Any electro-magnetism experts around?

[timhoward]

I wasn't quite sure where to post this random question, but I'm fairly sure that some of the geniuses on this forum will have some ideas.

I'm trying to find a way of re-magnetising some 50 year old small magnets used in open-frame motors for model railway locomotives.

There is a machine available for quite a chunk of change specifically for this purpose. I'm not quite ready to part with the £150 yet is it feels like it shouldn't be too hard to do another way.
A physicist friend says the concept is simple enough - basically put the magnet between two strong electro-magnets. Or put the magnet inside a solenoid and give it "lots of amps".

I've done a few experiments with a current-limiting car battery charger (10A limit) and a few home made coils and the concept is certainly working but not quite well enough; the magnet ends up stronger but not as strong as it needs to be. In the pursuit of more amps I also very carefully tried an 18v lithium (cheap parkside) battery which was better again, but still not good enough.

I'm thinking the inefficiencies of my own attempts at coil building are the main problem and I should buy some commerical electro-magnets. The question is, how strong? Some reasonably priced 12v units that I've seen say they will lift 50N/500 Kg. Does anyone have a view as to whether they would create a strong enough field for my purpose?

For info, this is the rather lovely machine that I'm trying to avoid buying:

Some people do just buy new replacement Neodymium magnets but that isn't anywhere near as much fun!
Any thoughts most welcome (other than I should get out more!)

 

[marconi]

You might like to purchase a couple of 12V solenoid operated heavy current contactors like these:

Amazon.co.uk : 12v 200a solenoid switch - https://smile.amazon.co.uk/s?k=12v+200a+solenoid+switch&crid=2J1C9FQRC280J&sprefix=12v+solenoid+switch%2Caps%2C222&ref=nb_sb_ss_ts-doa-p_3_19

and then carefully dismantle them to separate the solenoid and its plunger/armature from the contacts.

Solenoid 101: What is a Solenoid? | TLX Technologies - https://www.tlxtech.com/articles/solenoid-101-what-is-a-solenoid

You could then place them in line with a gap as in the video and energise them using 12Vsupply from a car battery using a momentary on switch of suitable amperage. The current has to be direct current. An alternating current will demagnetise. you would then place your train motor magnets in the gap. The clever thing would be to make some plastic restraining system for the train motor magnets so that the the weak magnets are kept face to face as flat and close as possible with the solenoid armatures.

Or here is a maker of solenoids based in the USA; there are others:

Solenoids and Electromagnets | MSS Magnetic Sensor Systems solenoidcity.com - http://www.solenoidcity.com/default.html


Keep your watch away from the magnets.

 

[timhoward]

@marconi thank you very much for your thoughts.
Using contactors / starter solenoids seems a very good starting point that I will pursue.

I'm also wondering if it would work to replace the plungers with iron bolts so they actually touch the weak magnet and convey the flux. I guess that's how it's working in the video. My limited understanding is that iron is needed so it doesn't become magnetised itself.
I will play some more! Thanks again.

 

[David Prosser]

Well I watched the video and I'm now curious, why does using "better" neodymium magnets cause a problem with the little trains running gear. In my mind you just reduce the voltage on the track reducing run speed and the associated wear?

 

[marconi]

What Is a Solenoid Plunger? (with pictures) - https://www.aboutmechanics.com/what-is-a-solenoid-plunger.htm#:~:text=They%20typically%20consist%20of%20a,made%20from%20various%20ferrous%20alloys.

 

[Avo Mk8]

Well I watched the video and I'm now curious, why does using "better" neodymium magnets cause a problem with the little trains running gear. In my mind you just reduce the voltage on the track reducing run speed and the associated wear?

I think it makes the motor very 'ratchety' in that any inertia of the armature is swamped by the pull of the magnet, so at low speed the motor tends to move in a series of jerks - so spoiling low speed smoothness of the locomotive.
Also I've read some people think it increases the wear of the motor bearings, which may be true if a stronger magnet increases the sideways, sorry 'radial', force on the motor shaft.

 

[timhoward]

I think it makes the motor very 'ratchety' in that any inertia of the armature is swamped by the pull of the magnet, so at low speed the motor tends to move in a series of jerks - so spoiling low speed smoothness of the locomotive.
Also I've read some people think it increases the wear of the motor bearings, which may be true if a stronger magnet increases the sideways, sorry 'radial', force on the motor shaft.

My hobby is mainly fixing / servicing other peoples locos (honest!) so I can't really speak about any long term neo magnet issues. However I can tell if anyone cleans their track with wire wool, as the strong magnet picks up an incredible amount of debris that they don't manage to clean up!

 

[marconi]

Here are two nice articles summarising some of the technology and engineering of hobby locomotive motors. You will note the use of flywheels to reduce jerkiness at low speed. There are also multipole armature motors which are smoother because they have more than one magnetic neutral axis as compared with a simple 2 pole motor:


Hobbytown Motor Engine HO Scale Model Train Replacement Part #614 | eBay - https://www.ebay.co.uk/itm/234397548346?_trkparms=ispr=1&hash=item36932ebf3a:g:RQgAAOSwF7hh9Lgw&amdata=enc:AQAGAAAA8KIQYnqYa7KpJNJNoyn0jAGCZask5Ooa6y8Aj/8JlZIUQyORQq8rBA0bLTjGbhICcpECwx/p7zWK6HiO2lnWJuddCdWFG8+nKsMYRvBJfFU2ym+iluId/HZU5BJPwy8c7Td78HwCAGAOue0xVF5tfxDYw54ntiFeJUnF5QSimtB6yv8+V0VA4eszPcN0CYiyOOMum1R6QaFNQW9NaUsbud67Lv+D0pYKGIfC3Z4356h9LRCiR8XI2gdhUsAPj0D7dwDdkIH6QaFcssBQdxlx/QzaAcu1rhWKAQVKG81TdoepN+t/UYoeuslhfC37C15SUA==&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338691609&customid=d-0420-CjwKCAjwu_mSBhAYEiwA5BBmfxyrdQp_2hMFebYSEZUSGwF67Yxe-pn5tVEid_OE3oRSoK5-6o43PhoC2BkQAvD_BwE&toolid=10049

and:

DC Motors -- principles of operation - http://solarbotics.net/starting/200111_dcmotor/200111_dcmotor2.html

 

[littlespark]

Train sets! What a nerd!

You need a real hobby….. like Lego!

 

[timhoward]

Train sets! What a nerd!
You need a real hobby….. like Lego!

Nah, not train sets! In fact my only track is a large boring test loop around a shed, I get far more enjoyment fixing them and fitting decoders to old models (for modern digital control systems) than running them or modelling.

But you'll be pleased to know my son is VERY into Lego, normal and technic. Also fischer-teckhik which is a brilliant and often overlooked engineering platform, often cheap on ebay.

 

[timhoward]

Here are two nice articles summarising some of the technology and engineering of hobby locomotive motors. You will note the use of flywheels to reduce jerkiness at low speed. There are also multipole armature motors which are smoother because they have more than one magnetic neutral axis as compared with a simple 2 pole motor:

Quite a common upgrade I do is replacing 3 pole Hornby ringfield motors with a 5 pole motor, it makes an enormous difference.

 

[littlespark]

My neighbours company sells little trees that go on model railways…. No joke

They buy them in from China, touch up some paint to make them more realistic and unique… box them up and sell them on.
I give all my spare bubble wrap for their packaging.


That’s. His. Actual. Job

 

[Moley]

Quite a common upgrade I do is replacing 3 pole Hornby ringfield motors with a 5 pole motor, it makes an enormous difference.

I remember when Portescap coreless motors with gearbox first hit the model scene and I started changing the motors in my white metal kits. My Kays GWR 28XX 2-8-0 was a beast with one of those.

 

[Lucien Nunes]

the magnet ends up stronger but not as strong as it needs to be. In the pursuit of more amps...

Also pursue:

• More turns
• More core cross-section
• Higher permeability core material
• More compact and leakfree magnetic circuit

Obviously you are aiming to get enough amp-turns to drive the core of your magnet charger into saturation, but once you get past the knee of the curve, more MMF won't give you much more total flux so you will simply make more heat.

I am not sure why the device shown in the first pic appears to have an open magnetic circuit between the outer ends of the solenoid cores. It works because it's physically much larger than the magnet it needs to charge. If you google 'magneto magnet charger' you will see versions aimed at engine magneto magnets which need more total flux (although not necessarily a higher flux density) and therefore will typically have a more efficient closed magnetic circuit.

Magnetic circuits work like electric ones, except that the conductors don't conduct very well and the insulators are quite useless. Imagine trying to work with a circuit of 100 ohms loop impedance and 200 ohms insulation resistance. The magnet charger in the first pic is like a circuit with no neutral, where the return current from the load has to find its way back via terrible insulation leakage.

 

[timhoward]

More compact and leakfree magnetic circuit

@Lucien Nunes thank you for getting me thinking about magnetic circuits. You triggered a light-bulb moment.
I'd picked up a 2nd hand mag-lock unit a while ago thinking it might help with this project, but I abandoned this as I couldn't work out how to utilise the magnetic field. It only seemed strong when two sections were bridged.
Your comment helped me realise that the principle of operation is the strike plate completes two magnetic circuits. There are two coils inside and the central plate is common to both.



All I actually needed to do in the end is place the magnet between two L section lengths, which in turn are on the central and outer plates on the mag-lock. A couple of quick blasts from an 18v battery did the job.
Thanks again to all who commented.

 

[marconi]

May we see the model railway you have constructed please?

 

[timhoward]

May we see the model railway you have constructed please?

Sure, though there isn't much/any "model" to it. My enjoyment comes from repairing the locomotives and the 'railway' is just a loop around a shed with a lift out bridge for the doors. I'll take some photos / video later.

 

[Moley]

Are you a fan of diesel or kettles? Personally I prefer the early diesels, class 31, 37 etc.

 

[timhoward]

Are you a fan of diesel or kettles? Personally I prefer the early diesels, class 31, 37 etc.

I'm a bit of an all-rounder really. I agree that early diesels are nice machines.
Possibly my quirkiest interest is early railcars/DMUs, Wickham's, 101's, 102's etc. I consider them a very ingenious fault-tolerant and flexible design.
I like seeing kettles at mainline speed doing the job they were designed for; I'm not so bothered about seeing them potter about on heritage lines.

I didn't have much time this afternoon as I had to work on the car - but here's a quick video of a class 43 HST set that I repaired this week rattling past with my son adding sound effects:
View attachment IMG_3401.MOV