Abstract :
A magnetic bearing
is a bearing which supports a load using magnetic levitation.
Magnetic bearings
support moving machinery without physical contact, for example, they can levitate
a rotating shaft and permit relative motion without friction or wear. In active
magnetic bearings (AMB) a stable equilibrium is achieved by means of one or more
control loops. The use of control loop for maintaining the gap between the shaft
and bearing differentiate the active magnetic bearings (AMB) from passive ones.
They are in service in such industrial applications as electric power generation,
petroleum refining, machine tool operation and natural gas pipelines'.
The typical
AMB system diagram is illustrated in above figure. Besides the controller, the general
control system also includes the sensor, A/D and D/A conversion and power amplifier.
The rotor’s displacement along one of the axes is detected by the position sensors
and converted into signals of standard voltage. Then compared with the setting value,
the error signal enters the controller. After A/D conversion, the controller processes
this digital signal according to a given regulating rule (control arithmetic) and
generates a signal of current setting. After D/A conversion, this current signal
enters the power amplifier, whose function is to maintain the current value in the
electric magnet winding at the current level set by the controller. Therefore, if
the rotor leaves its center position, the control system will change the electromagnet
current in order to change its attraction force and, respectively, draws the rotor
back to its balance position.
Active Magnetic
Bearings (AMBs) with their control system inherently offer the possibility of continuously
recording bearing forces and rotor displacements. This allows on-line monitoring
of critical process parameters and early detection of incipient faults, such that
reliability is increased. Furthermore, AMBs can actively influence a machine's behavior.
Thereby, operation can be adjusted and optimized
according to process changes.
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according to process changes.
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