CLC Closed Loop Control

Increases in compressor efficiency and pressure ratio can be achieved by controlling the blade tip-to-shroud clearance. The design and test results of a fail-safe active tip clearance control system for a centrifugal compressor are presented. An electromagnetic actuator is used to pull the compressor impeller axially to reduce the tip clearance. The actuator acts against a preloaded Belleville spring. The controlled variable is an averaged, tip-to-shroud clearance which is measured with a capacitance displacement sensor and averaging electronics. A scheduled digital PI control law was used to account for the nonlinear effects of the electromagnet with electrical current and gap size. Closed-loop tip clearance control is demonstrated over the clearance range from 0.005 inches (0.127 mm) to 0.012 inches (0.305 mm). A simplified model of the system is presented along with open and closed-loop dynamic test results. Changes in the compressor performance due to tip clearance control are also presented