I would say that if the generator was polarized backwards, that would be the case. I think this could happen if one were to motor it with the polarity reversed. By the way I have never tried this.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!