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  • 0T is this a motor speed control breakthrough

    I previously posted about a good cheap variable speed BUFFER motor with feedback since then I have opened up this motor and to my surprise there is nothing in there that senses the rpm of the spindle I am not an electronics person but I assume that most good speed control motors had have some sort of a device that senses the actual speed of the Armature and sends the signal to the controller I have included some pictures and this seems not to be the case with this motor there are only two wires going into the motor and it appears to be a conventional universal motor with two brushes and a field winding just thought others just thought others might be able to shed some light on how this works .I have had my share of router speed controls that only cuts the power and as soon as you apply any load to the motor hey just slow down .This is not the case with this buffer, if you apply some some pressure to it it tries to maintain the speed very well .Edwin Dirnbeck


    Sent from my iPhone using Tapatalk
    Last edited by Edwin Dirnbeck; 01-20-2017, 01:13 PM.

  • #2
    Most of those will sense the motor back EMF, which is dependent on speed. So they DO sense speed, but not with a tachometer type device
    CNC machines only go through the motions

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    • #3
      Most DC motor controls have I^2r compensation which adjusts the output of the drive based on the amps the motor pulls. Likely what they are doing here. My Hitachi router though has a hall effect tachometer it uses to hold a constant speed.

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      • #4
        First, speed control and speed regulation are two different things. Speed control can be an open loop system, which means that there is no feedback. A circuit that regulates the speed is some kind of a servo and it does employ feedback to control the part of the circuit that controls the speed.

        As others have said, it is possible to sense, to some extent, the motor's speed using the same two wires that provide the power to it. Generally speaking, this is not a very good way of regulating the speed. In theory, it would even be possible to have a tach signal piggybacked on the Voltage/current that drives it, on those two wires. I don't know of any examples of this and I doubt that it would be practical, but it is possible.

        I haven't examined the details of the circuit shown, but I do not think it is any breakthrough. Unless perhaps in terms of cheap.
        Paul A.
        SE Texas

        And if you look REAL close at an analog signal,
        You will find that it has discrete steps.

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        • #5
          About the only way to control RPM for a Universal motor is with some kind of rpm sensor on the shaft etc, these motors, being a series motor, operate in a runaway condition, there are series motor controllers out there such as the SuperPID etc that use such a sensor.
          Max.

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          • #6
            I expect it would be possible to sense the speed of a brushed motor through the commutator hash that would be on the supply wires, maybe someone does this. It is probably just easier to use the back EMF method.

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            • #7
              I can't view the pictures for some reason, but I know that a universal motor does produce a frequency as it commutates. This can be sensed and turned into a dc control voltage that in turn controls the conduction of the power device that feeds current to the motor. There doesn't need to be anything in the motor for this to work. A similar thing is done with tape deck motors, and you can certainly sense if there's a speed variation there.

              I know these are two different types of motors, one having permanent magnets and the other having a field winding. The permanent magnet type produces its own 'back emf' which opposes the voltage feeding the motor and causes it to reach a certain rpm, depending on applied voltage. This isn't speed regulation, but does a reasonable job of keeping the motor rpm within limits from no load to full load. Often this is good enough. My lathe with the DC motor works this way. I can use the audible drop in rpm as an indicator of how loaded down it is, and my power supply can simply be switched to different output voltages for an assortment of speed ranges. Not perfect by any means, but very workable.

              To control a permanent magnet motor for a tight speed regulation you need a circuit, and the same is true for a universal motor- it's just a bit different of a circuit.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                There are quite a few up sides to running a Universal motor on DC, Extended motor life is high on the list.
                Max.

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                • #9
                  Maybe it is the comutation

                  Originally posted by darryl View Post
                  I can't view the pictures for some reason, but I know that a universal motor does produce a frequency as it commutates. This can be sensed and turned into a dc control voltage that in turn controls the conduction of the power device that feeds current to the motor. There doesn't need to be anything in the motor for this to work. A similar thing is done with tape deck motors, and you can certainly sense if there's a speed variation there.

                  I know these are two different types of motors, one having permanent magnets and the other having a field winding. The permanent magnet type produces its own 'back emf' which opposes the voltage feeding the motor and causes it to reach a certain rpm, depending on applied voltage. This isn't speed regulation, but does a reasonable job of keeping the motor rpm within limits from no load to full load. Often this is good enough. My lathe with the DC motor works this way. I can use the audible drop in rpm as an indicator of how loaded down it is, and my power supply can simply be switched to different output voltages for an assortment of speed ranges. Not perfect by any means, but very workable.

                  To control a permanent magnet motor for a tight speed regulation you need a circuit, and the same is true for a universal motor- it's just a bit different of a circuit.
                  Darryl,thanks for your thoughts on how this might work. I too ,often do impractical things ,just because I want to. Edwin Dirnbeck
                  Last edited by Edwin Dirnbeck; 01-20-2017, 05:05 PM.

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                  • #10
                    Here are some links about speed control of universal motors:

                    http://www.brighthubengineering.com/...d-controllers/
                    (Shows only centrifugal mechanical control, series resistance, and field tapping)

                    https://hackaday.io/project/1280-sim...eed-controller
                    (Shows a phase-controlled AC controller using an AVR device and ZC detection)

                    http://rick.sparber.org/Articles/Motor/Motor052306.htm
                    (Shows simple means, as well as a rather complex DIY circuit)

                    http://www.onsemi.com/pub_link/Colla...TDA1085C-D.PDF
                    (This is a speed control IC that uses a tach/generator for speed feedback)

                    http://www.mouser.com/catalog/specsh...alill004v1.pdf
                    (Another microcontroller and TRIAC based circuit for AC control)

                    http://electronics-diy.com/1000w-ac-...controller.php
                    (This shows a very simple TRIAC control using only a few parts)

                    http://www.quan-diy.com/projects/dri...lspeedcont.htm
                    (Shows how to use current feedback to maintain torque, but with linear DC control)

                    http://www.hobby-hour.com/electronic...ed-control.php
                    (Somewhat complex control)

                    http://www.littelfuse.com/~/media/el...n_note.pdf.pdf
                    (Various circuits and information on triac phase control)

                    The board you show in the picture seems very simple, but hard to tell what the circuit actually is. If the power device has markings, it may help. The electrolytic capacitors indicate it may not be a thyristor, so it may work on DC as well as AC. I don't think it has any current feedback or sensing of rotation by commutation hash. It is probably PWM of some kind, which provides a fairly constant average voltage and thus constant speed. Motor speed is proportional to voltage, while torque is proportional to current. A series resistance for speed control causes the motor voltage to drop when it supplies torque.
                    http://pauleschoen.com/pix/PM08_P76_P54.png
                    Paul , P S Technology, Inc. and MrTibbs
                    USA Maryland 21030

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                    • #11
                      Originally posted by PStechPaul View Post
                      Here are some links about speed control of universal motors:

                      http://www.brighthubengineering.com/...d-controllers/
                      (Shows only centrifugal mechanical control, series resistance, and field tapping)

                      https://hackaday.io/project/1280-sim...eed-controller
                      (Shows a phase-controlled AC controller using an AVR device and ZC detection)

                      http://rick.sparber.org/Articles/Motor/Motor052306.htm
                      (Shows simple means, as well as a rather complex DIY circuit)

                      http://www.onsemi.com/pub_link/Colla...TDA1085C-D.PDF
                      (This is a speed control IC that uses a tach/generator for speed feedback)

                      http://www.mouser.com/catalog/specsh...alill004v1.pdf
                      (Another microcontroller and TRIAC based circuit for AC control)

                      http://electronics-diy.com/1000w-ac-...controller.php
                      (This shows a very simple TRIAC control using only a few parts)

                      http://www.quan-diy.com/projects/dri...lspeedcont.htm
                      (Shows how to use current feedback to maintain torque, but with linear DC control)

                      http://www.hobby-hour.com/electronic...ed-control.php
                      (Somewhat complex control)

                      http://www.littelfuse.com/~/media/el...n_note.pdf.pdf
                      (Various circuits and information on triac phase control)

                      The board you show in the picture seems very simple, but hard to tell what the circuit actually is. If the power device has markings, it may help. The electrolytic capacitors indicate it may not be a thyristor, so it may work on DC as well as AC. I don't think it has any current feedback or sensing of rotation by commutation hash. It is probably PWM of some kind, which provides a fairly constant average voltage and thus constant speed. Motor speed is proportional to voltage, while torque is proportional to current. A series resistance for speed control causes the motor voltage to drop when it supplies torque.
                      Wow,thank you PSTECHPAUL.This will give me enough reading till at least midnight. It comes at a perfect time as I had shut the tv off for this entire day and night . EdwinDirnbeck

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                      • #12
                        Originally posted by MaxHeadRoom View Post
                        About the only way to control RPM for a Universal motor is with some kind of rpm sensor on the shaft etc, these motors, being a series motor, operate in a runaway condition, there are series motor controllers out there such as the SuperPID etc that use such a sensor.
                        Max.
                        Thanks for your reply. HOWEVER,imagine telling someone like Steve Jobs of Apple that this couldn't be done.It just hasn't been done yet. Edwin Dirnbeck

                        Comment


                        • #13
                          I found some information on using BEMF for speed sensing. This is probably most effective for brushed DC motors with permanent magnet stators, where it generates a voltage proportional to speed. It is used in PWM drives where one driver is held at a low level (ground) while the other is floating. The DC level generated under those conditions is measured with a quick sample using an ADC.

                          http://ww1.microchip.com/downloads/e...tes/00905B.pdf

                          I don't know if it is possible to detect the noise created by the commutator and brushes and use that for speed feedback. There is a certain level of noise just from the brush riding on the commutator, and that can vary depending on their condition and how much current is being drawn. And a universal or series wound motor may not generate voltage when the drive is floating.

                          Here is an interesting discussion of BEMF as used for model train speed control, although they use fixed magnet brushed DC motors:
                          http://www.sumidacrossing.org/ModelT...Decoders/BEMF/

                          Here is an article about using the waveform produced by the commutation of a brushed DC motor for speed/position feedback, but requires IEEE membership or payment for the full article:
                          http://ieeexplore.ieee.org/document/4510598/

                          Here is something else about the technique:
                          http://uni-obuda.hu/e-bulletin/Gyorok_Mako_Lakner_1.pdf

                          And this may be helpful:
                          https://www.researchgate.net/publica...AND_VALIDATION

                          Yeah, I don't much want to watch or listen to the news today...
                          http://pauleschoen.com/pix/PM08_P76_P54.png
                          Paul , P S Technology, Inc. and MrTibbs
                          USA Maryland 21030

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                          • #14
                            I was building SCR based speed controllers for universal drill motors back in the 70's... couple of resistors, cap or two and a germanium transistor (low base emitter drop) around the gate and we had "emf feedback". Worked fine, within limits. An old GE SCR manual (book) has the circuit. Two of them back to back gave full speed; one SCR was about 1/2 max. Triacs were new fangled thingies and didn't work correctly (common gate) for what we need - full speed but poor regulation.

                            The OP circuit will be similar... dead simple and cheap.
                            Last edited by lakeside53; 01-20-2017, 08:18 PM.

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                            • #15
                              That speed control is just a light dimmer, basically.

                              The small bosch wood lathes have a tacho feedback to the speed control to close the loop and maintain speed. The better 7x mini lathes had a PWM speed controller that did back emf feedback for torque control.

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