[Amps] Working on my Darwin award
bcarling at cfl.rr.com
Thu Feb 27 07:38:52 EST 2014
This sounds like a nightmare, worse than taking antenna rotators apart!
Best regards - Bry Carling, AF4K
On Feb 26, 2014, at 3:01 PM, Manfred Mornhinweg <manfred at ludens.cl> wrote:
>> 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.
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