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MG Midget and Sprite Technical - polarising dynamo

Gents: I know I know that this has been out before but the wiser heads than mine on the BBS seem to have to patience of Job so I'm risking asking it again...
How does one polarise a dynamo ['66 Mk III + earth]

many thanks
Dave
David Cox

http://www.youtube.com/watch?v=wH_yIeKsBSM
Steven Devine

many thanks!
Dave
David Cox

And just for the hell of it, here's a far less wordy video version from Moss, that describes just the polarising. http://www.youtube.com/watch?v=PPUfQtVEJGg

Here's what your actually doing.
You're "momentarily"(flashing) touching the field connection of the dynamo, to the NON earthed side of the battery, whilst the body of the dynamo is connected to the earth of the car. You can do this even before you fit the dynamo to the car.

So to polarise for +VE earth car(battery positive terminal connected to chassis), you momentarily connect the field winding of the dynamo to the NEGATIVE terminal on the battery.

To polarise for -VE earth car (battery negative terminal connected to chassis), you momentarily connect the field winding of the dynamo to the POSITIVE terminal on the battery.

Here's why you're doing it.
You're doing it to establish a small residual magnetic field, "of the correct polarity", or "reverse the polarity" of a residual magnetic field, in the iron core that the field winding of the dynamo is wound onto. It's this residual magnetism that induces a small voltage in the armature (the shaft connected to the pully) windings of the dynamo, when the engine first spins the dynamo pully. It's not enough to charge the battery, but it's enough for the field windings to draw current - via the regulator. As the field current increases, the magnetic field increases. That induces a greater voltage in the armature, and the process continues until the dynamo is producing maximum output, as controlled by the regulator.
Lawrence Slater

Thanks to you, too, Lawrence!
David Cox

Yes, I posted it and then watched it. I usually just flash off the battery and look at the charging light.
I didnt realize twist was going the longway around the world with his teaching lecture. I hope it was informative for you and not just blather.
Steven Devine

I thought it was pretty good actually Steven, but he does jaw jaw a lot on some of his vids.

Christ though, imagine how long my vid's would be if I made one. I like to spin out a conversation a bit too. How about I make a vid about engine breathers or better yet, front wheel bearings? There's so much material to draw on with both. All of the 400+ posts on those long threads --- now slumbering quietly in the archives -- could be referenced constantly throughout the course of the video, soley to entertain, inform, and generally bore the ar*e of anyone unfortunate to want to watch them to the end. Some may even succomb to the grim reaper -- http://www.youtube.com/watch?v=cMYSWiPm7E0 -- before the end, so a reference could be provided in the video for the discoverer of the body, on what to do with it. Have you ever found someone dead at a computer, caused by watching a boring video? Imagine you did, would you too be tempeted to watch it, and risk the same fate? I don't believe in fate myself. I believe I control my own destiny, and life's better lived not hanging around for fate to have it's way with you. Speaking of having it's way with you, ---.

But I digress. Yes I agree that John Twist vid was too much about other aspects other than just polarisation.
Lawrence Slater

Hey Lawrence,
I got a chance to meet John about 10 years ago. I had a friend that dropped his name all the time and how he had been factory trained by MG.
I was feeling Quite intimedated meeting him from my buddy building him up so much.

The thing that really struck me about him was he came off like a regular guy.( Not pompus at all.) After talking with him for a short while you got the feeling you were talking to an old friend.

He reminded me of a lot of the old time mechanics I knew as a kid. If you had an interest in something more than willing to show you the ropes. Just a nice guy doing what he loves doing, fixing MGS,the cars he fell in love with as a kid.

I think what really made me see who he was is when his son Brooks was working on an alternater learning the ropes. John was sweating it out trying not to interfere while Brooks was starting to bugger up the pieces involved. It would be like watching Prop put the piston rings in your motor. We all have to make some mistakes before we learn, but its hard to watch
some one do it in front of you with out saying anything. I liked that he was letting the kid learn on his own terms. Feeding the kids curiousity.

I enjoy reading the posts on this site. Sometimes its like having another set of eyes looking at a problem with you. Its good to get another opinion and look at things from a different perspective. Imigine all the things that had to go right for all of US here to pick MGs as our cars and get to actually communicate and know each other! Mindnumbingly cool!

I also like it when people get to the point! Ha ha ha!



Steven Devine

I saw B.O.C. about 20 times back in the day! Great song!

http://www.youtube.com/watch?v=JVni1v0Ee1s

This always makes me laugh! LOL!

I got a fever!
Steven Devine

Hi Steven. I wasn't really knocking J Twist. His vids always contain something interesting. It's just as you said, sometimes you need patience. My mates old man was like Twist. He could fix anything you took to him, but he'd sometimes take all day whilst he told you about this and that. lol. One of the nicest blokes I ever met, a real genius, but I often made excuses to come back later when he'd fixed it. :). That said, him and people like him, taught me loads.
Lawrence Slater

This thread was discussed between 23/10/2013 and 28/10/2013

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