Estimation of rotordynamic seal coefficients using active magnetic bearing excitation and force measurement

In high-speed rotational machinery such as pumps or compressors, contactless seals are commonly used to separate different fluids or gases and pressure levels. However, the presence of a leakage flow through the seal gap exerts forces on a rotor. These can culminate in stiffening, restoring, and damping effects as well as in unstable, self-excited vibrational behavior. The JEFFCOTT rotor model and rotordynamic seal coefficients are put under investigation to prevent instability in the rotating machinery and to determine the rotor-seal systems dynamic behavior. This paper focuses on an experimental methodology, examined on a flexible rotor-seal test rig using an active magnetic bearing for excitation. Coefficient identification problems due to unknown random force (noise) in the experiment are shown and a solution is described in detail and validated on the test rig. The presented methodology leads to a calculation of rotordynamic seal coefficients during safe operating conditions. They are ultimately used to describe the system’s behavior and to predict the onset speed of instability.