What did Chester say?

I think I was once taught why this happens, but it was over 40 years back and I've forgotten most of it so all that follows may be nonsense…

The magnetic field generated by the rotor windings is distorted by various factors that I freely admit I don't understand, but the net result is that in a motor which will normally run in one direction the windings are either advanced or retarded (my guess would be advanced as the field will need time to build and collapse – an Electrical Engineer would be handy right now but I dropped out after the first year) to compensate and obtain maximum efficiency. When the motor runs backward this compensation is the wrong way around and the motor is much less efficient, so it would probably run hotter as well as slower.

I suppose it is analogous to running a two-stroke engine backwards, the porting would still be symmetrical but the spark would be retarded instead of advanced.

Brian G