Abstract: A synchronous rotary electrical machine. According to the first aspect of the present invention, the electrical phase difference between the stator teeth and the nearest rotor teeth is a stepwise function of azimuthal stator tooth position. In a preferred embodiment of the present invention, the stator teeth grouped into two or more sets, and the machine is multiphase. Each set is associated with an armature winding wound in a plane perpendicular to the rotational axis of the machine. The rotor teeth are spaced at the same pitch as the stator teeth of a single set. When direct electrical current is provided to an excitation winding concentric with the rotational axis, an m-phase alternating EMF is induced in the armature windings. With two tandem groups of such sets of stator teeth, each with its own tooth pitch, alternating EMFs of different frequencies are generated. Two alternating EMFs of different frequencies are heterodyned to provide alternating EMF at the difference frequency.