Tip clearance actuation with magnetic bearings for high-speed compressor stall control

Magnetic bearings are widely used as active suspension devices in rotating machinery mainly for active vibration control purposes. The concept of acti ve tip-clearance control suggests a new application of magnetic bearings as servo-actuators to stabilize rotating stall in axial compressors. This pap er presents a first-of-a-kind feasibility study of an active stall control experiment with a magnetic bearing servo-actuator in the NASA Glenn high-sp eed single-stage compressor test facility. Together with CFD and experiment al data a two-dimensional, incompressible compressor stability model was us ed in a stochastic estimation and control analysis to determine the require d magnetic bearing performance for compressor stall control. The resulting requirements introduced new challenges to the magnetic bearing actuator des ign. A magnetic bearing servo-actuator was designed that fulfilled the perf ormance specifications. Control laws were then developed to stabilize the c ompressor shaft. In a second control loop, a constant gain controller was i mplemented to stabilize rotating stall. A detailed closed loop simulation t it 100 percent corrected design speed resulted in a 2.3 percent reduction o f stalling mass flow, which is comparable to results obtained in the same c ompressor by Weigl et ell. (1998. ASME J. Turbomach. 120, 625-636) using un steady air injection. The design and simulation results presented here esta blish the viability Of magnetic bearings for stall control in aeroengine hi gh-speed compressors. Furthermore, the paper outlines a general design proc edure to develop magnetic bearing servo-actuators for high-speed turbomachi nery.