The constraints to eddy current induced in damper bars is different because of the different connection for the damper end winding of electric machine, and the current distribution of damper winding is also different. These make effects on the air gap magnetic field and no-load EMF. For the powerformer prototype machine in this paper, field-circuit coupling time-stepping FEM is proposed to calculate no-load EMF waveform of salient pole synchronous machine under different constraints for damper end winding. Then the results are compared with the experimental results. It shows that different constraints on damper winding produce different harmonic content in no-load EMF waveform of powerformer. The sine distortion factor also changes in some extent, and this changes need to be considered in accurate calculation.