Small Brushless Motors – can they generate?

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Small Brushless Motors – can they generate?

Home Forums Electronics in the Workshop Small Brushless Motors – can they generate?

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  • #370417
    Ian P
    Participant
      @ianp

      Tim

      You are partly right with your understanding of motors, but its not so simple. There really are hundreds probably of different motor designs that would fill shelves with books and I would find it difficult to break them down into a few simple groups.

      Just as one example, the little fans used to cool electronics/laptops etc have very simple but 'brushless' motors that their electronics internally.

      Ian P

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      #370419
      Ian P
      Participant
        @ianp
        Posted by Neil Wyatt on 04/09/2018 16:45:04:

        Posted by Ian P on 04/09/2018 14:33:07:

        A normal brushless motor only runs as a motor when it is combined with some electronic commutation scheme, if you took out the electronics, whatever windings it had could be used to generate an alternating voltage.

        The electronics are normally separate from the motors, and using one to generate AC is exactly what we are discussing.

        In effect a stepper motor is just a BLDC motor with an awfully large number of poles.

        Neil

        I used the term 'took out' in the sense of took out of circuit.

        There are however lots of brushless motors with integrated commutation electronics.

        Ian P

        Ian

        #370421
        Robert Atkinson 2
        Participant
          @robertatkinson2
          Posted by Tim Stevens on 04/09/2018 16:28:59:

          I am sure that when Robert Atkinson referred to 'back to back brushless DC motors', the DC was a mistake for AC. This whole debate has been about motors with rotating magnets and a fixed set of coils, no commutator etc. And I am sure too that what Ian P refers to as a 'normal brushless motor' cannot be the same thing as used on drones and RC models generally.

          In a drone motor, there is a clump of electronics which so far as I can tell, does the same job as a 'full size' VFD but starting from a small Lithium battery, not the mains. But that does not stop the same motor with no electronics attached responding to a 3 phase input from a similar motor used as a generator, by rotating in synchrony. And surely a stepper motor is the same thing, but with accurately placed magnets etc, designed to go round in a series of cogs, and fed with counted pulses rather than 3 phase continuous?

          Regards, Tim

          No, I ment Brushless DC Motors. The type used in drones and RC cars. ALL motors are Alternating Current at the coil, they have to be to rotate more than 180 degrees. In brushed DC motor the brushes and commutor switch the direction of current. In a Brushless DC motor the switcing is done by electronic switches. In things like small fans these are internal so you put DC in. With he larger model / drone motors the switches are external in a "controller" if you take the basic motor it has 3 stator windings 120 degrees apart and a permanent magnet rotor. The controller produces 3 phase "AC" but it's typically not a sinewave. It also needs feedback from the motor. In earlier designs and simple items (fans) this is by sensors typically hall effect sensors. in newer designs the sensethe back-emf from the coils when the switches are off.

          So if you have a Brushless DC motor without a controller you can use it as either a pemanent magnet 3 phase AC generator or a permanent magnet 3 phase AC motor. So connect 2 "back to back" , turn one fast and the other will turn at the same speed.
          With the aircraft tacho-generators you change the strength of the magnet (old Alinco not modern rare earth) with a coil and capacitor to set the output voltage to the correct level.

          Stepper motors will work back to back, but not nearly so well because of their magnetic design. They tend to stall and then won't "pick-up" again.

          Robert.

          #370423
          Robert Atkinson 2
          Participant
            @robertatkinson2
            Posted by Ian P on 04/09/2018 13:13:07:

            Tim,
            Your proposed "back to back" brushless DC motors driving a mechanical tachometer will work just fine. This is EXACTLY how aircraft remote tachometers work. I have repaired aircraft tachometers, generators and designed and built test equipment for them. Ignore comments about selsyns and synchros, these are for transmitting anglular information not speed (and the power need to drive the tacho).

            Robert

            Something in that description does not make sense. I have never seen a 'brushless DC motor' in any aircraft instruments I have seen. A brushless motor needs a power supply to run (to generate the rotating field) and it would not works as a generator.

            Ian P

            notBLDC

            In the aircraft Tachometer systems they are not called brushless DC becuase they are not run from DC they are called Permanent Magnet 3 Phase Generators / Motors. Magnetically and electrically they are the same as the "motor" part of a drone style Brushless DC motor. Some tacho indicators indicators use induction motors but PM ones are more common. the speed to pointer mechanisim is a magnetic drag cup the same as a traditional car speedo or tacho.

            Robert G8RPI

            #370462
            Neil Wyatt
            Moderator
              @neilwyatt
              Posted by Tim Stevens on 04/09/2018 16:28:59:

              I am sure that when Robert Atkinson referred to 'back to back brushless DC motors', the DC was a mistake for AC. This whole debate has been about motors with rotating magnets and a fixed set of coils, no commutator etc. And I am sure too that what Ian P refers to as a 'normal brushless motor' cannot be the same thing as used on drones and RC models generally.

              The confusion arises because brushless DC motors work by simply switching DC to the different coils of the motor rather than using 3-phase AC (which would work just as well, but takes much more sophisticated electronics).

              Running a BLDC motor as a generator does, however, produce AC not DC.

              (In effect the DC is used to generate three out of phase square waves, but as these are always positive relative to 0V, rather than symmetrical about 0V, they are technically DC not AC, even though the direction of current in the coils reverses).

              Neil

              #370477
              Werner Schleidt
              Participant
                @wernerschleidt45161

                Brushless DC Motors are normaly synchron engines with a permanent magnet.

                They are excelent to use as an alternator! For use with Lamps you have to use at minimum for each phase a diode.

                Usually the are wound in a star setup.

                Stepper motors are also good generators .I used it with my son with a wilesco power plant.The stepper was from a defect epson printer. There were diodes and a 2000 µF Condensator to run are small transistor radio as a steam radio. The boy had lot of fun with it.

                Years ago i modified a small Servo DC motor to a 3 Phase Outrunner alternator for a steam turbine. That works very good with four LEDs .My mistake was that I had to much thermal conection from the steam turbine to the magnets so they got hotter then 80° C and the magnet was dead.

                Werner

                #370486
                Neil Wyatt
                Moderator
                  @neilwyatt
                  Posted by Ian P on 04/09/2018 16:59:50:

                  Posted by Neil Wyatt on 04/09/2018 16:45:04:

                  The electronics are normally separate from the motors, and using one to generate AC is exactly what we are discussing.

                  There are however lots of brushless motors with integrated commutation electronics.

                  Indeed, but I did say normally. Integrated ones are usually built into something (computer fan, HDD) as an integral part of the design rather than as a stand alone motor as used in RC models, spindles etc. where you need a more sophisticated level of control.

                  #370492
                  colin wilkinson
                  Participant
                    @colinwilkinson75381

                    Scitsu supply self contained rev counters, I use one on my Greeves. Just tape the sender wire to a ht lead, that's all.

                    https://www.scitsu.com/about-us-1-w.asp

                    Colin

                    #370532
                    Michael Gilligan
                    Participant
                      @michaelgilligan61133

                      Posted by Neil Wyatt on 04/09/2018 20:00:42:

                      [ … ]

                      The confusion arises because brushless DC motors work by simply switching DC to the different coils of the motor rather than using 3-phase AC (which would work just as well, but takes much more sophisticated electronics).

                      Running a BLDC motor as a generator does, however, produce AC not DC.

                      [ … ]

                      .

                      Once … I was confused … Then I saw the light idea

                      Then … I looked on ebay **LINK**

                      https://www.ebay.co.uk/itm/Mitsumi-3-Phase-Mini-Brushless-Motor-External-Outer-Rotor-DIY-fan-RC-Drone-Model/253175557807

                      "Brush Motor" according to the caption on the photo crying 2

                      MichaelG.

                      #370538
                      Joseph Noci 1
                      Participant
                        @josephnoci1

                        WRT using RC Model style Brushless motors in this application:

                        Tim Asked:

                        But now I have a further question – just confirmation if you like. If I connect the three wires of two identical drone motors, colour to colour, and spin one of them, the other will spin at the same speed – won't it? And swapping two of the pairs would reverse the drive? This would give us a further option, in effect replacing a too-convoluted cable run with three wires, driving a genuine vintage mechanical rev-counter head.

                        Just say Yes, it will work, or no, and explain. Thanks

                        Regards, Tim

                        Robert Atkinson Answered:

                        Your proposed "back to back" brushless DC motors driving a mechanical tachometer will work just fine. This is EXACTLY how aircraft remote tachometers work.

                        And also:

                        So if you have a Brushless DC motor without a controller you can use it as either a pemanent magnet 3 phase AC generator or a permanent magnet 3 phase AC motor. So connect 2 "back to back" , turn one fast and the other will turn at the same speed.

                        And IanP said:

                        A brushless motor needs a power supply to run (to generate the rotating field) and it would not works as a generator.

                        So, a lot of stories about..

                        Tim, Connecting a NORMAL RC Brushless motors back to back does not seem to work reliably.

                        Those motors have very powerful magnets working against a metal ( magnetic) stator, and the magnetically induced mechanical detent is immense. To overcome this, the electrically generated magnetic field needed must be greater that the permanent magnetic field. That requires a large ampere-turns ration of the rotor. These motors typically have from 4 to 50 turns of wire per winding – a 4 turn winding will need many amps to generate an opposing magnetic field able to repel the permanent magnets. A 50 turn will require less amps, but many more volts. Whatever the turns, the motor needs to spin quite fast ( a few 100 to a few thousand RPM) to generate enough voltage to drive sufficient current into the coupled slave motor. The resulting magnetic field in the slave may be sufficient to overcome the permanent magnet field, but the slave will simply sit there vibrating – it CANNOT catch up in RPM to the driven motor. And at low drive speed, the voltage is to low for the slave to be bothered. It does not work well at all. The slave motor cogs, sometimes starts, and generally jerks about. At higher drive motor speed the slave motor 'sometimes' gets going and then keeps pace – not deterministic I fear.

                        Generally, the type of master/slave generators have the slave as a coreless motor, that is, the rotor has magnets, but the stator has only copper coils, no metal stator, so there is NO magnetic detent.

                        The simple permanent magnet attraction between metal rotor/stator keeps the motor from being able to rotate at low voltage inputs, and when the field is strong enough, the rotor cannot catch up in speed, ie, it loses steps!

                        Robert:

                        The slave motor does not seem to start reliably at all…I know the A/C Tacho's well – we repaired many on old Harvard's at the flying school neat where I lived – The slave motor exhibited hardly any magnetic detent at all.

                        IanP:

                        See my post in the very beginning – These motors make excellent AC generators – a 200KV (RPM/Volt) motor of 70mm diameter, 70mm long, easily delivers 900watts at 5000 RPM.

                         

                         I am trying to do a video that does not make us all sea-sick – using a few different RC brushless motors..Will post it soon.

                        Joe

                        edit – the usual darn typo's…

                        Edited By Joseph Noci 1 on 05/09/2018 13:51:40

                        Edited By Joseph Noci 1 on 05/09/2018 13:52:01

                        Edited By Joseph Noci 1 on 05/09/2018 14:17:58

                        #370539
                        Joseph Noci 1
                        Participant
                          @josephnoci1

                          Just to illustrate the reason for magnetic detent ..

                          The conventional RC brushless with magnetic core:

                          brushless.jpg

                          And the Corelss style – The 'coil' being the rotating part, with no magnetic material. Note that this image is for a DC coreless motor, but the principle remains.

                          coreless motors.jpg

                          #370543
                          Joseph Noci 1
                          Participant
                            @josephnoci1

                            The videos..

                            I used AXI brushless motors for the tests – all the same size, 36mdiameter x 40mm long.

                            The slave motor was always a 990KV motor

                            I used a 900KV motor as driver in the 1st video 'Motor-1'

                            This did not work well, very jerky and intermittent.

                            I then used a 1200KV AXI motor as the drive motor – this was much better, but still not reliable startup. Motor-2.

                             

                            Perhaps using a very small motor as the slave, say a 15mm or 20mm diameter motor, of around 800KV, and a larger ( similar to the ones I used) motor, of around 1200 to 2000KV might work? Bottom line is..Maybe, with a lot of fiddling and mucking about – Use two plain, small dc motors, of the old ferrite magnet variety – that should always work!

                            Joe

                            EDIT – just to say how I drove the drive motor:

                            The drive motor was driven in my lathe which has a tacho readout – I varied the spindle speed from 0 to 800 RPM.

                            Motor-1 Video –  jerked all the way up to 500 RPM and then started to try turning.

                            Motor-2 Video – jerky up to approx 350RPM and then work turn well up to 450RPM or so, and then jerk again, and then smooth out at 700 RPM, with a few occasional jerks…

                            Edited By Joseph Noci 1 on 05/09/2018 14:56:50

                            #370544
                            Muzzer
                            Participant
                              @muzzer

                              To answer the question in the header – yes in general if you force the motor to spin faster than the no-load speed, the back EMF will exceed the supply voltage and the motor will generate. This applies to all of the above, including the various brushless motors – as well as"normal" AC induction motors.

                              There's a lot of ambiguity in motor descriptions but what is usually described as "brushless DC" is driven by 3 trapezoidal ie flat topped phase voltages ("6 step" control). If you drive the motor and observe the waveforms, they will look sort of flat topped. In contrast, an "AC brushless" will generate something approximating to a sinusoid. Both are synchronous machines with permanent magnet rotors and need to be commutated by some form of electronics.

                              You can commutate brushless motors without absolute position indicators (such as hall sensors, resolvers etc), based on the back EMF but the challenge comes when you wish to start the thing moving. Without movement you have no back EMF, so can't determine where the rotor is. In some applications you can just ramp the speed up and let it follow open loop but for loads with high starting torque or where positioning is critical this doesn't work so well.

                              Joseph – if you look at the phase voltages between your motors, you'd probably understand why they might have difficulty synchronising. I think you'll find they are a long way from either trapezoidal or sinusoidal.

                              Werner suggested using a PM motor as a generator but the voltage will vary proportional to speed. If you are driving with a variable speed (eg engine), this may be a problem! With a claw pole alternator ("Lundell" machine) that has a wound rotor, you can change the voltage constant by varying the rotor current, otherwise it behaves in a very similar fashion. That's what the alternator regulator does of course.

                              Murray

                              #370564
                              Robert Atkinson 2
                              Participant
                                @robertatkinson2

                                Posted by Joseph Noci 1 on 05/09/2018 14:53:43:

                                The videos..

                                I used AXI brushless motors for the tests – all the same size, 36mdiameter x 40mm long.

                                The slave motor was always a 990KV motor

                                I used a 900KV motor as driver in the 1st video 'Motor-1'

                                 

                                This did not work well, very jerky and intermittent.

                                I then used a 1200KV AXI motor as the drive motor – this was much better, but still not reliable startup. Motor-2.

                                 

                                 

                                Perhaps using a very small motor as the slave, say a 15mm or 20mm diameter motor, of around 800KV, and a larger ( similar to the ones I used) motor, of around 1200 to 2000KV might work? Bottom line is..Maybe, with a lot of fiddling and mucking about – Use two plain, small dc motors, of the old ferrite magnet variety – that should always work!

                                Joe

                                EDIT – just to say how I drove the drive motor:

                                The drive motor was driven in my lathe which has a tacho readout – I varied the spindle speed from 0 to 800 RPM.

                                Motor-1 Video – jerked all the way up to 500 RPM and then started to try turning.

                                Motor-2 Video – jerky up to approx 350RPM and then work turn well up to 450RPM or so, and then jerk again, and then smooth out at 700 RPM, with a few occasional jerks…

                                Edited By Joseph Noci 1 on 05/09/2018 14:56:50

                                This speed is too low for reliable operation of a drone type Brushless DC motors. They will not generate enough voltage. Using lower kVP motors will help.

                                Robert

                                Edited By Robert Atkinson 2 on 05/09/2018 19:18:24

                                #370571
                                Joseph Noci 1
                                Participant
                                  @josephnoci1

                                  But I suppose the original question was what motor could be used to generate a voltage to drive a voltmeter as a RPM indicator – A brushless motor will work fine – just choose one with a lowish KV, say 350 or so, add 6 diodes – 1N4001, maybe cost a pound or so, and you will have a nifty RPM indicator. Use a potentiometer to scale the output to suit a low-volt DC voltmeter.

                                  Joe

                                  #370577
                                  Neil Wyatt
                                  Moderator
                                    @neilwyatt

                                    I think you will find a major issue with any setup that uses a diode rectifier. The diode drop will be about 0.4V so no signal at low speeds (unless you use schottky diodes, and even then it will be enough to result in significant non-linearity).

                                    I would suggest using the unrectified output across two of the motor connections into a moving iron meter. A variable resistor can be used to calibrate the meter and it will give a linear response from 0 to whatever you want.

                                    Neil

                                    #370590
                                    Werner Schleidt
                                    Participant
                                      @wernerschleidt45161

                                      If you want to use it as tachometer with an analog or digital read out it is good to use a magnet on a shaft with a reed contact or hall sensor. Then with a NE 555 circuit as triggered monoflop you can convert pulse to DC voltage.

                                      This is a small and cheap electronic circuit . I you want i can provide the plan.

                                      Werner

                                      #370594
                                      Joseph Noci 1
                                      Participant
                                        @josephnoci1
                                        Posted by Neil Wyatt on 05/09/2018 21:45:26:

                                        I think you will find a major issue with any setup that uses a diode rectifier. The diode drop will be about 0.4V so no signal at low speeds (unless you use schottky diodes, and even then it will be enough to result in significant non-linearity).

                                        I would suggest using the unrectified output across two of the motor connections into a moving iron meter. A variable resistor can be used to calibrate the meter and it will give a linear response from 0 to whatever you want.

                                        Neil

                                        Not relevant in this case Neil – That's why I said use a low KV motor – it generates a higher voltage at low speeds – and the measured engine surely will not run well below 200rpm or so..A 300KV motor will easily register on the voltmeter at 300rpm through the diodes..

                                        Joe

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