Vibrational Forces in Salient Pole Synchronous Machines Considering Tooth Ripple Effects

In rotating electrical machines, the air-gap magnetic field causes radial forces on the stator core physically based on the Maxwell stress. These radial force waves do not contribute to the torque generation of the machine but may lead to dangerous vibrations. This problem must be especially researched when designing salient pole synchronous machines with a high number of poles and fractional slot windings. At full load operation, the stator winding magnetomotive force (MMF), the stator tooth ripple effects, the saliency of the rotor poles, the induced currents in the damper winding, and the field winding MMF generate a set of harmonic field waves circulating around the air gap. The waves interact by modulation effects and therefore create different magnetic force wave modes. This paper presents an analytical computation model for force waves and compares results with finite-element calculations as well as measurements. Attention is paid to tooth ripple phenomena and reduction of parasitic forces with nonstandard stator winding phase belt distributions.