I work in a large electrical machine shop. We have a test that no one can explain to me. We energise a dc field frame (in this case 6 pole) with ac and record the current draw, then reverse the polarity of connection, energise and compare values. If a pole is reversed polarity then a difference in these values appears. Why is that? My gut says it must be related to inductive reactance but I can't find an answer or quite visualise it. Any help would be greatly appreciated.