eDrive - October/November 2011 - (Page 18)

FEaturE artiClE Magnetic Bearing Applications & Economics John Rama, P.E. Synchrony, Inc. Magnetic bearings offer a myriad of operational advantages and efficiency improvement over traditional oil lubricated bearings. They have been in use for the past 30 years, but for several reasons, including cost, have been limited to niche applications such as high-speed turbomachines, that required their unique capabilities. Recent advances in the technology are making it more practical to apply these bearings in a broader range of applications. They have become much smaller, simpler and more economical. ment of the rotor toward the stator along the axis of control. This entire process is repeated thousands of times each second, enabling precise control of machinery rotating at speeds in excess of 100,000 rpm. These bearing systems are available for both radial and axial applications. In high-thrust axial applications, bearing losses can be reduced from 2 percent to 5 percent of machine power to near zero. Applications of Magnetic Bearings A Background of Basic Principles A non-contacting technology, magnetic bearings have zero friction (just windage), no wear and high reliability. They are capable of previously unachievable surface speeds and eliminate many of the rotor dynamic complications of hydrodynamic bearings. Lubrication is eliminated, meaning that these bearings can be incorporated into processes that are sensitive to contamination, such as the vacuum chambers in which many semiconductor manufacturing processes take place, and making other applications inherently greener. Commercial/industrial applications for magnetic bearings are various and growing. Natural gas pipeline and storage compressors (and associated drivers) comprise many of the existing applications of magnetic bearings in large industrial machinery. Magnetic bearings have almost become the default choice for turboexpanders (and accompanying generators) where, in addition to their high-speed capabilities, they enable the elimination of oil in the process system (where the fluid is liquid air, oxygen, nitrogen, or a chemical such as ethylene). New high speed motor direct-driven centrifugal compressors (figure 2) with magnetic bearings are used in “friction-less” chillers produced in large sizes (up to 550 ton) and in high volumes (400+ units/yr). Figure 1. Simplified magnetic bearing diagram. Attractive electromagnetic suspension is used to elevate the rotating equipment by application of electric current to an array of stationary electromagnets (stator) interacting with a ferrous material integrated or mounted on the shaft (rotor), effectively suspending the shaft in the magnetic field (see figure 1). The control system continuously monitors and maintains the position of the rotor within this airgap. The control process begins by measurement of the rotor position with a position sensor that is similar to the probes used for non-contact vibration monitoring. The signal from this device is received by the control electronics, which compares it to the desired position. Any difference between these two signals results in calculation of the force necessary to pull the rotor back to the desired position. This is translated into a command to the power amplifier connected to the magnetic bearing stator. The current is increased, causing an increase in magnetic flux, an increase in the forces between the rotating and stationary components, and subsequently, move18 October/November 2011 Figure 2. 400 hp, 20000 rpm, integrated permanent magnet momo tor and centrifugal compressor w/ magnetic bearings. Magnetic bearings are an enabling technology expanding the use of electric motors to new applications and stretching application beyond previous operating boundaries. One advantage is the ability to vary the stiffness and damping of the bearings as a function of shaft speed, allowing the rotor dynamics to be tailored over the entire operating range (see figures 3a and 3b). Other capabilities allow resolution of rotor dynamic issues that can occur with hydrodynamic bearings, such as oil whirl instability which, if uncontrolled, can lead to catastrophic equipment failures. Oil whirl phenomenon occurs due to the generation of destabilization forces in an oil lubricated journal bearing system and can put a limit on the top operating speed of machinery. Although lobe, pressure dam and tilting pad bearing designs show greater stability for higher speeds than cylindrical bearings, www.e-DriveOnline.com http://www.e-DriveOnline.com

Table of Contents for the Digital Edition of eDrive - October/November 2011

eDrive - October/November 2011
IMS Research Forecasts Chinese Low Voltage AC Motor Revenues to Triple by 2016
ON Semiconductor and Stegia Collaborate to Bring Ultra Small Smart Motor Solutions to Market
Coordinated Circuit Protection Solutions For Power Supplies, Relays, Solenoids And Controllers
New, Quieter Stepper Drives Boost Performance
Tools for Advancing Motor Drive Technology
Next Generation Variable Frequency Drives
Magnetic Bearing Applications & Economics
Motors & Drives
Sensors & Encoders
Components
ICs & Semiconductors
Industry News
Calendar of Events
Marketplace

eDrive - October/November 2011