Extreme Temperature, High Load, Active Magnetic Bearings for Turbomachinery and ISG Applications

Technology Overview The active magnetic bearing is a rotor support that uses magnetic force to hold the rotor in place as opposed to the forces of a rolling element or air foil bearing. Like other bearing types, the magnetic bearing can be characterized in terms of stiffness, damping, and load capacity, thus the forces that apply these properties are somewhat analogous for each bearing. As shown in figure 1, a magnetic bearing consists of multiple electromagnetic coils attached to a ferromagnetic stator. The coils are arranged such that opposite poles are adjacent, maximizing magnetic flux through the rotor. A ferromagnetic, laminated rotor stack is attached to the shaft to provide the flux path and attractive magnetic forces while minimizing eddy current formation. Position sensors are fixed a certain distance from the shaft, on the order of thousandths of an inch. The voltage output from the position sensors and subsequent signal conditioning relays position information to the microprocessor controller, which uses this information to produce a command signal. The command signal is transferred to a proportional current through power amplifiers and output to the magnetic coils, providing an attractive magnetic force to the rotor. Typically, control algorithms treat the rotor support system as a mass/spring/damper interaction on two axes, usually vertical and horizontal. The controller will output signals proportional to the shaft’s displacement from center. Advantages and Limitations Using a magnetic bearing for turbine engine applications results in three major technological advantages: oil-free operation with no air requirements, operation in extreme temperature environments, and active control. Proceeding from these advantages is a laundry list of desirable improvements to the turbine engine. They include reduced weight; no bearing contact, no wear, and less maintenance; operation in high altitudes; the subtraction of an oil, lube, and cooling system; bearing placement in the engine’s hot sector; shorter, thicker and highly damped shafts; blade tip clearance control and stall suppression; fault tolerance; control of shaft imbalance; and dynamic stiffness and damping. The magnetic bearing also enables integral starter-generator (ISG) technology that could replace the bulk, complexity, and cooling/lubrication needs of a separate shaft/gearbox driven generator.