After a long discussion some considerable time ago on this board, I was left with the impression that NO AC current whatsoever is involved, that the current is pulsed DC. Which still leaves the question as to whether pulsed DC can damage DC motors in the long term. I know this has been of concern in some low quality train motors but I don't know how or even if it has ever actually been established authoritatively. This is important because most of the motors used in our cars ARE low quality.