The Principle
of Operation of the Magnetic Bearing
A Magnetic Bearing is an electro magnetic device,
which maintains the relative position of a rotating
assembly (rotor) with respect to a stationary
part (stator). The electro magnetic forces implemented
for this are controlled by an electronic control
cabinet. An active Magnetic Bearing is, therefore,
made up of three distinct parts:
• the bearing itself
• the electronic control system
• the auxiliary bearings (known as
RDS, Rotor Delevitation System).
The configuration of the component
parts is as follows:
Radial Bearing Configuration
Four electro magnets are arranged around the rotor
to form the bearing. Each electro magnet is driven
by an amplifier. In horizontal applications, the
magnet centre lines are orientated at 45°
to the perpendicular such that forces due to gravity
are acted on by the upper two adjoining magnets.
This adds to the load capability and increases
the stability of the system.
Opposite electro magnets are adjusted
to pull against one another in the absence of
any externally applied force, (the bias force).
When an externally applied force causes a change
in position of the rotor it is sensed by position
transducers, which, via the electronic control
system, cause an increase in one current and a
decrease in the other current flowing through
the respective electro magnets. This produces
a differential force to return the rotor to its
original position. The signal from the position
transducers continuously updates the differential
force to produce a stable system.
Axial Bearing Configuration
A flat, solid ferro magnetic disc, secured to
the rotor is used as the collar for the axial
thrust bearing. Solid disc electro magnets are
situated either side of the collar and operate
in a similar manner to the radial bearing above
but in one dimension only.
Position
Transducers
Two dimensions are controlled at each radial bearing
location and one dimension is controlled at the
axial bearing. One transducer could be used for
each dimension if it were totally linear and free
from drift due to aging or temperature effects.
Two transducers per dimension require only that
a balance or difference be maintained, thus cancelling
unwanted offsets. A passive bridge system such
as this is therefore preferred as it greatly increases
the accuracy and reliability without undue increase
in costs or complication.
The Electronic Control System
The purpose of the electronic control system is
to control the position of the rotor by varying
the current in the electro magnets. The electronic
control system defines the stiffness and damping
of the magnetic suspension and these values can
be adjusted and adapted to the disturbance frequency
of the machine itself.
Auxiliary Bearings
Auxiliary bearings are used to support the rotor
when the machine is stationary with the electronic
control system switched off, or in the event of
a failure in the magnetic suspension system to
allow the rotor to run down without damage to
the rotor itself or the stator of the Magnetic
Bearings. Dry bushings are employed as the primary
technology for this purpose by Waukesha Magnetic
Bearings for this purpose with a clearance between
the stationary and rotating parts of half the
Magnetic Bearing. This unique patented WMB system
known as an RDS (Rotor Delevitation System) is
much more rugged and reliable than the older roller
bearings originally used. |
