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ACTIVE MAGNETIC BEARINGS (AMB)
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
BASIC OPERATION
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.
IMPROVED MACHINE PERFORMANCE USING ACTIVE CONTROL TECHNOLOGY
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.
IDENTIFICATION AND CONTROL
AMB systems are often used to control structural resonance frequencies coming from the rotor or from elastic supports. The resonance frequencies may vary significantly with the rotational speed. Controller design for AMB systems is therefore important for the system performance. Controller design requires a plant model. Identification, i.e. modeling based on dynamic measurements, is a fast way for obtaining such a model. Both controller design and identification are topics of current research.
SYSTEM COMPONENTS
MAGNETIC BEARING
The radial and axial magnetic bearings are located in the generator. In order to reduce the range of products, magnetic bearings for generator rotor and turbocompressor rotor are designed as the unified size according to the generator rotor load in operation condition. The radial bearing radial gap is 0.15mm considering the gap of 0.4mm between the compressor stator and blades in order to protect the compressor.
POSITION SENSOR
The rotor displacements in radial and axial are monitored by the position sensors, which are of induction type. The sensor consists of sensitive elements located on the stator and an acting element located on the rotor in front of the sensitive elements. The sensitive element is an annular magnetic circuit with 24 poles, of which each 6 poles are grouped to detect the radial displacements in X and Y directions. In such design, a kind of 2/3 redundancy working mode for sensor signals can be easily realized. The acting element is an extension made of the laminated ferromagnetic steel, which is fixed on turbomachine shaft. Windings around the stator perimeter are distributed in order to average and smooth the measure value. This kind of sensor has good sensitivity of no less than 10mV/μm and resolution of at least 1μm. Its cut-off frequency is enough so high (>5k Hz) that the phase lag at operation frequency can be neglected. The voltage signal after the sensor modulator can be transferred more than 200m without obvious attenuation.
CONTROLLER
The controllers, as well as all its peripheral equipment, including A/D, D/A, network card, etc., is standard industry type, usually selected as high speed Digital Signal Processing (DSP) computer, which has good stability and excellent hard real-time interrupt processing capability. For example, the new DSP product of TI 6713 has powerful floating-point operation of 1350 MFLOPS and can be adopted as the ideal micro processor of the controller. The A/D converter has 10 channels with 500kS/s rate and 16bit precision, while the D/A converter have 5 channels with 1MS/s rate and 14bit precision.
The controller shall have the following functions:
(1) Receive information about displacement, rotation speed and angular position of the machine rotor from the sensor converters;
(2) Receive the control commands from the operation computer to change some parameters of the AMB control system;
(3) Generate and release the current control signals in coil windings according to the specified algorithms and control commands;
(4) Diagnose the states of the elements of the AMB system and transmit this information to the operator computer via networks;
(5) Release signals about alarm and emergency protection.