the rotor of an electronically commutated dc machine , which may be operated in the motor mode or in the generator mode , comprises at least one permanent magnet having at least four poles disposed concentrically to the rotary axis of the rotor , said poles being polarized substantially normal to said axis of rotation and being disposed adjacent an annular return circuit member . the stator winding is arranged in the annular gap between rotor and return circuit members and comprises a meander - like conductive strip array whose linear forward or backward extending meander sections consist of a number of parallel conductor sections . means for detecting the angular position of the rotor relative to the linear forward and backward extending meander sections supplies signals to an electronic control system which in turn controls the flow of current through the conductive strip array such that the latter produces a magnetic field to drive the rotor . an electronically commutated dc motor of such structure and including , for instance , 40 poles exhibits an extremely constant rated motor speed and by virture of the high space factor in the magnetic gap produces a high e . m . f . such a motor may preferably be used as driving unit in winchester disk storages .