Electrical machines

An electrical machine comprising a rotor (79, 89) without windings, a stator (78, 88) having armature windings comprising at least two coils (A1, A2) having active portions positioned with armature winding slots (81, 91) in a stator iron, and field windings (F) having active portions positioned within field winding slots (80, 90) in the stator iron so as to generate magnetomotive forces in direction s extending transversely of the magnetomotive forces generated by the armature windings, and circuit means for controlling the currents in the coils (A1, A2) in synchronism with rotation of the rotor such that periods in which a magnetomotive force is generated in one direction by current flow in one of the coils alternate with periods in which a magnetomotive force is generated in the opposite direction by current flow in another of the coils, characterised in that the armature winding slots and the field winding slots are equal in number and alternate with one another in the stator iron, and in that, considering the width of each slot (80, 81, 90, 91) as being the maximum extent of the slot in the direction of rotation of the rotor and considering the depth of each slot as being the maximum extent of the slot radially of the rotor and the thickness of the back iron behind the slot as being the distance between the maximum extent of the slot and the maximum extent of the armature iron along the same radial direction, the width of each armature winding slot (81, 91) is greater than the width of each field winding slot (80, 90) and the thickness of the back iron behind each armature winding slot (81, 91) is greater than the thickness of the back iron behind each field winding slot (80, 90).