[Amps] Working on my Darwin award

[Manfred Mornhinweg]

> On a different note, I worked with Mass Spectrometers for a while.  I 
> don't know the rating of those BIG electromagnets, but if it tried to 
> take a wrench away, you just let have it.  It'd crush your hand 
> including bones.

Big magnets are great pranksters. There was a time when I had to service 
large servomotors, specialized for low inertia. Those things have a flat 
copper rotor, basically a set of printed coils and nothing else, 
rotating between two sets of really big and strong magnets. The magnets 
pull the housing of the motor together with great force. To open those 
motors, the technique is to remove the screws, then bolt some levers to 
the two end plates of the motor, and twist one motor end relative to the 
other. Unlike poles attract each other A LOT, but by rotating the plates 
by one magnet location, you have like poles facing each other, which 
repel each other just as strongly. When twisting the plates carefully, 
the motor easily comes apart as soon as the unlike poles no longer face 
each other fully. But if you twist them in some crooked position, so 
that the end plates jam in the housing, you can just keep twisting them 
until the repelling force becomes enough to overcome the jamming force. 
At that point, kaboom! You get the parts flying all through the room, 
maybe all the way through the roof, and hopefully not in your face. It's 
the well regarded magnetic cannon effect.

After reassembly, the magnets need to be remagnetized, because they 
loose about half of their strength when the magnetic circuit is opened. 
This is done with a one turn winding that twists between those magnets. 
You simply apply a few thousand amperes to that wire, for a short 
moment. I did that with two truck batteries in series. One time the wire 
welded itself to the battery post and I couldn't rip it off quickly 
enough (with pliers, of course). The wire evaporated, and the next time 
that motor needed maintenance, I had to install a new magnetizing wire. 
At the factory they probably have a nice pulse generator, rather than 
banging the wire with pliers against a battery pole...

It's interesting to note that when the magnets are weak, the motors run 
much faster than normal. It's logical to anyone knowing 
electromagnetics, but looks counterintuitive for laymen.

Manfred

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