Hi Eric,
I'm not familiar with the term stall in conjunction with motors. In aerodynamics a stall condition is present if the draft is higher than the lift, which makes an aircraft descend at a rate similar to that of a piano thrown out of the window.
Axial fans can stall as well, meaning there is no air movement along the longitudinal axis. This can be caused by a closed intake air duct or exhaust or just too high resistance (clogged up filters etc.) within an air system. If an axial fan stalls the probability loosing blades is very likely, because the vibrations move from the wingtip to the root.
Radial fans cannot stall, hence vacuum cleaners use radial fans. You can hear distinctively how it winds up to max rpm (no load) if you close the suction port.
May be you happen to have a link towards the mentioned stall sensor circuit.
Regards
Hans