from your motor from the unwanted noise of the environment.

                    2. The user, your customer will have to tune the radio receiver to an unused frequency in whatever area the tool is being used in. Total PITA from a consumer viewpoint.

                    3. The hash you receive will be very broad band and will have to be filtered with a low frequency filter. Designing this filter will require assumptions about the frequency of the desired signal and this will limit the speed range attainable.

                    4. You will have wires between the motor and the controller anyway: the power wires. So adding one or two more for feedback is no real problem.

                    5. A speed sensor, perhaps an optical one, can be easily added for little cost: LED for light and a photo-transistor. These components can be purchased in a single package that just mounts near the shaft. It will give you a nice CLEAN signal that will directly translate from frequency to revolutions per second. This will be a lot less expensive than adding an AM radio receiver. And a lot easier to design. And the user does not have to tune anything.

                    There is a reason why things are done the way that they are. You will drive yourself nuts trying to make that AM receiver work. Look into tach circuits and figure out where it can be mounted.
                    Paul A.
                    SE Texas

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


                    • #11
                      You could reinvent the wheel

                      Or get one of these

                      Baldor DC Motor Adjustable speed drive. The actual electronic device is rather ubiquitous under the maker's name.


                      • #12
                        Originally posted by CalM View Post
                        Baldor DC Motor Adjustable speed drive. The actual electronic device is rather ubiquitous under the maker's name.
                        Baldor relabel KB drives for their line.


                        • #13
                          If you really want to do exactly what you say. Its kind of like an electronic steam punk thing.

                          There are many Frequency to voltage converter chips out there. You could build a circuit that
                          "hears" the wine from the radio, converting that to a voltage that helps control the speed of
                          your motor.

                          If the wine starts lowering in Freq .. it would just pump up the juice to the motor to keep it
                          at that freq.

                          All the other suggestions here are great and how it "should" be done .. but .. your drummer
                          may want a different beat.
                          John Titor, when are you.


                          • #14
                            Edwin, I can see some advantages in your proposal the most obvious being that one controller unit could be used on different machines without alteration to the machine at all, just plug it through your magic box!

                            I have a device in my car which uses alternator hash to determine engine speed and it seems to work quite well but I have no idea of how accurate it may be.

                            In your situation I would consider a filter on the mains input to your box leaving a pure 60/50Hz followed by a take off for your detector circuit. Your detector would filter out the mains frequency leaving whatever noise is coming from the motor. Half wave rectify the noise and you have a pulse train representing motor speed.

                            So no need for the AM radio link but still using your idea of using brush noise(?) as the feedback signal to your controller.



                            • #15
                              The variable speed electric drills I have seem to have good torque at low speeds. Possibly you could use the electronics from a variable speed electric drill to control the speed of your devices.