Page 98 - Steam Turbines Design, Applications, and Rerating

Bearings for Mechanical Drive Turbines 79

netic bearing. Because the total resistance of the coils and current path
is quite small, the overall power loss is insignificant when compared to
conventional bearings.
A journal that floats in a magnetic field produced solely by bias cur-
rents is not stable. To ensure stability, the journal motion must be mon-
itored continuously and corrected instantaneously by superimposing a
small control current on each bias current.
Figure 3.29 illustrates the basic active magnetic bearing configura-
tion, showing the required sensors and control circuits in combination
with the bearing elements. If the journal moves upward from its cen-
tered position, the current in the top sector is reduced by i. The current
in the bottom sector is increased by the same amount. The control cur-
rent produces a net downward force that pulls the journal back to the
center. A modified version of this operating principle is incorporated in
active magnetic thrust bearings.
The topic of magnetic bearings is given additional coverage in the
companion volume to this text, A Practical Guide to Compressor Tech-
nology.