Comparison between the salient-pole synchronous machine and the double-fed induction machine with regards to electromagnetic parasitic forces and stator vibrations

Pumped-Storage Hydro Power Plants have been mainly equipped with fixed-speed Salient-Pole Synchronous Machines (SPSM). However the upgrade from fixed-speed technology to variable-speed technology, using Double-Fed Induction Machines (DFIM), brings many benefits: the control of the power in pump mode, operation at the best efficiency point in turbine mode, higher stability in case of a perturbation. Like the SPSM, the DFIM faces radial electromagnetic forces due to the non-sinusoidal airgap magnetic flux density. However these forces are not necessarily the same as the ones existing in the original SPSM essentially because of the different magnetic circuit properties. Vibration problems, due to matching between exciting force and stator eigen-mode, may occur in the DFIM although they did not exist in the original SPSM. The risk is even more relevant considering that the DFIM operates at different speeds, so that it leads to a shift in frequency in some force components.