Background: Electrical machines Material and methods: Finite element method Results: The great emphasis is placed on the physical phenomena existing the rotor because the measurement of rotor damper bar currents is difficult in practice. There are presented the comparisons of maximum magnitudes of stator current and electromagnetic torque determined during faulty synchronization with maximum magnitudes designated during sudden short circuit after both no-load and rated operation condition. In addition, the effect of synchronizing limits on faulty synchronization was presented. Conclusions: Faulty synchronization with inverse phase sequence causes an appearance of higher electromagnetic torque than in case of sudden short circuit and thereby huge mechanical stresses which can lead to rotor shaft damage. The most vulnerable construction element is rotor wedge localized near to rotor big edge. Huge included current in the conductive parts of the rotor which appear on account of stator current negative sequence can cause a failure of the retaining ring. The worst case is for 1.05 Un from the mechanical stress point of view. Whereas from the thermal point of view, the worst case is when speed is below synchronous speed.