Electromagnetic circulatory forces and rotordynamic instability in electric machines

The electromechanical interaction in electric machines induces additional forces between the rotor and stator. To study this interaction, a simple electromechanical model was developed. The mechanical behaviour was modelled by the Jeffcott rotor. The electromagnetic forces were described by a simple parametric model including two electormagnetic variables. The aim of the study was to investigate the effects of electromechanical interaction on rotordynamic instability in electric motors. If the new electromagnetic variables are interpreted as ‘quasi-displacements’, the interaction turns up in the equations of motion as additional damping, stiffness and circulatory terms. The electromagnetically induced damping and stiffness effects in electric motors have been studied previously. However, the effects of circulatory terms have been overrided in electric motors. It is well-known that the circulatory, i.e. cross-coupled stiffness, terms are a major source of instability in rotating machines. Thus, the presented model offers a new and simple explanation for the rotordynamic instabilty in electric motors. The numerical examples raised another source of instability. The system parameters may yield a negative-definite stiffness matrix (symmetric part), which destabilize the system without stabilizing forces.