Magnetic Bearings Come Of Age

Magnetic bearings come of age

Linda Widbro
Revolve Magnetic Bearings Inc.
An SKF Company
Calgary, Alberta, Canada

Magnetic-bearings, which support shafts with magnetic levitation rather than mechanical contact, have been in industrial use for decades. Magnetic bearings offer a host of advantages to users, including high-speed capabilities and the ability to operate lubrication-free and in vacuum environments. They generate no friction, experience minimal wear, and operate contamination free with extremely low vibration. And the bearings can precisely control shaft position, measure external forces acting on the shaft, and even monitor a machine's operating condition.

Recent technological developments, especially in digital processing and control, have made magnetic bearings a more-robust and cost-effective design solution than ever. Today's bearings are suitable for a wide range of applications, from semiconductorfabrication equipment to microturbines, from refrigeration compressors to vacuum pumps.

MAGNETIC-BEARING BASICS
Magnetic-bearing systems electromagnetically suspend shafts by applying electric current to a bearing's ferromagnetic materials. The systems have three main elements: bearing actuators, position sensors, and controller and control algorithms.

Typical units consist of two magnetic radial bearings and one magnetic thrust bearing. They control the shaft along five axes: two axes for each radial bearing and a fifth axis along the shaft. Magnetic bearings have stationary and rotating components — the stator and rotor, respectively. The radial magnetic bearing stator resembles an electricmotor stator.

The radial stator is formed by a buildup of laminations, each of which is shaped with poles. The laminations stack together, and coils of wire are wound around each pole.

Controlled electric currents passing through the coils produce an attractive force on the ferromagnetic rotor and levitate it within an air gap. The gap usually measures about 0.5 mm but, in some applications, can be as large as 2 mm. The rotor fits over the shaft, which is in the air gap but need not be centered. This is useful in applications where it is valuable to compensate for wear, or if the shaft oscillates — such as in machine-tool grinding processes where the wheel wears over time.

A magnetic thrust bearing provides axial control. The thrust-bearing rotor is a solid steel disk attached to the shaft and positioned at a preset distance from the stator on one or both sides. During operation, electromagnetic forces produced by the stator act on the rotor and control axial movement.

Magnetic-bearing arrangements also include touchdown or auxiliary bearings. Their main function is to support the shaft when the machine is idle and protect machine components in case of power outage or failure. The touchdown bearing's inner ring is smaller than the magnetic-bearing air gap to prevent potential damage if the shaft delevitates.

CONTROL SYSTEMS
The control system regulates bearing current and, thus, the force of the bearings. During operation, radial and axial position sensors feed data on shaft location and movement to the controller. It compares actual and desired shaft position, calculates the force required to maintain the shaft in the preset position and, if necessary, commands the amplifier to adjust the electric current to raise or lower the level of magnetic flux.

The main parts of the control system are the digital signal-processing (DSP) electronics, a power supply, and amplifiers. Generally, the larger the machine, the larger the amplifiers. Controller size also depends on the dynamic load capacity required, which is typically greater in heavy machines.

The shaft can be controlled through Single Input/Single Output (SISO) or Multiple Input/ Multiple Output (MIMO) algorithms for high-speed and more-demanding applications. The controller typically measures and processes position signals at 10-kHz frequency, enabling precise control of machinery rotating at speeds of 100,000 rpm and higher.

A significant benefit of magnetic-bearing technology is that the controller functions as a built-in condition-monitoring system, providing extensive real-time information and making other monitoring devices unnecessary. Software, such as MBScope from SKF, provides detailed diagnostic information about machine health and helps schedule preventive maintenance more effectively.

The software includes configuration tools for tuning input parameters and checking clearances prior to start-up. Its viewing tools include real-time monitoring of positions, currents, and forces; an alarm log that captures all system variables before and after an unusual event; and short or long-term trending. This lets users view information in various formats for bearing tuning and machine diagnostics. Adaptive vibration control (AVC) is another important tool. AVC computes the forces necessary to cancel out vibration in two ways. One is to let the shaft rotate around its geometric center and tightly control shaft displacement, eliminating runout caused by imbalance. This is useful in high-precision applications, such as machine tools.

The other way is to rotate the shaft around its center of mass to reduce vibrations transferred to the housing or casing (to <0.01 m m). This is a valuable feature in turbomolecular pumps and other semiconductor-manufacturing equipment.

AVC can increase machine reliability and the time between service intervals. Its adaptive feature minimizes vibrations even with rotor fouling over time and, by canceling out process disturbances, can extend equipment's operating range.

DESIGN CONSIDERATIONS
The ultimate goal of magnetic-bearing design is reliable, noncontacting rotation over the machine's entire speed range. It is also essential to meet OEM and end-user cost targets without compromising performance. Reducing the size of digital-control systems means more cost-efficient solutions, and compact magnetic-bearing designs can lead to smaller, more-robust machines.

When developing magnetic-bearing systems, main factors to consider are the speeds, loads, and operating environment. The mechanical strength of the shaft typically limits speed. Surface speeds of 3.5 X 10 ⁶ DN (diameter in mm 3 rpm) are possible.

Static capacity — the maximum force magnetic bearings generate to lift the shaft — is a function of variables such as amplifier current, surface area of the magnetic poles, number of coil windings, and air-gap dimensions. A good rule of thumb is 75 lb of force/sq in. of bearing.

Dynamic capacity — the rate at which magnetic bearings change the applied force — is determined by a single variable, amplifier voltage.

Consider, for example, a 150-N magnetic bearing connected to a 2-A/40-V control system. Switching to a larger, 200-N bearing with more coil turns, a larger magnetic pole area, and so on, will increase static capacity. If the controller remains the same, however, there will be no effect on dynamic capacity — the ability to handle shaft imbalances and other dynamic forces during operation.

Conversely, retaining the 150-N magnetic bearing but switching to a 3-A/50-V control system will increase the unit's dynamic capacity but have no effect on static capacity.

DIVERSE APPLICATIONS
The unique design and wide-ranging capabilities of magnetic bearings offer solutions in a host of diverse applications. One example is semiconductorfabrication, particularly front-end operationsinvolving the production of silicone wafers. Magnetic bearings can improve yields in these operations, which are highly sensitive to contamination and vibration. For instance, magnetic bearings permit edge rotation of 300-mm wafers, allowing convenient access to both wafer sides.

Because magnetic bearings have an air gap, they are ideal for certain biological and pharmaceutical applications. Blood cells or other liquids can pass through the air gap without damage.

Refrigeration compressors are another important application. Magnetic bearings can run at the high speeds required by new-generation refrigerants and, unlike conventional oil-lubricated bearings, they pose minimal risk of contamination. Magnetic bearings can also be hermetically sealed and are therefore attractive for processes handling corrosive fluids that would attack windings and laminations.

WHERE MAGNETIC BEARINGS MAKE SENSE
Magnetic bearings operate without contact. This results in many unique characteristics that are valuable in a wide range of equipment. Applications that require more than one of the following attributes are generally suitable for magnetic bearings.

Lubrication-free. Consider magnetic bearings when lubrication systems for other types of bearings are expensive, unreliable, or unsafe; the lubricant contains environmentally unfriendly components and disposal becomes an issue; or the lubricant is incompatible with or contaminates the fluid or process.

Reliability. The bearings offer superior reliability, comparable to that of electric motors, and it is reasonable to expect an operational life of 15 to 20 years. The control system has reliability typical of electronic components with conservative mean time between failures of 5 years.

Operation in vacuum. High vacuums are difficult environments for lubricants. Many systems in high to ultrahigh vacuums (to 10- ¹⁶ Torr) are sensitive to outgassing and contamination of volatile lubricants.

Low vibration. Magnetic bearings are suitable for applications sensitive to machine vibration. Typical casing vibration is 0.01 m m.

Force measurement. The controller can determine bearing load and force direction by measuring current and position within the bearings. This gives valuable information for machine designers when developing magnetic-bearing systems. Forces can be measured with accuracy generally better than 5%.

Shaft-position control. Because sensors monitor shaft location, the control system can reposition or oscillate the shaft while it is rotating. For example, the control system can compensate for wear and adjust the shaft axial position during operation to optimize the grinding-plate gaps and improve product quality in pulp refiners.

Precision. Tight control eliminates shaft runout caused by unbalance. This is accomplished with Adaptive Vibration Control. Shaft displacement at the running speed can be reduced to about 1 m m, important for precision grinding and machine-tool cutting operations.

Contamination. Processes sensitive to microcontaminants benefit from magnetic bearings with stainless-steel cans or barriers. With the advent of 300-mm wafers and 0.25-mm device sizes, it has become critically important to eliminate microcontaminants in all aspects of wafer processing.

Submerged operation. Magnetic bearings can operate directly in the process fluid and eliminate the need for mechanical seals. This reduces emissions, machine cost, and operating maintenance costs.

Reduced energy consumption. Magnetic bearings reduce frictional losses, resulting in higher overall mechanical efficiency. And the lack of a lubrication system eliminates the cost of operating pumps, cooling fans, reservoir ventilation fans, and so on.

Condition monitoring. Magnetic bearings have built-in condition-monitoring capabilities. This eliminates the need for devices like accelerometers and vibration sensors, as well as monitoring equipment and interface software. In addition, magnetic-bearing control systems directly observe shaft and process-fluid behavior with no need to interpret rolling-element and race frequencies.

Air gap. Some applications simply benefit from noncontact operation. For example, in biotech applications, heart pumps or mixers benefit by not damaging cells with contacting surfaces. In textiles, fibers can pass through the gap. Air gaps can be up to 2 mm.

High speed. Speed is limited by the mechanical strength of the shaft. Surface speeds on radial bearings are as high as 3.5 3 10 ⁶ DN (diameter (mm) 3 rpm). This attribute becomes more valuable as lubrication becomes more difficult.

Phase control. Today's DSPs do more than just controlling the magnetic bearing, performing functions that can easily reduce the cost of a system by more than the cost of the magnetic bearing. One example is phase control. This feature synchronizes shaft rotation with external timing signals. Synchronization positions the shaft (phase) to within 0.05 of its reference mark while rotating at speeds to 36,000 rpm. Phase control is used in applications such as neutron choppers.

MAKE CONTACT
Revolve Magnetic Bearings Inc.,
(403) 232-9292, www.SKFmagneticbearings.com