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The mechanical properties of the rotor lamination have to be good, in order to overcome the centrifugal stress due to high speed rotation. Since AMBs are actively controlled regarding to the sensor signal, the control performance strongly depends on the sensor performance.

Several sensor types are used in AMBs: inductive, eddy current, capacity and optical displacement sensors. They provide a great flexibility and high computation speed.

Digital controllers enable principally an adaptative control, unbalance compensation and provide a great tool for system diagnosis. AMBs are controlled in closed-loop. Different methods such as PD, PID, optimal output feedback or observer based state feedback are in use. Switching amplifiers are usually used because of their low losses. The amplifier is often the limitating component in an AMB system. Figure 1. The AMB system as shown in Figure 1. As a result, this sensor measures the relative displacement between the bearing stator and the rotor.

This position signal is then sent to the controller, which adjusts the current level to the actuators through the use of a current amplifier.

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The change in current within the actuator coils will either increase or decrease the force placed on the rotor based on this change, which adjusts the position of the rotor within the stator. The bearing actuators are electromagnets consisting of coils of wire wrapped around a ferromagnetic material. The force capacity of these electromagnets can be calculated based on simplified magnetic circuit analysis. The magnetic field across the air gaps results in an attractive force on the rotor.

This force is dependent on the geometry of the magnet, the current in the wires and the distance between the stator and rotor. From above Equation it is shown that the force is inversely proportional to the gap distance squared, indicating that as the rotor gets closer to the stator the stator pulls harder on the rotor.

This is an important observation related to magnetic bearings.

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The forces in a magnetic bearing are always attractive. As a result, a control system must be used to stabilize the system. The control system accomplishes this by counteracting the potential increase in the attractive force due to the gap becoming smaller by decreasing the current supplied to the magnet. In general the controllers will simply supply equal and opposite currents to these magnet pairs to provide the stabilizing force. They use a flat, solid ferromagnetic disc secured to the rotor as the collar, the axial thrust bearing.

Solid disc electromagnets are situated both side of the collar and operate in a similar manner to the radial bearing above but in one dimension only. These bearings are designed with a conical shaped stator and rotor. This is called closed loop feedback control and is necessary for the shaft to be held in a stable position. In simple terms, the control system reduces the upper bearing current when the shaft is Typically, magnetic bearing control is performed in a single input-single output SISO manner.

This means that the position information from one sensor causes only the control current in the corresponding axis to be varied. Control systems can also be multi-input and multi-output MIMO. MIMO is used when higher levels of control are required or when significant cross-coupling between axes is expected. The components of the control system include a position sensor and accompanying electronics, a controller, and amplifiers. These components are described below. Normally, the sensors are calibrated so that the when the shaft is in the desired position, the sensor produces a null voltage.

When the shaft is moved above this desired position, a positive voltage is produced and when it is moved below, a negative voltage results. The controller consists of anti-aliasing filters, analog-to-digital signal processor and Pulse- Width Modulation PMM generators. The voltage from the position sensors is passed through the anti-aliasing filters to eliminate high frequency noise from the signal. This noise can cause the signal to inaccurately represent the position of the shaft. This converts the voltage signal to a form that can be processed by the digital signal processor.

The digital information is then passed through a digital filter by the digital signal processor. This produces an output proportional to the amount of current required to correct the position error in the shaft. The requested current is compared to the actual current in the bearing, which is also sensed, filtered and sampled with an analog-to-digital converter.

Magnetic Bearings

The error between the actual and requested current is used to characterize the PWM signal sent to the amplifiers. This information is sent to the pulse-width modulation generators which create the PWM wave form sent to the amplifiers. The delivery of the control current request must occur well before the next sample of the shaft position is taken. The sampling and control delivery process is repeated at a frequency of 10 kHz. The amplifiers are simply high voltage switches that are turned on the off at a high frequency, as commanded by the PWM signal from the controller.

The operation of active magnetic bearings causes much less losses than operating conventional ball or journal bearings but nevertheless, the losses have to be taken into account and sometimes they lead to limitations. Losses can be grouped into losses arising in the stationary parts, in the rotor itself, and losses related to the design of the control. Losses in the stationary parts of the bearing come mainly from copper losses in the windings of the stator and from losses in the amplifiers.

The copper losses are a heat source and if no sufficient cooling is provided, can limit the control current and hence the maximal achievable carrying force. Losses in the rotor part are more complex and lead to more severe limitations. These losses comprise iron losses caused by hysteresis and eddy currents, and air drag losses. The losses heat up the rotor, cause a breaking torque on the rotor, and have to be compensated by the drive power of the motor. The relations of the various losses with respect to one another are shown in Figure 1.

In general, the eddy current losses are the largest ones. No wear takes place as there is no contact between stationary and rotating parts. Estelle Croot's research was the subject of three Australian patents [3] and was funded by Nachi Fujikoshi, Nippon Seiko KK and Hitachi, and her calculations were used in other technologies that used rare earth magnets but the active magnetic bearings were only developed to the prototype stage.

Kasarda [17] reviews the history of active magnetic bearings in depth. She notes that the first commercial application of active magnetic bearings was in turbomachinery. This reduced the fire hazard allowing a substantial reduction in insurance costs. The success of these magnetic bearing installations led NGTL to pioneer the research and development of a digital magnetic bearing control system as a replacement for the analog control systems supplied by the American company Magnetic Bearings Inc.

The company was later purchased by SKF of Sweden. The French company S2M , founded in , was the first to commercially market active magnetic bearings. Extensive research on magnetic bearings continues at the University of Virginia in the Rotating Machinery and Controls Industrial Research Program [5].

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During the decade starting in , the Dutch oil-and-gas company NAM installed twenty gas compressors, each driven by a megawatt variable-speed-drive electric motor. Each unit was fully equipped with active magnetic bearings on both the motor and the compressor.

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These compressors are used in the Groningen gas field to extract the remaining gas from this large gas field and to increase the field capacity. The motor-compressor design was done by Siemens and the active magnetic bearings were delivered by Waukesha Bearings owned by Dover Corporation. Originally these bearings were designed by Glacier, this company was later taken over by Federal Mogul and is now part of Waukesha Bearings.

By using active magnetic bearings and a direct drive between motor and compressor without having a gearbox in between and by applying dry gas seals, a fully dry-dry oil-free system was achieved. Applying active magnetic bearings in both the driver and in the compressor compared to the traditional configuration using gears and ball bearings results in a relatively simple system with a very wide operating range and high efficiencies, particularly at partial load.

As was done in the Groningen field, the full installation can additionally be placed outdoors without the need for a large compressor building. Meeks [18] pioneered hybrid magnetic bearing designs US patent 5,, in which permanent magnets provide the bias field and active control coils are used for stability and dynamic control.

These designs using permanent magnets for bias fields are smaller and of lighter weight than purely electromagnetic bearings. The electronic control system is also smaller and requires less electrical power because the bias field is provided by the permanent magnets. Allaire University of Virginia , and Prof. Okada Ibaraki University.

Dynamic characteristics of hybrid foil-magnetic bearings (HFMBs) concerning eccentricity effect

Since then, the symposium has developed into a biennial conference series with a permanent portal on magnetic bearing technology [6] where all symposium contributions are made available. The web portal is supported by the international research and industrial community. Joining the hall of fame and earning lifetime achievement awards in were Prof. Yohji Okada, Prof. Magnetic bearing advantages include very low and predictable friction, and the ability to run without lubrication and in a vacuum.

Magnetic bearings are increasingly used in industrial machines such as compressors, turbines, pumps, motors and generators. Magnetic bearings are commonly used in watt-hour meters by electric utilities to measure home power consumption.

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They are also used in energy storage or transportation applications and to support equipment in a vacuum, for example in flywheel energy storage systems [19]. A flywheel in a vacuum has very low wind resistance losses, but conventional bearings usually fail quickly in a vacuum due to poor lubrication. The book is organized in a logical fashion, starting with an overview of the technology and a survey of the range of applications. A background chapter then explains the central concepts of active magnetic bearings while avoiding a morass of technical details.

These system models and performance objectives are then tied together through extensive discussions of control methods for both rigid and flexible rotors, including consideration of the problem of system dynamics identification. Supporting this, the issues of system reliability and fault management are discussed from several useful and complementary perspectives.