This paper presents the analysis of the instabilities inception in a transonic
centrifugal compressor for different rotation speeds. The analysis was conducted from
experimental results obtained with unsteady pressure sensors implanted in the inducer,
vaneless diffuser and vaned diffuser. Beyond the stability limit the compressor enters
into a deep surge without any precursor, whatever the speed. The surge process is
initiated in the vaned diffuser by a massive boundary layer separation. For low speeds,
together with the surge which remains triggered in the diffuser, aerodynamic instabilities
are detected in the inducer. These instabilities can be understood as “tip clearance
rotating disturbances” because they are generated at the leading edge of the impeller main
blades and move along the tip clearance trajectory.