Small Brushless Motors – can they generate?

[SillyOldDufferModerator @sillyoldduffer]

For completeness, this is the NE555 circuit I was thinking of. It's from Don Lancaster's TTL Cookbook.

Actually I prefer Les Jones' suggestion which uses a purpose made chip and is simpler. This is the LM2917 circuit recommended by Texas Instruments in their Application Notes.

Dave

 

Edited By SillyOldDuffer on 10/08/2018 10:40:26

[duncan webster 1Participant @duncanwebster1]

Why not use a signal from the low tension side of the coil feeding either 555 or LM2917. It might be very spiky, some-one will know

The FET in SOD's 555 circuit looks a bit superfluous, a 555 will output 200mA, enough to blow the cover off a little meter.

Edited By duncan webster on 10/08/2018 11:05:58

[I.M. OUTAHEREParticipant @i-m-outahere]

Thats is what Les is talking about by picking up a signal from the points .

I was just thinking about how a micron data logger picked up its rpm signal on a junior dragster , it is just a piece of wire wrapped around the plug end of the HT wire , this is used to give an inductive pick up for the tacho .

Plenty of ways to clean up the signal , schmitt trigger rings a bell but i'm sure someone will know better than i !

[SillyOldDufferModerator @sillyoldduffer]

Posted by XD 351 on 10/08/2018 11:18:06:

…

Plenty of ways to clean up the signal , schmitt trigger rings a bell but i'm sure someone will know better than i !

One of the many things I like about LM2917 solution suggested by Les. The chip cleans up the signal with a built in comparator so you don't have to. Excellent!

Dave

[SillyOldDufferModerator @sillyoldduffer]

Posted by duncan webster on 10/08/2018 10:47:02:

…

The FET in SOD's 555 circuit looks a bit superfluous, a 555 will output 200mA, enough to blow the cover off a little meter.

Edited By duncan webster on 10/08/2018 11:05:58

I thought so too and a little digging reveals it's not strictly necessary. What the FET does is allow the meter to be calibrated so that max rpm registers at full-scale thus getting best use out of the meter's scale. A useful touch if you don't mind adding the extra components.

Dave

Edited By SillyOldDuffer on 10/08/2018 12:05:28

[CirclipParticipant @circlip]

Seem to remember that Jaguars had a generator fastened to the end of the camshaft driving a voltmeter for Rev counter.

Regards Ian.

[Tim StevensParticipant @timstevens64731]

Well, this has been an interesting debate – hasn't it?

The problem with some of your answers relates to my rather basic grasp of electronics, and the owner of the car himself is way behind me in this respect. So, we want something we can understand. Secondly, the car has very limited dynamo output, and negligible voltage regulation (being an early 3-brush system), and is a six volt system. So, any electronic device would need to be extra clever to cope with variable voltages, etc, and use next to no electricity. The intention was to make an independent (self contained) system, which could easily be removed if needed. And the option of a coil or HT lead pick-up system seems only to be available for 12volt systems (and expensive).

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

Edited By Tim Stevens on 10/08/2018 14:09:40

[Tim StevensParticipant @timstevens64731]

XD 531 asks about the generator – Yes, there is one, 6 volt, three-brush, and like all vintage and pre-war dynamos (so far as I know) it is completely enclosed. No cooling, no fan – because the roads were very dusty back then.

But – the rotating drive is accessible and one option is to add a rotating magnet or two, with a pick-up coil nearby, as he suggests (but my question was exploring a different option).

Cheers, Tim

[Nick Clarke 3Participant @nickclarke3]

Posted by Tim Stevens on 10/08/2018 14:03:09:

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.

Sorry but no – for that to work with any accuracy you would need two 'selsyn' type devices.

 

As to why not consider these two cases:
Firstly turn the 'sender' motor slowly – it will generate little or no electricity and so the 'receiver' motor is unlikely to turn at all.

As an other example if the motors are designed to run on say 5 volts at 3000rpm spinning the 'sender' motor very quickly may generate more than 5 volts and damage the 'receiver' motor.

 

Between these two extremes the results are unlikely to be linear and from 0 rpm to max rpm definitely not.

[Jon LawesParticipant @jonlawes51698]

Just to further complicate things, remember that older cars especially have very "noisy" ignition systems, from an EMC point of view. The simpler the circuitry the less likely this is to be an issue.

[Neil WyattModerator @neilwyatt]

Posted by Nick Clarke 3 on 11/08/2018 20:34:57:

Sorry but no – for that to work with any accuracy you would need two 'selsyn' type devices.

 

Reality check, I doubt a genuine vintage mechanical rev counter head is going to win any awards for accuracy!

[SillyOldDufferModerator @sillyoldduffer]

Hi Tim,

I said in an earlier post ' Nothing wrong with your idea apart from there may be a cheaper easier alternative'.

Rejecting electronics takes us back to the beginning. The answer is if you take almost any permanent magnet motor and spin it as a generator, it will generate a voltage proportional to the speed. Exactly what comes out will depend on the type of motor used, but I think they will all make enough volts to work a meter. The power output, efficiency and waveform are all irrelevant because you're going to calibrate the scale, which is unlikely to be linear but so what. All the system has to do is crudely move a needle, which is low-tech.

I'd get hold of a motor of a suitable physical size to fit the car, hold it temporarily in a vice, connect a multimeter, and then drive the spindle with a hand or electric drill. That'll give you good indication of how the motor would work as a generator attached to a real engine, ac or dc, and also the maximum number of volts the voltmeter needs to measure.

Dave

[Nick Clarke 3Participant @nickclarke3]

Posted by Neil Wyatt on 11/08/2018 21:07:16:

Posted by Nick Clarke 3 on 11/08/2018 20:34:57:

Sorry but no – for that to work with any accuracy you would need two 'selsyn' type devices.

 

Reality check, I doubt a genuine vintage mechanical rev counter head is going to win any awards for accuracy!

Agreed the magnetic heads which came in during the 1950s were notoriously inaccurate, but I assumed that a chronometric head was being referred to. These are usually accurate but are far more expensive to produce and don't give an instantaneous reading. They measure the revs, display the result measure again etc etc, the needle moving in a series of steps.

[Tim StevensParticipant @timstevens64731]

The magnetic eddy-current devices as speedometers (and rev-counters) were used in the vintage era, too – notably by Stewart Instruments. But the main point in relation to not 'keeping up' well from very low speeds is not relevant here for two reasons:

1. The engine (like all IC engines) will not run at very low rpm – that is why we have a tick-over, around 500 – 1000 rpm.

2. The only time that you really need an rpm meter to work well in this sort of use is at high speed, so low speed inaccuracy and very low speed failure is of no account.

These factors are recognised in law – which requires a speedo to work above 10mph and to be accurate within 10%.

Regards, Tim

[Michael GilliganParticipant @michaelgilligan61133]

Tim

This looks like a bargain-priced motor, probably suitable for your requirement: **LINK**

https://www.ebay.co.uk/itm/DC-5V-12V-16-Pole-Three-Phase-Brushless-Motor-External-Rotor-Car-Quadcopter-RDR/153087819158

MichaelG.

[Tim StevensParticipant @timstevens64731]

Thanks, Michael

I have already ordered from a similar source a couple of a slightly different version at not much more money, so I will have a play with them first. Meanwhile I do feel that these little devices could be of more use to earth-bound modellers (and vintage motorists) than they seem to be so far.

Regards, Tim

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Tim Stevens on 14/08/2018 12:05:43:

… Meanwhile I do feel that these little devices could be of more use to earth-bound modellers (and vintage motorists) than they seem to be so far.

.

Agreed !

MichaelG.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Tim Stevens on 10/08/2018 14:03:09:

Well, this has been an interesting debate – hasn't it?

The problem with some of your answers relates to my rather basic grasp of electronics, and the owner of the car himself is way behind me in this respect. So, we want something we can understand. Secondly, the car has very limited dynamo output, and negligible voltage regulation (being an early 3-brush system), and is a six volt system. So, any electronic device would need to be extra clever to cope with variable voltages, etc, and use next to no electricity. The intention was to make an independent (self contained) system, which could easily be removed if needed. And the option of a coil or HT lead pick-up system seems only to be available for 12volt systems (and expensive).

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

Edited By Tim Stevens on 10/08/2018 14:09:40

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

[Ian PParticipant @ianp]

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

not

[Ian S CParticipant @iansc]

This is a brushless alternator I made from an old shaded pole motor. A new claw type armature with ferite magnets(ex speakers). The shading rings have been cut.

A brushless motor will give you an AC current, if you ant DC direct from the machine, you need a brushed motor.

Ian S C

Edited By Ian S C on 04/09/2018 13:55:16

[SillyOldDufferModerator @sillyoldduffer]

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

…

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

Ian P

I think it does work as a generator! At the beginning of the thread I said "Just tried flipping a 4-wire stepper motor as used in a printer and it does make volts."

This type of brushless motor contains a permanent magnet. Turning the axle moves the magnetic field relative to the coils and generates a species of AC on each pair of windings.

No idea how efficient a generator it is. Nor have I confirmed a brushless motor driven as an alternator has enough oomph and the right waveform needed to turn another brushless motor connected to it. If I can find a matching pair of motors I'll give em a whirl.

Dave

Edited By SillyOldDuffer on 04/09/2018 14:17:08

[Ian PParticipant @ianp]

Dave

Whilst a stepper motor has no brushes its not what most folk (or industry) would call a 'brushless motor'. I dont think I'm being pedantic in pointing that out.

Mechanically driven standard (but usually only small ones) are sometimes used as generators in things like radios or torches.

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.

Ian P

[Tim StevensParticipant @timstevens64731]

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

[Tim StevensParticipant @timstevens64731]

Just to be quite clear, the 'clump of electronics' I refer to above is wired to the motor, but is not a part of the motor assembly. I should have said 'With a drone motor' , not 'In …'

Tim

[Neil WyattModerator @neilwyatt]

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

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