I think that the idea of adding a DC component to an AC induction motor
might be that the DC tends to magnetically saturate the poles of the
stator.  This would tend to reduce the change in the stator magnetic
field that alternately pulls and releases the rotor teeth.

 [ Good observation, Mark, I didn't think of that!  -- Robbie

There must be five zillion kinds of tiny AC induction motors.  They're
fun because you can get away with all sorts of power-wasting tricks
that would be prohibitive in a big industrial motor.

Mark Kinsler