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Old 25th Dec 2022, 2:15 pm   #41
kalee20
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Default Re: Bicycle Dynamos

I have stripped a Dynohub apart with the magnet on a steel plate, but it lost some of its power all the same.

I have a squat cylinder of mild steel, of the full thickness of the magnet, and a close fit inside, machined to size for me by a local machine shop. Whether it's any better remains to be seen!

More significantly, does anyone know how easy it would be to remagnetise? I'd imagine a close-fitting insert, with slots and wound individually (a bit like a motor's rotor), hit by a pulse of current... but the devil's in the detail. I get the idea that the magnet steel has a high remenant flux density, but not a very high coercive force (else it wouldn't demagnetise so readily) - and therefore should be amenable to remagnetising.

Incidentally I have stripped apart a 3-speed Dynohub, and it's not much different from the non-dynohub 3-speed. The dynamo is completely separate, no cross-contamination, the 3-speed 'works' bulges into the hollow space in the middle of the dynamo windings.
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Old 25th Dec 2022, 2:28 pm   #42
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Default Re: Bicycle Dynamos

If 'magnets' are demagnetising after dismantling, then surely they're not very good magnets in the first place?
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Old 26th Dec 2022, 1:31 am   #43
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Default Re: Bicycle Dynamos

Quote:
Originally Posted by LyntonP View Post
Sturmey Archer actually supplied a keeper as an accessory.
One hub I never had was the type that was combined with the 3 speed gear mechanism for the rear wheel. I bet that was fun to service!
Lynton
But not quite as much fun as the 4-speed one I played with as a teenager in the '60s. ISTR that there were specific timing setup instructions for the gear sets in those. I never did dismantle the dynohub part itself having no idea where a keeper might be obtained.
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Old 26th Dec 2022, 11:15 am   #44
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Default Re: Bicycle Dynamos

Quote:
Originally Posted by stevehertz View Post
If 'magnets' are demagnetising after dismantling, then surely they're not very good magnets in the first place?
As Peter said it is probably the nature of the steel magnet, a ferrite component would be able to hold its magnetism better but from a standpoint of being weaker by unit volume in the first place. The same effect happens if early steel speaker magnet assemblies are removed from their pole pieces. The convenience of not having to use a keeper for a rare disassembly would probably not justify the cost of a rare earth magnet.

The charging electromagnet for these units is normally a cylindrical coil with pole pieces to match the stationary stator, I did have one of these magnets re energised for my Phillips roadster through an acquaintance at a cycle shop who sent off the unit, though it had to be reassembled by the people who did the work, it wasn't particularly cheap but was very effective, and about 8 years ago it is still good. I assumed that a previous dismantler had failed to follow instructions about the keeper. You would need to be a very enthusiastic amateur restorer to warrant creating the coil and stator to do this job, however I have been told that rewinding an old stator with heavy gauge wire and feeding it with a high current low voltage DC will get the job done, though I have never since had the occasion to try.

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Old 26th Dec 2022, 3:15 pm   #45
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Default Re: Bicycle Dynamos

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Originally Posted by G.Castle View Post
I have been told that rewinding an old stator with heavy gauge wire and feeding it with a high current low voltage DC will get the job done, though I have never since had the occasion to try.
It's the sort of thing I'd wondered... it might be a Spring project! A big capacitor, a chunky SCR, an antiparallel diode to allow the current to freewheel and decay gracefully, and a series diode to stop it reversing in a decaying oscillatory manner with circuit capacitances and undoing the good work...

The whole thing could be immersed in an oil or paraffin bath to keep the coil cool. I suspect that the current would pull the magnet poles into alignment so it wouldn't need clamping, though if anyone declared that this is wrong and the current would cause repulsion, I'd listen... I do occasionally get my phasings and polarities completely reversed
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Old 26th Dec 2022, 7:42 pm   #46
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Default Re: Bicycle Dynamos

I was asked the question, "Can you ensure the lights stay on when I stop at traffic lights on my bike", I answered "Yes".
I used a four NiCad cell pack, a blocking diode, a zener diode and a charge limit resistor fitted in a small box that Velcro fitted under the saddle.
Worked on dynamos and alternator hubs as well.
At rest after a 2 minute charge period, it lasted about 5 minutes before fading out and the batteries lasted a long time. Not sure Lithium would ... they don't like being left flat.
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Old 27th Dec 2022, 12:49 pm   #47
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Default Re: Bicycle Dynamos

In my other interest you are warned that dismantling a motor is at your peril, unless you have either a re-magnetiser to hand, or know someone who has one!
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Old 27th Dec 2022, 2:27 pm   #48
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Default Re: Bicycle Dynamos

A keeper is traditionally 'soft iron' which I take to mean that it doesn't retain any magnetism after it's removed. (Because if it DID.....this force would surely be deducted from the magnetism of the permanent magnet)

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Old 27th Dec 2022, 3:12 pm   #49
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Default Re: Bicycle Dynamos

Normally magnetically 'soft' material are used for keepers yes, but you do not 'use up' a magnet in the way you do say a battery, a strong permanent magnet made with a low remanance material, will lose its strength easily though when it's poles do not have a magnetic path through some form of keeper.

The keeper is Normally magnetically soft but is only a convenience for re use, two suitable magnets can be used to 'keep' each other, N to S, N to S, for instance. There is a protocol how a keeper should be removed from a magnet too I understand. It should be slid sideways across the lines of magnetic force rather than pulled away along them, to preserve the magnetism.
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Old 27th Dec 2022, 5:08 pm   #50
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Default Re: Bicycle Dynamos

In general it's best if you can use the keeper to displace what was normally in contact with the magnet poles (the stator of the cycle dynamo, for example) and vice versa.

When I dismantled the motors in the TS05 magnetic tape drive on my VAX11/730, I turned a keeper from steel rod. The outside diameter was much the same as that of the motor's armature. I drilled a hole down the middle to fit round the spindle and turned a recess in one end to fit over the armature windings. Then I removed the end plate without the brush gear, fitted the keeper over the shaft, and pushed the armature and other end plate out using the keeper. Then I could clean the commutator and check the brushes and ball races for wear, leaving the keeper inside the field magnet. Reassembly was much the same, fit the armature to the brush gear end plate, insert it into the keeper (still inside the field magnet, push the armature home, ejecting the keeper, Then fit the other end plate.

The worst part was measuring up the size to make it without dismantling the motor. A scrap motor of the same type would have made life a lot easier.
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Old 28th Dec 2022, 1:15 pm   #51
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Default Re: Bicycle Dynamos

Quote:
Originally Posted by JonSnell View Post
I was asked the question, "Can you ensure the lights stay on when I stop at traffic lights on my bike", I answered "Yes".
I used a four NiCad cell pack, a blocking diode, a zener diode and a charge limit resistor fitted in a small box that Velcro fitted under the saddle.
Worked on dynamos and alternator hubs as well.
At rest after a 2 minute charge period, it lasted about 5 minutes before fading out and the batteries lasted a long time. Not sure Lithium would ... they don't like being left flat.
I used an oil/carbide combination to achieve the same effect...
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Old 3rd Jan 2023, 4:04 pm   #52
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Default Re: Bicycle Dynamos

A phoenomenon I experienced as a kid with the bottle dynamo off my bicycle was that when I touched the case and terminal and ran it along the coal bunker top I got a belt: a mild, telephone-magneto type tingle, but still a belt.

I put this to good effect by connecting the dynamo to two aluminium Hoover tubes and getting my friends* to hold them, then running the dynamo quickly along the coal bunker top.

The dynamo only produced 6 - 8 Volts on the bike in normal operation, so whether it was the lack of loading or the frequency that made us more sensitive to feeling it, I don't know.

This wasn't the dynamo I analysed in Post 21 but was a British-made Miller model identical externally.


* They were still my friends afterwards!
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Old 3rd Jan 2023, 4:12 pm   #53
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Default Re: Bicycle Dynamos

How good a sine wave would it have produced? I seem to recall it being said that the spikiness of the output from a telephone magneto "enhances" the shock value.
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Old 3rd Jan 2023, 4:52 pm   #54
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Default Re: Bicycle Dynamos

Quote:
Originally Posted by Dave Moll View Post
'How good a sine wave would it have produced?'
I never did find out, Dave, as that particular dynamo got lost or mislaid or thrown out yonks ago. The modern Miller dynamo I took to bits in the last decade was indeed sinusoidal, though. But I suspect it has a more modern magnet (cylindrical and looks ferrite-y) than they used to have.

Quote:
Originally Posted by Dave Moll View Post
'I seem to recall it being said that the spikiness of the output from a telephone magneto "enhances" the shock value.'
The spikiness certainly enhances the 'ting' value, and a type 59A bell movement is far more sensitive on a peaky magneto output than on a sine wave, so rate-of-change L (Di/Dt) would be quicker, hence the enhanced shock value. If I find an old bicycle dynamo I'd like to open it up and compare with the new one.
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