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The multiple ‘attractions’ of a magnetic bearing

January 10, 2006
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Clever, clean and well-behaved, a new breed of magnetic bearings is providing an effective solution for an increasing number of sophisticated and demanding applications

When a bearing has a controller that can position a shaft rotating at extremely high speeds (0-100,000 rpm) within microns of movement and compensate for mechanical vibrations that are inherent in rotating equipment - that’s clever. When a bearing has no lubricant and never sheds wear particles – that’s clean. And when a bearing’s own built-in conditioning system eliminates the need for routine maintenance – that’s well-behaved. Small wonder then that SKF’s range of magnetic bearing solutions is attracting attention from engineers with application problems from a wide range of industries.

Of course the idea of magnetic levitation, the idea at the heart of magnetic bearings, is far from new. As far back as the 1800’s, experiments were being made. What stopped any development towards a functioning magnetic bearing was the need for sensing and control systems far beyond the technology of the time. So how do today’s magnetic bearings work?

The basic magnetic bearing system

In some ways a basic radial magnetic bearing system is similar to an electric motor. The differences appear when you consider its three main parts:

• Bearing actuators;

• Sensors; and

• Controller and control algorithms.

Rather than generating a torque, this arrangement produces an attractive force to levitate a shaft. A typical radial stator consists of a laminated iron core with copper coils, creating a series of north and south poles around the shaft. When the coils are energized it becomes an electromagnet, which produces an attractive force that acts upon a ferromagnetic shaft (laminated or solid). The radial air gap between the stator and the shaft is typically 0.5 mm to 2 mm. In contrast to the radial bearing described above, a magnetic thrust bearing has a solid ferromagnetic disc attached to the shaft, with an electromagnet at one or either side.

A third arrangement is the conical magnetic bearing. This combines the characteristics of a radial bearing and a thrust bearing. Conical bearings successfully control both radial and thrust motion in machine that have only a modest thrust load. Eliminating the need for a separate thrust bearing gives opportunities to reduce the overall machine length. In all three types of bearing, positioning of the shaft is achieved by an arrangement of inductive sensors in five axes (four radial and one thrust). Signals from the sensors pass to the controller where advanced control algorithms measure shaft position and regulate current to the bearing’s actuators.

A close look at the controller

SKF magnetic bearings are developed and manufactured by its business unit in Canada. As a recognised technical leader in magnetic bearings SKF Magnetic Bearings creates active magnetic bearing systems for a wide range of industries and has been involved in the development of a variety of magnetic bearing applications. Much of the company’s reputation stems from their unique expertise in control system hardware and the associated control algorithms which vary bearing current and forces to control and operate the shaft at pre-set targets.

The controller hardware

The controller hardware has three main parts: the digital signal processor (DSP) electronics, the power supply and the amplifiers.

• The DSP electronics provide the “brains” behind a magnetic bearing system. Advanced control algorithms measure shaft position and regulate current to the actuators 10,000 times per second. Machine-specific tuning parameters can be modified and saved in a file using a PC and SKF software called MBScope. This software also allows the user to monitor the bearing and shaft performance by looking at parameters such as vibration, balance and speed, etc.

• The power supply converts the supplied AC power to DC power used by the bearing’s amplifiers. The larger the amplifier, the larger the power supply required.

• The amplifiers regulate current to the bearings based on the set points provided by the DSP electronics. Amplifiers are sized according to the machine requirements. Generally, the larger the machine, the larger the amplifiers.

The DSP software

The SKF software provided for the DSP offers unique possibilities

• Unparalleled unbalance control – SKF has developed a functionality named Adaptive Vibration Control (AVC), which controls the bearing system’s response to shaft unbalance. This can be used in two ways. One way is to allow the shaft to rotate around its geometric centre and tightly control the shaft to eliminate the runout caused by unbalance. This is applied in applications with high precision requirements such as machine tools. The other way is to rotate the shaft around its mass centre to reduce vibration transferred to the casing (to less than 0.01 µm). This is a very high-value feature in turbomolecular pumps and other front-end semiconductor manufacturing equipment.

• Magnetic bearing systems may be used as an exciter, where the bearing force is modulated for deliberate excitation of vibration. The excitation force is applied to the rotor without contact. In addition, it can be precisely measured. In this way magnetic bearings are a valuable tool in equipment design, development and testing, as well as in rotor dynamic research. This method was recently used to test and verify a new seal design, and has also been used in a similar concept to test the performance of machine tool spindles.

• The ability to actively move the shaft within the air gap and even to oscillate the shaft is possible. Uses include the compensation of tool wear in grinding processes or adjustment of the grinding plate due to wear in a pulp refiner.

• Condition monitoring: Magnetic bearings have an intrinsic condition monitoring system, so the operator has access to full machine diagnostics without additional hardware. The DSP electronics along with MBScope allow viewing of the position and current (force) information in various formats for bearing tuning and machine diagnostics.

• Stiffness (inverse of position compliance) and damping of magnetic bearings can be adjusted in situ, allowing a machine to safely pass critical speeds and bending modes.

A range of controllers with varying power outputs (current and voltage) is available to match the required machine performance. A powerful application SKF has provided magnetic bearings to support the rotating parts of a new 1.2 MW high-speed generator. The generator is coupled to an industrial gas turbine and produces electricity for primary power, backup power or combined heat and power for a distributed power system. The system has been designed and built by Turbo Genset Co. Ltd, a UK company that specializes in high-speed machines with intelligent power control systems.

Justin Hall, operations manager of Turbo Genset, says, “We knew by using magnetic bearings we would be beating the conventional solution on life-cycle costs, by reducing maintenance and power consumption. The magnetic bearing system is an intelligent device. Microprocessor controlled, it offers an ability to monitor the condition of machines in service. It’s ‘fit and forget’ for life. You never have to service it.”

Why magnetic bearings are so different

Magnetic bearings operate without a lubricant. This makes them particularly suited to machines operating in a vacuum; at high or low (cryogenic) temperatures; or in corrosive process fluids. In fact, any machine, or process, with no tolerance for contamination by lubricants or wear particles may be a candidate for a magnetic bearing solution. Two typical examples are the semi-conductor capital equipment industry and the food and beverage companies. Lubrication-free operation also means that all the equipment associated with lubrication, such as pumps and filters is no longer required. This gives a worthwhile reduction in overall ownership cost.

Low shaft rotational losses is another characteristic of magnetic bearings. This results in the possibility of reducing motor power and achieving higher efficiencies. Low loss means operating temperatures are generally lower than those in other bearings, such as rolling element or fluid film bearings. This results in a reduced requirement for cooling equipment. Magnetic bearings work with a controlled air gap and even this can be an advantage. In certain processes there is a requirement for a process liquid or material to pass through the bearing. The air gap makes this possible. Biological and pharmaceutical applications involving life cell processing are just two examples. Magnetic bearings can also be hermetically sealed and are therefore attractive for processes handling corrosive fluids that would otherwise attack windings or laminations. They can also be submerged in process fluids under pressure without the need for seals. This makes them extremely useful in sensitive processes such as in the food industry.

Compared to rolling element or fluid film bearings, magnetic bearings enable much higher surface speeds. Up to 250 m/s or about 4.5 million “n x d” (rotational speed in min-1 and diameter in mm) can be achieved. This level of high speed enables many new applications such as advanced machine tool spindles or a hydrogen circulator that was developed and manufactured for the US National Laboratories.

The true worth of a magnetic bearing

A revealing way to realize the full value of a magnetic bearing in say, a large turbo machine application is to consider the life-cycle cost. The first savings are achieved by leaving out the oil lubrication system, cooling system, gearbox (a variable, high-speed motor directly coupled to, for example, a compressor), condition monitoring equipment and spare parts. Losing these ‘accessories’ also increases machine reliability.

Two further savings can be made. First from the decreased requirement for regularly scheduled maintenance and second from savings in energy. A magnetic bearing system consumes only a fraction of the energy consumed by a hydrodynamic bearing. With so many benefits, magnetic bearings now become an extremely attractive option for many applications that previously were not considered.

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