Electrical machines

Abstract

An electrical machine comprises a rotor without windings, a stator having an armature winding (24,25) and a field winding (10) for generating a magnetomotive force in a direction extending transversely of the magnetomotive force generated by the armature winding. An electric circuit (40) is provided for controlling the current in the armature winding (24, 25) such that periods in which a magnetomotive force in one direction is associated with a first current pulse alternate with periods in which a magnetomotive force in the opposite direction is associated with a second current pulse. A position sensor is provided for monitoring the rotational position of the rotor and for supplying output signals dependent on the speed of rotation of the moor. Furthermore a control system (32) supplies control signals to the circuit (40) to control the current in the armature winding (24, 25), each control signal being produced in response to detection of a respective one of the output signals from the position sensor and being maintained for a length of time determined by the duration of the output signal. Such an arrangement enables control of acceleration, no-load speed and loaded torque-speed characteristics (and braking of the machine) to be achieved with simple on/off control of armature and field switching devices (if present), so that the control circuitry can be produced at relatively low cost. Simplification of the control circuitry is further ensured by the fact that such control can be effected without current sensing.

Dependencies are inferred from the claim text.