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Small Brushless Motors - can they generate?

If driven, do they produce AC?

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Joseph Noci 110/08/2018 07:21:15
397 forum posts
718 photos

Tim, apologies for my diatribe on the big alternator in my unmanned aircraft - that obviously was not applicable to your request at all, but I suppose if you do not give enough info in the initial request as to your intended application, anything goes...

As many have said, a small DC motor is the simplest, easiest and most trouble free for your needs - there are some with ball bearings - Ebay, Banggood, etc..

Sorry..

Joe

Edited By Joseph Noci 1 on 10/08/2018 07:21:46

XD 35110/08/2018 09:07:01
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1115 forum posts
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There is a  recent video on the great scott channel  on youtube where  he tested the efficiency of , stepper, brushless and brushed motors as generators .

Tried to embed the video here but was couldn't get it to work , i will have to search on the forum as im sure the answer is there .

Edited By XD 351 on 10/08/2018 09:21:22

Edited By XD 351 on 10/08/2018 09:22:24

Les Jones 110/08/2018 09:25:17
2041 forum posts
141 photos

You could use ignition pulses taken from the contact breaker and feed them into a frequency to voltage converter IC (Such as an LM2917) and connect the output from that to your voltmeter.

Les.

XD 35110/08/2018 10:30:56
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Is the output of a generator or alternator purely linear in relation to the rpm ? Probably not .

I think Les has hit the nail on the head , you could also use an inductive pick up off the coil lead to drive it either way a wire is easy to hide , the only glitch i see is scaling and this would depend on how accurate you want the tacho to be .

With such a car wouldn't you want the least conspicuous fit out ? Does it have a generator on it ? If so does the generator have a cooling fan or impeller ? If yes could you use a proximity sensor to pick up the fan / impeller blades ?

I wonder how one would go using a micro to convert a voltage derived from a frequency to voltage converter to a pwm and drive the meter from that ? May help to make the meter reading track the rpm better .

Edited By XD 351 on 10/08/2018 10:32:25

SillyOldDuffer10/08/2018 10:37:19
3512 forum posts
687 photos

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

tacho555.jpg

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.

tacholm.jpg

Dave

 

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

duncan webster10/08/2018 10:47:02
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1776 forum posts
31 photos

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

XD 35110/08/2018 11:18:06
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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 !

SillyOldDuffer10/08/2018 11:57:31
3512 forum posts
687 photos

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

SillyOldDuffer10/08/2018 12:04:12
3512 forum posts
687 photos
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

Circlip10/08/2018 12:19:38
898 forum posts

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

Regards Ian.

Tim Stevens10/08/2018 14:03:09
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943 forum posts

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 Stevens10/08/2018 14:17:24
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943 forum posts

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 311/08/2018 20:34:57
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95 forum posts
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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 Lawes11/08/2018 20:52:57
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225 forum posts

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 Wyatt11/08/2018 21:07:16
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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!

SillyOldDuffer11/08/2018 21:12:36
3512 forum posts
687 photos

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 313/08/2018 09:58:41
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95 forum posts
2 photos
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 Stevens13/08/2018 11:08:46
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943 forum posts

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 Gilligan14/08/2018 11:49:29
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12061 forum posts
525 photos

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 Stevens14/08/2018 12:05:43
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943 forum posts

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

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