Rather B. Fishing · 24 April 1992
Magnetic Bearings (this might get a bit pedantic, so you may just
want to skim...we techno-weenies can to be a bit
much at times...)
Since you (well, one of you) asked, here is some more info on magnetic
bearings which are currently available. The government owned
corporation in France which owns the rights to magnetic bearings is Le
Socie'te' Europe'enne de Propusion (SEP). A partnership between SEP
and SKF (the big Swedish bearing company) called Le Socie'te' de
Me'canique Magne'tique (S2M) manufactures the mag bearings for the
european market. A partnership between S2M and Kollmorgen Corp (yes,
the same people who make periscopes) called Magnetic Bearing Inc (MBI)
manufactures mag bearings for the North American market. <I used to
work for Kollmorgen, just in case anyone was wondering where my info
was coming from...>
I've just dug through too many pages of technical manuals (in English
and French, and boy is my French ever rusty!!!) on mag bearings, and
have reduced a brain-frying amount of mechanical engineering down into
some simple statements and statistics.
A mag bearing is like a funky cross between an induction motor, an
eletromagnet and a PID controller. The idea is to pick up a rotor in
a magnetic field. No rotor is rigid really - even big shafts have
resonance modes, and bouncing modes, and bending modes... If you've
done any engineering mechanics, you're familiar with this sort of
behavior (and if you're not familiar, take my word for it please, or
curl up with your favorite physics book, followed by Fung or Shanes or
Den Hartog...) So considering that the rotor wants to bend and bounce
and expand and flex and misbehave in general, the magnetic bearings
act on the rotor, pushing and pulling it with magnetic force to keep
it in a desired position. The control on the mag flux is achieved
with an active ANALOG PID controller. A digital controller is
apparently too slow, since the variable that's measured and acted on
to control the flux is current and there's a SQUARE relationship
between current and flux. The current changes too fast for any
digital sampler to deal with - the aliasing problems are unreal! So
the controller is a good old fashioned analog filter...awesome,
state-of-the-art 1930's technology...
In terms of force and power, it takes 8000 watts to get a bearing
force of 5 tons (english units) per bearing quadrant. This means to
elevate and control a 20 ton shaft, you need 8000 x 5 watts ( four
radial bearing units and one thrust bearing) = 40000 watts. All
things considered, in terms of power consumption, that's pretty cheap.
That's about the same amount of power as running 60 microwave ovens at
full power simultaneously... Mind you, a set of the really big mag
bearings are to the tune of a million bucks american... The RPMs
available to big shafts with mag bearings are in the low single
thousands (which may seem high to you mechanical types, but remember
that mag bearings allow a shaft to survive the passage through the
critical - the only limitation on the shaft rotation is the strength
of the material when subjected to centrifugal forces).
The itsy bitsy mag bearings, for specialty items like turbomolecular
pumps, small cryogenic compressors, neutron choppers (don't ask me
what it is - I'm just quoting some S2M literature here) use up only a
little power - 120 watts per bearing quadrant for shafts less than a
kilo, But the RPMs are in the 100,000s! A set of little mag bearings
is cheap - in the tens of thousands range of dollars.
(So you're probably all thinking that that's the last time you
encourage me to expound on something... Sorry if I got pedantic - I
suffer from nerd attacks every now and then... I can usually tell
when I'm in the middle of an excessive nerd attack at my weekly
traveller game, since Jim the GM and everyone else is giving that long
suffering look again... Life is so hard for us nerds... I shoulda
listened to my Mom and gone to music school...)
Catie
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Catherine Helm
Finder Graphics, Inc. - A Schlumberger Company
Corte Madera, California, USA
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