Electromagnets (Split From Stuart Dynamo)

[noel shelley]

MODERATOR NOTE (SillyOldDuffer).   This new topic is a split, first time it’s been tried, so fingers crossed!

To keep Stuart Dynamo on topic please use this one for related material.   For context, this one starts with posts copied from Stuart Dynamo.

On noel shelley Said:

Thanks to the OP Steve for starting this thread. Having read “The ABC of dynamo design” from V2 and V3 of ME 1899 and 1900, normally in bed and late I realise that A, the maths is very complexed and B, getting the materials in this day and age would be not easy !

…
Noel.

I’ve hit the same problem with my latest endeavour, which is a Morse Sounder.  This is a classic telegraphic device,  powered by an electro-magnet, that taps out morse as a series of clicks, not beeps.  Something like this:

It’s a simple form of electric motor, analogous to a beam steam engine used to work a pump, not rotary motion.

The problem is designing and building the electromagnet!  The Victorian original called for best Swedish soft Iron and Silk Covered magnet wire, both of which are difficult to source today.

In the 19th Century insulating Copper wire was a major problem, with a host of different materials in play, none of them ideal.   Paper is good until it gets wet.  Cotton OK, except bulky.   Shellac is excellent except it tends to crack, melts easily, and is hard to apply without leaving bubbles in it.  Silk is excellent, but expensive and double or triple layers are needed.  Rubber perishes, Gutta-percha, good but over consumption destroyed the source.  A good compromise for telegraphic, wireless, and other light work was shellac coated Copper protected inside a silk sheath.   As well as being a good insulator, the silk protected the wire from mechanical abrasion, which occurs every time a magnet coil is energised electrically, and also due to vibration from a running motor.

Silk covered wire has two major disadvantages:  it’s expensive, and – worse – it separates the wires physically, when, in a magnet, they must be as close together as possible.   So a lot of effort and research was put into alternatives, essentially replacing Shellac with tougher, less brittle synthetics that could take more heat, coupled with process improvements that don’t leave bubbles in the insulation.   Took about 50 years, but new-made Silk wire disappeared after WW2.

So, unless appearance is important, ordinary enamelled copper wire is fine.  Avoid second-hand wire because winding and unwinding it is likely to damage the insulation and to work-harden the Copper.  New wire de-risks magnet winding considerably!

Swedish Iron, or soft-iron of any variety, has become almost unobtainium.  Main use seems to be laboratory work, so not cheap.  The modern equivalent is ‘Electrical Steel‘, which contains Silicon rather than Carbon.  It’s a speciality alloy, not available in small quantities, and ‘our’ suppliers don’t carry it.  Plenty of it in old transformers and motors, but the laminations are cut into shapes I’ve not been able to re-use.

I’ve ordered an educational electromagnet in hope the core will be soft-Iron or Electrical Steel that I can repurpose, but I’m expecting it to be mild-steel.   Mild-steel is unsuitable for my project because it has high remanence, i.e. magnetism persists after the current is turned off, and low permeability makes it a poor choice  for motors and dynamos: inefficient!

If anyone knows of a source of soft-Iron or electrical steel in rod form, please let me know!

Dave

[SillyOldDuffer]

On SillyOldDuffer Said:

[…] If anyone knows of a source of soft-Iron or electrical steel in rod form, please let me know!

Dave

I ordered some [12mm diameter] yesterday, Dave

… supposedly ‘soft-iron’

I will let you know what turns-up

MichaelG.

[noel shelleyParticipant @noelshelley55608]

As A young lad DCC could mean Double Crank Compound, or if your interests lay with electrical matters it was Double Cotton Covered  and in the late 50s/early 60s was still available ! Now ?

As for movement between turns and damaged insulation potting will help but not always pretty.

For SPECIAL purposes ships sunk before 1945 are valuable for their steel as it has NOT been contaminated by atomic radiation. Scapa Flow here I come ! On that subject Many here would enjoy reading “Coxes Navy” or how a scrap man from the midlands who knew NOTHING about marine salvage, invented it, as he raised the German High Seas Fleet. Noel.

[JasonBModerator @jasonb]

I just used mild steel and lacquered copper wire for this one and it seems to work OK so should do for your Morse Project Dave

 

You could possibly try pulling an ATV coil apart as they buzz quite well. That is what I used for this buzz coil ignition

 

[JasonB]

On JasonB Said:

I just used mild steel and lacquered copper wire for this one and it seems to work OK so should do for your Morse Project Dave

 

You could possibly try pulling an ATV coil apart as they buzz quite well. That is what I used for this buzz coil ignition

 

Excellent video of another delightful engine.  Just right with breakfast coffee – I shall enjoy ‘Lecky’ again tomorrow!  The ignition coil is a good idea too.  Wonder if my local garage has any scrap…

I may be in fusspot mode with the sounder in that I’m trying to design it for effectiveness rather than just making a noise, and in hope of getting it to work first time rather than experimenting.  One concern is that the device shouldn’t mangle the timing of the morse signal.   Therefore the electromagnet and the return spring both have to take the same time to cause start and end clicks, and balancing the two may be tricky.  Easy enough to make a sounder that clicks merrily, but I’m not so confident about making one that doesn’t bias the signal such that it can’t be decoded by a human operator.   Difference between a model and an operational device.

Probably ought to start a topic on it.  Much interest in the aspects of the design that are intuitively “good enough”, and those, like the number of turns of wire needed to produce a given force, that need wet-towel brain work.

Dave

 

 

 

[JasonBModerator @jasonb]

Dave, if you can’t find it in rod form you have two other options.

1. A stack of “washers” cut from sheet or salvaged

2. Laminate strips again cut from sheet or salvaged and then turn them into a round rod. If you used a couple of oron rivits about 1.5mm dia you could turn almost round without cutting the heads away particularly if they were Csk

[noel shelleyParticipant @noelshelley55608]

In Jasons picture of the Gleichstrom dynamomaschine The part marked as Bh would appear to allow the brush holders to be rotated round the com  to find the spot that produces least arcing as commutation takes place. Interesting. Noel.

[Michael GilliganParticipant @michaelgilligan61133]

… also [it would appear] in the video that I linked

MichaelG.

[JasonBModerator @jasonb]

The “reaction compensator” is quite a common feature, one on an Alyn Dynamo

 

[duncan webster 1Participant @duncanwebster1]

Cores from old relays?

[Robert Atkinson 2Participant @robertatkinson2]

Duncan beat me to it.

Old industrial or telephone relay cores would be my go-to source for soft iron rod.
The other option is the core does not have to be round. A square or rectangular section core could be made from the “I” laminations from a a transformer. A polepiece cap could be used if it has to look round. A relay armature can supply metal for a cap.

 

[JasonB]

On JasonB Said:

The “reaction compensator” is quite a common feature, one on an Alyn Dynamo

 

Another good reason to use slip-rings and a rectifier if you want a efficent generator. Of course that does not give you the vintage look.

[JasonB]

On JasonB Said:

Dave, if you can’t find it in rod form you have two other options.

1. A stack of “washers” cut from sheet or salvaged

2. Laminate strips again cut from sheet or salvaged and then turn them into a round rod. If you used a couple of oron rivits about 1.5mm dia you could turn almost round without cutting the heads away particularly if they were Csk

Just glue laminations together and then machine to round. Leave a small flat on 2 opposite sides so the outer 2 laminations have some structural strength

[Keith Rogers 2Participant @keithrogers2]

As  this thread seems to have grown out of all proportion I thought some of you would find these ME articles of interest. The notes seem to cover what’s being discussed here.

I must admit I came across them by accident in the ME Index this morning  and have not looked at any of them  through lack of time.

Lammas

I hope this works!

Keith

[Michael GilliganParticipant @michaelgilligan61133]

Works a treat, Keith … thanks for the references

Now all we need is the text

MichaelG.

[JasonBModerator @jasonb]

Dave, we now seem to have generator posts in one that is meant to be about magnets.

At least with thread drift they all wash up in the same place

[duncan webster 1]

On duncan webster 1 Said:

On JasonB Said:

Dave, if you can’t find it in rod form you have two other options.

1. A stack of “washers” cut from sheet or salvaged

2. Laminate strips again cut from sheet or salvaged and then turn them into a round rod. If you used a couple of oron rivits about 1.5mm dia you could turn almost round without cutting the heads away particularly if they were Csk

Just glue laminations together and then machine to round. Leave a small flat on 2 opposite sides so the outer 2 laminations have some structural strength

I’m not convinced a stack of washers will be effective because magnetic flux doesn’t like jumping gaps. Laminate strips should be good because the flux runs end to end.  The hard part is an interrupted cut into a laminate!  Worth a try though.

Duncan’s suggestion of reusing a relay core is excellent.  GPO relays were common as muck in my youth, and there are some are being auctioned on ebay at the moment.  I’m watching the bids!

I’ve had a post from MichaelG with a link to a old book full of practical suggestions.  One of them is Iron wire, which Florists once used, though I suspect modern stuff is mild-steel covered in plastic.

Meanwhile, the educational electromagnet I ordered has arrived:

Not sure what it’s made of!  Shiny, with a helix running around it.  Next test is to power it up and see if there’s any residual magnetism left after powering down.   Mild-steel and other wrong metals will retain some magnetism.  Should be none if this is soft Iron or Electric Steel.

Dave

[Peter Cook 6Participant @petercook6]

As an additional data point on Swedish iron, in 2019 Neville Michie posted on the Electric clocks forum about making a replica Eureka clock. The originals used Swedish Iron cores. He had done experiments on some replica clocks in Sydney and had some words of wisdom about the issues related to the magnetic reluctance.

The post can be accessed here if anyone is interested

[Michael GilliganParticipant @michaelgilligan61133]

Back to Basics

This is probably all in the ‘Teaching Grandma’ category; but the teacher is reputable:

https://spark.iop.org/simple-electromagnet

MichaelG.

.

Edit: __ shame on them for giving dimensions in centimetres though !

[Peter Cook 6]

On Peter Cook 6 Said:

As an additional data point on Swedish iron, in 2019 Neville Michie posted on the Electric clocks forum about making a replica Eureka clock. The originals used Swedish Iron cores. He had done experiments on some replica clocks in Sydney and had some words of wisdom about the issues related to the magnetic reluctance.

The post can be accessed here if anyone is interested

Thanks for the link, Peter

I recall watching a video of some discussion, but the post is concise.

MichaelG.

[BazyleParticipant @bazyle]

Wrought iron being low carbon is the stuff you want. Swedish iron was just that as they were were good at making puddled iron using their clean iron ore and charcoal instead of dirty sulphurous coal before people understood how steel was stronger than cast or pure iron.

The last open hearth furnaces were in Russia about 35 years ago after which nobody made wrought iron, since Bessemer steel what industry wanted.

So see if you can find genuine 19th century iron railings or suchlike in a heritage recycling yard. Most blacksmiths nowadays don’t understand the difference between real wrought iron and hot bent bits of mild steel so not easy to find the real thing. BTW also a clue is that real wrought iron rusts very little. However another gotcha is that the old blacksmiths heavily worked iron by folding and hammering repeatedly to incorporate both iron oxide particles and clean carbon from charcoal to make stronger things and eg steel knives.

Before Bessemer many ironworks were making ‘pot steel’ when they found out how to blow air through small quantities of cast iron, a couple of cubic feet at a time, to burn out the carbon. This is still done I think for demonstrations in the Iron Bridge museum. It wasn’t taken all the way to remove all carbon because the target was strong steel not bendy soft iron. However the museum if they still do it ought to be able to take it all the way. Someone needs to have a word with them.

 

[Grindstone CowboyParticipant @grindstonecowboy]

These people have soft iron wire, if that’s any use, but also noticed that they do silk-covered copper wire (although the silk is artificial). I recall that soft iron wire used to be used (maybe still is) for tying reinforcing rods together.

Rob

Links to https://www.wires.co.uk/acatalog/fe_black.html

[SillyOldDufferModerator @sillyoldduffer]

Hats off to China!  The yoke of their educational magnet doesn’t retain magnetsism after the current is switched off.  (8A @ 6V)  I tested it with the pole in a dismantled 5V minature relay.

I followed up Peter’s link to Neville Michie’s comments on the merits of Swedish Iron to find it is indeed special!   Turns out that a tiny difference in purity makes an enormous difference to Iron’s magnet performance.

My CRC Handbook of Chemistry and Physics (54th Edn, 1973) makes the difference clear: 99.91% Iron has a maximum permeability of 5000, whilst 99.95%  purified Iron has a maximum permeability 180,000.  Ninety times better from a 0.04% change!

What I’ve got is either Silicon Iron or Electrical Steel, which is also Silicon based.  Although similar to Bazlye’s Wrought Iron, they are considerably purer.  How well, or badly, Wrought Iron performs will depend on how it was made and where.  I’d like to try it but none in my junkbox.

For what I’m doing mild-steel would also work, but the action would be sticky and need more power.  On the other hand, I don’t need the high-performance of best soft Swedish Iron.

Trying to understand magnetism has blown my tiny mind; it’s hard!

Dave

[FulmenParticipant @fulmen]

Understanding magnetism is what separates a competent electrical engineer from the rest of us mortals. My father is one of those, so I know a bit about how smart those guys can be. My best advice is to figure out how to repurpose transformer cores.

[Author]

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