An electronic controlled switch, for example a bipolar thyristor, is included in the armature circuit between the brushes, conduction of the thyristor being controlled by a pulse source providing pulses in synchronism with the rotation of the machine for selectively timed energization of the thyristor and hence connection of the armature winding. The pulse source may be a mechanical interrupter coupled to the machine shaft, a separate pulse source, or the like, and the thyristor can be directly controlled from the pulse source by means of slip rings, or non-contacting pulse transfer circuit elements, such as inductive elements.