Three phase motor for current limiting

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Sean Taylor" <sstaylor-at-uiuc.edu> 

Hi all,

I vaguely remember hearing something about using a 3 phase induction motor 
for current limiting.  Something like using 2 phases in series, and 
rotating the shaft would change the current limit.  I don't see how this 
would work, unless the rotor were modified like we do for making a motor 
synchronous.  If it was modified, perhaps lining up the "missing" part of 
the rotor with the poles from one of the phases would reduce the flux 
linkage between them, and limit the current alot.  Increasing the area that 
the poles from each phase share would lead to increased current, until the 
rotor's normal area (unmodified) was under the poles of both of the phases.

I'm thinking that the rotor would have to be modified kind of "oddly", and 
it would depend greatly on how the motor was wired.  If one were to have 
access to all 3 phases individually, then 2 of them could be hooked in 
"anti-series" in order to maximize current when the flux between the two 
phases are shared completely, and relatively little current when not.  In 
order to minimize the current at the minimum setting, it would be 
benificial to have all the core possible near one phase, but no flux 
linkage between the two phases - this would maximize inductance, and limit 
the current the most.  So maybe a rotor with 1/3 of it taken off - 1/6 on 
either side, 180 degrees apart would work.  Would it be best to actually 
remove the core material, or is it possible to just remove parts of the 
squirrel cage in these areas (it is possible to do this with some rotors . 
. . and as a side note, would that work for making a motor synchronous?).

The main problems I can thin of with this are: 1) getting a 3 phase motor 
big enough to handle the power (approximately 12 to 15 HP should do for a 
10 kVA pig) 2) moving this motor around (along with a pole pig, variacs, 
etc.), and lastly keeping the shaft from moving, and allowing adjustment.

Please comment!!

Sean Taylor