Circuit arrangement for powering an electric motor

Abstract

A circuit arrangement for powering an electric motor having at least three windings from a direct voltage source via a commutation circuit which, for each winding terminal of the electric motor, comprises a commutation branch including a first and a second switching element by which the relevant winding terminals can be connected selectively to a first terminal of the direct voltage source or, via a measurement impedance, to a second terminal of the direct voltage source. A ring counter device periodically changes over the switching elements of the commutation circuit in accordance with a signal series which is cycled through the ring counter device under control of a clock signal, one period of the ring counter device corresponding to one complete cycle of the signal series through the ring counter device. A clock frequency generating stage supplies the clock signal whose clock frequency is dependent on the measurement voltage. A motor-current dependent commutation which operates reliably for all operating conditions of the motor, particularly all speeds, using a circuit of low complexity, is obtained in that a sample and hold circuit has an input coupled to the measurement impedance in order to sample a measurement voltage appearing across the measurement impedance and in that the ring counter device, in each of its periods, controls the sample and hold circuit so as to acquire a sampled value of the measurement voltage at a presettable instant outside the time interval for the change-over of the switching elements of the commutation circuit.