Small Brushless Motors - can they generate?

Hello, me again

I need a small generator - about the size of a bicycle dynamo - to be driven at up to 5000 rpm. The web is overflowing with drone and toy motors - and I wonder if I could use one to work 'in reverse' - generating a small output (AC, probably - this would be fine.) I can change wiring etc, if necessary.

Regards, Tim Stevens

Yes Tim, I know one of the members of Malden & District ME uses 3 in his gas turbine powered locomotive.

See  also https://www.youtube.com/watch?v=PwJ-Q-NfZg0

Edited By KWIL on 09/08/2018 15:46:10

Just tried flipping a 4-wire stepper motor as used in a printer and it does make volts. I'd have to drive it with another motor and put the output into a load to see if it's efficient, but I don't see why it shouldn't be. As stepper motors are multi-pole the AC will be high frequency compared with an ordinary alternator, which might not be acceptable.

Any motor with a permanent magnet should generate, not sure how well or for how long though. I tried spinning the motor out of a dishwasher pump with a hand-drill once and that lit a small lamp on the bench. I didn't investigate further.

Well worth a try.

Dave

Tim,

What voltage are you looking for?

The kv of the motor will be determined by your requirements.....for example, a motor with a kv of 1000 will generate 1v when spun at 1000rpm. Other attributes of the motor have a bearing on how much current you want too....

Edited By John Rudd on 09/08/2018 16:44:53

If you want to keep it simple just use a bridge rectifier on two of the three output wires. It's not the most efficient way of doing it but its the most basic.

If you don't mind making things very slightly more complicated then a 3 phase rectifier can be wired up, its very simple but not near as simple as option 1!

**LINK**

Obviously this gives you unsmoothed DC, so depending on what you want to use it for you might want to add smoothing capacitors.

Tim, a brushless Rc type motor works very well as an alternator. In one of my big unmanned aircraft I use a 110cc twin boxxer engine and have fitted a large 220KV motor as alternator to power on-board electronics. The motor is about 70mm diameter and 90mm deep. At 5000RPM I get 18volts at 42amps max, and near 21volts at 5 amps.

Simple 3 phase bridge and a 1000uf capacitor gives very clean unregulated DC.

Not sure what voltage you require, but find a motor around 40mm to 50mm diametr, similar in length, divide the RPM by the desired voltage which will give you the KV motor that should be ok.

If you can indicate your required volts and amps, I can give better guidelines..

Joe

Dave,

Your ingenuity never fails to amaze me ! Only you could think up something like that !

Keep them coming !!

The application is to add a rev-counter to a vintage motorcar - an Essex of 1918, and six volt. What I need is a simple alternator driven by the engine (from the fan-drive pulley, perhaps) and the output will go to a voltmeter. The current needed is minimal, and the voltmeter can be chosen (or easily modified) to make the reading sensible. Of course, the scale will need redrawing, but that is easy, once we have run a test. So, ideally, a device where the voltage output (driving only a voltmeter) is proportional to rpm, and ideally, with ball bearings that should last long enough at perhaps 3000 miles per year. And the max revs of the Essex engine is about 3000 (yes it is that old) so there is no need for a device to run at F1 speeds.

Why not do it the old-fashioned way? Well, running a cable drive is complex, and long, and things get in the way. So, a drive using electrical wires is much easier and should waste less power.

My main concern is to know whether the switching circuitry of a drone motor is in the motor or separate, and from what you say the answer is separate, and so winding connections are accessible without destroying stuff.. So, I will invest and see how we go.

Thanks, everyone. Every question answered - wonderful.

Cheers, Tim

Tim,

Given your criteria, it would seem a small 100 watt motor with a kv of 500- 600 would give you the required ac volts over the anticpated engine speed( for a voltage range of 0-5v if my maths is right...) ....generally brushless motors come without any electronics.... The three wires are brought out into 'the wide world'

Edited By John Rudd on 09/08/2018 17:40:52

In that case I'd dump the fan driven alternator in favour of a Hall Effect or Optical Sensor. These sensors produce a voltage pulse each time they're triggered as the engine rotates. In the usual arrangement the pulses are counted and the rpm displayed on a digital display completely inappropriate for a vintage car.

However, you could dump the digital display and instead convert the pulses to a voltage level by integrating them with a capacitor. (The output voltage is proportional to the pulse rate. ) Simple circuit - if I can find the book, I'll post and example later.

A similar circuit uses a wire near the distributor input to inductively detect spark events. These also produce pulses at a rate proportional to rpm that can be integrated to drive an analogue voltmeter.

Dave

I don't think it's quite that simple is it Dave? If you just let the HE or opto device generate a pulse then the pulse width and rate are both inversely proportional to speed, so the average is constant. You need to allow each sensor pulse to trigger a constant width pulse T, so when you integrate those the average is proportional to rate up to the point where T is equal to the internal between sensor pulses. Still a simple circuit though.

Silly-old-duffer, John Haine

I'm not sure I understand your point(s) - I am looking at a device which moves wires through a (permanent, unchanging) magnetic field, or more likely, moves the field around the wires. Nothing is done with the output except supplying a (high resistance) voltmeter. So, doesn't Faraday's Law apply in any event, without creating pulses and then turning them back into a proportional output - the voltage from the alternator relating directly to the rate of cutting of lines of force? No generation of pulses, no average, no integration, and the only circuit is a few rectifiers and perhaps (but I doubt it) a smoothing capacitor.

And for the avoidance of doubt, yes, I know there will be a small non-linearity because of the diodes, but I'm sure we can take account of that in designing the scale for the voltmeter.

So, what am I missing, please?

Tim

Yes you're right John - it's not as easy as I first thought.

I found a circuit that fixes the problem with loads of discrete components - far too complicated for what it does.

Still looking, I half remember a simple circuit that uses an NE555 to generate constant width pulses from the input. All I have to do is find it in the muddle I call a study!

Dave

Any ball raced DC motor will be fine the voltage required is tiny.

My dad made an anemometer using an ordinary small DC motor and it worked for about 20 years, with nothing more clever than a variable resistor to calibrate it.

Using two wires of a brushless motor will be fine.

Use a 50mV or 100mv fsd meter movement and you won't be short of volts.

Neil

Hi Tim,

We're discussing a simpler way of generating the rpm voltage that doesn't need the car to drive an alternator. The advantage is elimination of the need for a £ brushless motor and belt drive. Nothing wrong with your idea apart from there may be a cheaper easier alternative

At it's simplest, when a stream of DC pulses are used to charge a capacitor through a resistor (Vin) , the DC voltage across the capacitor at Vout is proportional to the pulse rate.

So if a sensor is arranged to emit a pulse on each rotation of your cars flywheel, then the pulse rate will be low when the engine is idling and the capacitor will only partially charge causing the voltmeter to read low. When the engine is accelerated the pulse rate increases, causing the capacitor to collect more charge. Then the voltmeter reads a higher voltage.

As the voltage appearing across the capacitor is proportional to engine rpm, the voltmeter can be calibrated directly in rpm.

John's pointed out it's not quite that simple because the input pulses all need to be the same width. That's achieved with some simple electronics. At least I think it is! I'm hoping to find a tested circuit rather than design one from scratch.

Dave

+1 on Neil's suggestion of an ordinary permanent magnet DC motor with decent bearings for this RPM sensing job. One out of a toy car or even a hair dryer made in the last ten years is probably made by Johnson in China and will probably work for a very long time. Connect it to a suitable DC voltmeter, make a new scale background for it displaying rpm, and there's your tach.

You may be able to find an old style bicycle generator driven off the bike tire - the old style of these make a DC output varying with speed.

I would not go for complex circuitry or high tech brushless motors for this job, you do not want your leisure time in the Essex car cut short by troubleshooting high tech circuits and over-complicated gadgetry gone Tango Uniform.

I used to get pupils in O level and A level Physics classes to connect a bike dynamo to an oscilloscope when showing them how to use a scope - the output is AC and roughly (VERY roughly!!) sinusoidal not DC. The faster you spin it the higher the frequency of the output obviously.

As Jeff suggests almost any small PM motor will do the job (generate a voltage to drive an indicator). I would suggest looking at the meter/display end of the system and see what you need to drive that.

If you use a moving coil meter it will have a FSD of milliamps at most (robust ones used in car dashboards before the introduction of digital a stepper motor driven displays were about 10 or 20mA FSD). Even the smallest motors will require a series resistor to reduce its output and not burn out the meter coil.

If there is a requirement to conceal the motor (its a 'Tacho Generator' in this application) you could recess a small motor in the cavity bored into the end of some convenient rotating engine part. Even a motor out of a cellphone vibrator (4mm diameter) would have enough weft to drive a meter. Certainly no need for a motor anyway near the size you were considering and the only advantage of a stepper or brushless motor would be the elimination of brushes, in practice even a cheaply made brushed motor would last a lifetime as its doing virtually no work.

Ian P

wire the 555 as a monostable triggered by the Hall effect, then you get a mean voltage proportional to speed, no need for capacitors, the meter can't move that quickly so it smooths it out for you.

See **LINK**

for more info. Set the pulse length so that at max engine revs there is still a small space between, then have a series resistor to calibrate the meter

Edited By duncan webster on 09/08/2018 22:29:16

Must it be an analog gauge to fit in with the original decor, or could the owner tolerate a digital display?

For about $10 on the net you can buy a little digital tacho display unit the size of a matchbox that is triggered off a wire wrapped around one sparkplug lead. It runs off its own internal AA battery so no problem with a 6v car system etc. There are probably analog units out there that operate the same way too.

Edited By Hopper on 10/08/2018 02:29:41