Dear Electricpete, thank you for your thoughts...your first input was the right one (unfortunately)!
I tried with an impact test (accelerometer on stator stack, hammer at about the same point, motor still) and a very sharp peak structural resonance at 520 Hz came out.
Repeating the same impact...
Dear GregLocock, thank you for your inputs. I took my time to try what you said.
I can't hear 500Hz component noise by myself (I tried, referring my ear to a pure 500Hz sine),
because it isn't predominant in the overall sound pressure spectrum...more important are 49 Hz (running speed), 100Hz...
I'm studying the airborne noise emitted by a single-phase small (less than 100W) electric motor for dishwasher machines.
It is a 2 poles motor with capacitor connected to an auxiliary (2 poles) winding very similar to the main winding (capacitor and auxiliary winding are always connected during...