A note on internal damping induced self-excited vibration in a rotor by considering source loading of a DC motor drive

It is well known that rotors become unstable beyond a certain threshold spinning speed due to the non-conservative circulatory forces, which arise out of rotating internal damping. In this note, it is shown that if the source loading of the non-ideal drive is considered then this instability manifests itself as a constant rotor spinning speed and a constant amplitude whirl orbit about an unstable equilibrium. A DC motor drive is considered and the corresponding steady-state spinning frequency and whirl orbit amplitude are analytically derived as functions of the drive and the rotor system parameters.