Stuart Dynamo

[Steve RowbothamParticipant @steverowbotham77083]

Does anyone have experience of making the Stuart dynamo, I have just finished machining / assembly but am getting very little output (approx 100mV).

The concentricity and air gap seem fine, the armature spins freely by hand with magnet not fitted.

 

The only issue is that the pole pieces were not quite molded in symmetrically which has resulted in a thin layer of gun metal on the right hand side when bored to the specified diameter (as required to achieve the correct air gap).

 

 

The electrical connections are good (ish), but the magnet does seem very weak.

Just wondering if anyone else has been here before?

 

 

 

[SillyOldDufferModerator @sillyoldduffer]

Wild guess: are you spinning it fast enough?  Generators like to run fast, with one source giving 4V at 3000rpm from a Stuart.

Dave

[Steve RowbothamParticipant @steverowbotham77083]

good thought Dave, I ran it at about 2000 rpm using a drill so should have had over 2V

[noel shelleyParticipant @noelshelley55608]

If the magnets are cast in as appears to be the case then unless magnetised after casting they will have been taken above the Currie point and have little magnetic force. Small permag motors and alternators normally have the magnets clipped or glued in to a casing. An old bike dynamo makes an excellent unit for such jobs, the type that rubs on the side of the tyre. Noel

[Steve RowbothamParticipant @steverowbotham77083]

Noel, you are correct, the separately supplied permanent magnet fits into a recess machined into the bottom of the casting where it attaches to the cast-in pole pieces

 

 

 

[noel shelleyParticipant @noelshelley55608]

AH ! Now I understand. An interesting way of doing it, all be it not I fear a very efficient way to generate electricity. It might be worth trying to find a pair of half moon ceramic magnets of the right ID x thickness and mill out the pole pieces. Araldite will hold them in. Noel.

[Michael GilliganParticipant @michaelgilligan61133]

I have no idea whether this will be of an use or interest to you, Steve … but we did have a go at understanding the contraption a while ago:

https://www.model-engineer.co.uk/forums/topic/stuart-dynamo-rotation/

MichaelG.

[Speedy Builder5Participant @speedybuilder5]

Silly question, have you tried running it as a motor?  If it won’t spin with a few DC volts applied, and just flicks to a fixed position, then I would suspect that there was a problem with the windings or angular position of the commutator.

Bob

[Speedy Builder5Participant @speedybuilder5]

Ah, just read the old link – its an A/C dynamo but still, applying a few volts should make the armature flick to a stable position ??

 

[JasonBModerator @jasonb]

Make sure your meter is set to AC volts

The old stuart description says it will make a lamp glow when rotated by hand so you should be seeing something at your 2000rpm test speed.

[John HaineParticipant @johnhaine32865]

A thin layer of non-magnetic metal over a pole piece probably won’t do much for it’s efficiency…

[JasonB]

On JasonB Said:

Make sure your meter is set to AC volts

…

Fingers crossed that’s the reason.   Maybe on a DC setting the meter is reading 100mV because of the protective diodes.

I assumed a ‘dynamo’ would output DC from a commutator and brushes.   Despite being trained to know that assumptions are always dangerous!  Ho hum!

In the olden days 6V bicycle lamps were good for testing small dynamos because they light fully on 40mA (rear light) or 60mA (front).   Ordinary 3V torch bulbs are much less sensitive, needing 300mA for full brilliance.  I don’t know if filament cycle bulbs are still available, LEDs being much better,

If not AC, my other nasty overnight thought was shorted turns in the winding.  As far as I know there isn’t a simple way of detecting them without dismantling.   Can be done with a Time Domain Reflectometer, a tool rarely found in a home electronics lab.  Though a basic TDR could be home-made, an oscilloscope is needed as well.  Too much fuss.

Dave

[duncan webster 1Participant @duncanwebster1]

I agree with JH that the mom magnetic coating to the pole piece won’t be helping, at the very least it will increase the effective air gap in the magnetic circuit. The OP mentions that the magnet seems weak. Is it an old alnico type that has got tired? The link MG provided mentioned not removing the rotor without fitting a keeper, which you wouldn’t need with a modern magnet. Not sure how you test a magnet, but someone here will know. There was an article way back in around 2005 in ME about making a remagnetiser, but finding a modern magnet that would fit would be a lot easier. I wonder if a motor reeinder would have a remagnetiser?

[noel shelleyParticipant @noelshelley55608]

With the out of centre poles the output may be interesting to view on an oscilloscope, since as has been said the bronze over the pole will effectively be an air gap to the magnetic surface. IF the bar magnet is not very strong it might be worth finding a ceramic magnet of the right size DO NOT try to cut or break one to make it fit. The surface the magnet sits on needs to be as flat as possible to not introduce another air gap. An other point will be the centrifugal force on the rotating coil do not be tempted to run it at to higher speed, it may disintegrate.

You don’t say what you will drive it with but as it will be of low power and the genny of poor efficiency with AC output a simple bridge rectifier and a capacitor for smoothing + a limiting resistor to suit the voltage will let you use LEDS . Probably more impressive than a barely glimmering bulb ? Good Luck, Noel.

[Michael GilliganParticipant @michaelgilligan61133]

Purely for convenient reference … I am re-posting the Stuart advertisement dated 1965

.

.

MichaelG.

 

IMPORTANT EDIT:

Please also see here:

http://www.stuartturnersteam.com/Machines/Bottone/Bottone.html

This is a different one ^^^

A sanity-check please: What exactly is being built ??

[JasonBModerator @jasonb]

In the first photo you can see the boss at about 11 0’clock which is where the 300rpm bit is so it is the one in the old advert

[SillyOldDufferModerator @sillyoldduffer]

New worry from me after reading Michael’s screen clip of the Stuart advert, which Neil Wyatt noted in #525915

The Note says ‘On no account remove the Armature without first bridging the field magnet poles with a piece if iron.‘, and Neil said ‘I think it’s worth drawing attention to the warning not to disassemble the ‘device’ without bridging the magnet pole pieces first.‘

This may be because the magnet is of the self-destruct variety.   Magnetic chuck instructions used to warn users not to dismantle the unit.  I believe the problem is that designing Alnico magnets for maximum pull requires the poles be positioned in a way that causes the magnet to eat itself if the flux has to jump a large gap.  These magnets are OK when fitted inside a machine that limits the gap, but the magnetic circuit has to be bridged if the magnet is removed.  How quickly the magnet eats itself depends on the pole design:  anything from rather slowly on an ordinary horseshoe magnet to almost instantly on a high-performance set-up.

I believe this self-destruct behaviour is peculiar to Alnico alloys, and modern super-magnets just keep going.

Steve mentions his magnet being “very weak”. Is it possible it was left for some time without a keeper before being fitted?   If so, replace it.

🙁

Dave

 

[Michael GilliganParticipant @michaelgilligan61133]

Worth mentioning [I think] is that the 1965 advert appears to be for a ready-built item, not for a kit which Steve is presumably building.

MichaelG.

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More references:

http://stuartturnersteam.com/Machines/Dynamo/Dynamo.html

https://www.stuartmodels.com/product/stuart-dynamo-unmachined/

 

and this should be the list of spare parts for the current kit:

https://www.stuartmodels.com/product/stuart-dynamo/

 

[Robert Atkinson 2Participant @robertatkinson2]

Leaving aside switching the meter to AC   and the magnet strength the location of the pole pieces in the gunmetal casting is very poor and certainly not helping things.
A multimeter alone is not a good test because it provides no load on the generator (Dynamo is a imprecise term). Use a small torch bulb if you can find one or connect a resistor of about 100 ohms in parallel with your meter.

It does sound like a combination of all three things. A DC meter will give no meaningful reading. A weak magnet will also produce very low output.
I’d be looking at getting a modern rare earth magnet for a trial at least. Make up a couple of iron or mild steel pole pieces to make it fit across the cast in pole pieces. A cyindrical one with a diameter similar to the width of the existing Alinco one would do. Make sure it has poles on opposite faces and isn’t a holding type with poles across the face.
Was the kit a recent purchase from Stuarts or old stock from somewhere? If a recent purchase I’d be asking them for a new casting based on the poorly located pole pieces.

Robert.

[Steve RowbothamParticipant @steverowbotham77083]

Thanks all for the excellent responses, the device I have built is indeed the ‘Stuart Dynamo’ AC generator described in the 1965 catalogue provided by Michael, here it is!

I have made a little progress today, I stripped it back down and machined the magnet recess a little more to remove any vestiges of gun metal around the edges to ensure best possible contact with the poles, and also increased the tension on the Phosphor Bronze springs which make contact with the slip rings, and got an improved output:

 

 

The bulb is 6V and is glowing slightly. It is also nearly 60 years since it was last illuminated!

 

The gun metal ‘skin’ on the right hand pole is likely having a significant effect due to increasing the air gap as has been pointed out by others, it is quite noticeable when holding a calibrated steel rod between the poles, so this certainly won’t be helping. I did wonder about the positioning of the poles when I received the castings, but impossible to work out how it would end up before machining.

 

 

The magnet really does seem quite weak, I have experience of Samarium Cobalt magnets of similar size, and from memory they were a lot more powerful than the magnet supplied which can easily be pulled away from the pole pieces. Definitely an area for potential improvement, particularly when considering the air gap issue I have.

 

I’m also now wondering about the speed of the drill I was using to drive the generator, I am seeking to find a better defined arrangement to drive up to 3000rpm.

 

And to answer Noels point, I do intend to rectify the output once I can get the requisite Voltage out of it to drive LED ‘streetlights’ as part of a steamplant model.

 

On a final point, I decided to fully document the build of this because the other 3 Stuart kits I have built (Oscillator, 10V and Beam) all had excellent books of constructor notes available which I found to be of immense value as a novice. I couldn’t find any construction notes on the ‘Dynamo’, and so having spent quite a while working out how to hold and machine the castings to achieve the required concentricity I decided to write it up to provide a bit of guidance for future novices. I can also add to the notes the fun and games of ‘commissioning’ it!

[Michael GilliganParticipant @michaelgilligan61133]

Great progress, Steve !

“ One small step for Man … etc. “

MichaelG.

.

Please forgive me for mentioning it, but that casting looks disgraceful
… they should be ashamed.

[Michael GilliganParticipant @michaelgilligan61133]

Here’s a very short video about rebuilding an old one:

https://youtu.be/1R3sSemN5z4?feature=shared

The results look encouraging.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

You mention street lights (plural) so instead of a bridge rectifier you could have 2 leds in parallel but opposed polarity, each with a single diode. This will save you 0.6 of a volt and I doubt you will see any flickering as the frequency is quite high. If the generator peak voltage never exceeded 5v you could manage without the diodes, but I hope you’ll do better than that

[noel shelleyParticipant @noelshelley55608]

Michael has mentioned it and so will I ! Were that a new kit I would have sent the castings back to be changed, they are very poor, either the wrong sand was used or the ramming up was not hard enough. Now seeing the finished item one thought is that the wire pick up might be better done with a small carbon brush to avoid point contact and any resistance. This would probably need a pair of insulated brush boxes to be made.

Never the less it is good to see a project working. Noel.

[John HaineParticipant @johnhaine32865]

Were the magnets really cast in-situ?  They look like ferrite to me.

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