Classic cars forum & vehicle restoration.

[oldtimer]

In the course of"evaluating my automotive assets",described by Management as "sorting out that junk",I built up an old dynamo for myVolvo 120.The rotor and the casing have been apart for years and,for some reason,the remaining brain cell is telling me that, over such a lengthy,the dynamo may well have "demagnetised"!
I cannot recall the source of this claim and it may well be"a false creation
proceeding from a heat oppressed brain",like Macbeth.
However,"a nos moutons",as Greeney may say.Will some of those kind forum members who are knowledgeable in this particular Black Art please
do some, or all,of the following.
1 Confirm or demolish this assertion
2 If true is the condition "fixable".I am aware of the procedure for changing polarity and wondered if something similar may apply in this case.
My thanks,in anticipation,

oldtimer

[Phil - Nottingham]

The dynamo retains a residual magnetic field when it is "off" ie no voltage going through the coils becuase the Cutout contacts are open. Any electric current going through a coils or coils of wire will give a magetic field - this is restored to full when it is re-energised - it may have to be repolarised to make it charge again though - this is where the myth comes from

The strength of the field is used to regulate the output voltage and this is done by the regulator which in effect switches the 12v battery voltage on/off many times to give a lower mean field votage and thus weaker magnetic filed and thus lower out voltage
_________________
Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2

[oldtimer]

Hello Phil,
My thanks for your reply in a way I feel I can understand!When I spoke to a chap Ihave known for some time he was adamant that the situation was irretrievable but I shall go with your more optimistic approach.

Thanks again
oldtimer

[PAUL BEAUMONT]

Hey, Oldtimer, are you sure that your informant isn't in the pay of "The Management"??
PAUL

[Phil - Nottingham]

Dynamos can depolarise themselves for unknown reasons although usually there is a wiring or internal fault. So if it keeps happening after repolaring investigate.

Altenators actual require some residual magentism to start charging but will self energise.
_________________
Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2

[Rusty]

You may be getting mixed up with open bar magnetos ! When you pull an armature out of a magneto you have to put a keeper (a small piece of steel) across the polls of the magnet or it will lose its charge and need to be re magnetised, "most" of the time a generator will be fine.

Graham.

[oldtimer]

Hello Graham,
Thank you for your input from across the world!
You may well have hit upon the reason for the few remaining brain cells
scrambling various bits of information from different sources on matters which are vaguely related and inventing a new myth as a result.

Regards
John (oldtimer)

[Phil - Nottingham]

This how myths are created - magnetos are before my time with cars - had one on my James Cadet motorbike with a Villiers 150cc two stroke though - very feeble
_________________
Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2

[oldtimer]

Hello Phil
Like you my involvement with magnetos was minimal and lost in the mists of time.In fact Graham sparked off that train of thought and made me recall that my brief flirtation with magnetos was at a time in my youth when a few of us were "messing about on the river" and involved with boats,sailing, and those with an "iron mainsail", some with magnetos Iseem to remember.

Regards,
John (oldtimer)

[47Jag]

Oldtimer,

The magneto has a permanent magnet made from high quality steel which will stay magnetised indefinitely as long as it isn't taken apart. If it is then a 'keeper' has to be fitted as Rusty said. The dynamo's magnets are the pole shoes and the casing which are made of mild steel. Neither are very good at retaining magnetism fo a long period. The dynamo however re-magnetises every time the engine is started so it rarely is a problem with regular use. The total loss of residual magnetism will take at least years. The solution is simply to remove the FIELD wire and apply battery power to the terminal for a couple of seconds.

Art

[Keith D]

So this post has degenerated (excuse the pun!) to magnetos.

These things cause grown men to cry. They defy all known laws of physics and fall definitely into the realms of witchcraft.

My 1928 Cowley has one made by Mr Lucas. This combination has to guarantee regular nightmares. This object does not need to be actually wet to die. It only has to THINK it's wet. Regular prayer helps!

Thank heavens for coil ignition!

Keith

[oldtimer]

Hello Keith
It was never my intention to create an international incident but it looks like I may have done just that as it looks like "magneto"is a dirty word!
In my defence I can only plead insanity and claim that a certain gentleman from the Antipodes used the word first!
Seriously if my memory is correct,about this item being used in boats,then, from what you say, they must have been a constant source of frustration and some real dirty words in a constantly damp environment.

Regards,
John (oldtimer)

[Keith D]

Hi John,

I am ashamed to admit that I have just retired from a lifetime in electrical engineering, and yet I still have problems in keeping magnetos sparking! That's the only reason it's a "dirty" word!!

Keith