stepping motor techniques are made viable for heavy duty machines with variable speed operation by reason of the simplification and gain in specific output and efficiency which results from constructions ensuring maximum saturation of magnetic flux at the stator pole faces and with minimal airgap between the associated stator and rotor pole faces . the density of the ferro - magnetic material forming the arcuate faces of the rotor poles may be varied circumferentially of said faces and the poles relatively shaped and arranged so as to render possible uni - directional current , 2 - phase operation . energy stored is minimised and the reduced energy inductively stored is returned to the source by vestigial coupled windings . uni - directional currents only flow in the windings , and the switching circuitry , although not fundamentally novel , is greatly simplified .