An electronically commutating motor having a phase winding connected in at least one of the branches of a bridge circuit. A comparator measures the voltage across one of the diagonals of the bridge and switches the voltage across the other bridge diagonal as a function of the polarity of the first-mentioned voltage.

A stator has a pair of magnetic pole portions, and a rotor having N- and S-poles is disposed between the two magnetic portions. Each magnetic pole portion of the stator has two static magnetic poles and a cut-away portion or indent formed therebetween. The stator is quadripolar statically, and bipolar dynamically.

A driven rotary shaft system has a rotor including a core with a generally cylindrical surface, made from a stack of circular laminates of magnetic material which are clamped together in alignment. Permanent magnets are mounted in aligned windows formed in each laminate near diametrically opposed pole pieces for providing at least a pair of magnetic fields in positions which interact with magnetic fields of an adjacent stator. The permanent magnets and the windows have an arcuate cross section generally corresponding to the contours of the generally cylindrical surface. At least a pair of opposed air gaps are formed on diametrically opposed sides of the core, and between said pole pieces. These air gaps have internal concave contours which follow, correspond, and conform to the contours of flux lines produced by said permanent magnets. In one embodiment, the air gaps are completely within the perimeter of the circular laminates. The portions of the cylindrical surface which are between the air gaps form the pole pieces for said rotor. The portions over the air gaps form an aerodynamically smooth shape, containing a limited amount of the laminate material in order to limit magnetic flux therein.

A diametrical cross-sectional peripheral shape of each of the teeth of a rotor of a stepping motor is made to be more acute than the conventional peripheral arcurate shape that corresponds to an arc having a radius that extends from the axis of the rotor.

In a two-phase salient pole variable reluctance motor in which the number of stator poles is four or a multiple of four, alternate stator poles carry windings of different phases and each stator pole has an elongated pole shoe, so that the pole tips of each adjacent pair of stator poles are closely spaced. Each rotor pole has an iron depletion layer in the vicinity of its poleface surface which defines a saturating zone, across which the greater part of the magnetomotive force produced by stator windings is developed by uniform flux build-up with rotor angle throughout almost the entire duration of pole overlap. A working stroke approximating a stator pole pitch is achieved. The circumferential extent of each rotor pole is matched to that of the stator poles so that when the rotor poles are fully aligned with a pair of stator poles, each rotor poletip is located in the vicinity of a pair of spaced apart stator pole tips, the relative disposition of the pole tips being such that when both phases of the motor are then simultaneously excited, fringing flux passes through the rotor poletips, this flux being relatively strong for one poletip of each rotor pole and relatively weak for the other poletip. When the rotor poles are initially positioned so that there is partial overlap between rotor and stator poles, the motor is started by a conventional reluctance motor phase sequence, but when the rotor poles are initially fully aligned with stator poles, in which position neither phase acting alone is capable of producing torque, both phases are excited simultaneously, so that torque is applied to the rotor by the flux asymmetry to move the rotor into normal phase energization. The motor is bi-directional and self-starting in both senses from any rotor position.