Parasitic forces in large synchronous machines considering tooth ripple effects

In large salient-pole synchronous machines the magnetic field crossing the air-gap causes radial forces on the stator core physically described by the Maxwell-stress. These force waves do not contribute to the torque but may lead to dangerous vibrations. The stator winding magneto-motive force, stator tooth ripple effects, the saliency of the rotor poles, induced currents in the damper winding and the field winding magneto-motive force generate different harmonic field waves circulating through the air-gap. These field waves interact by modulation effects and create different force wave modes. This paper describes an analytical model for computing force waves and compares results with finite-element calculations and measurements. Attention is paid to reducing parasitic forces with special non-standard stator winding phase belt distributions.