Method and circuit for D.C. braking a three-phase asynchronous machine

Claim 1

In a circuit for the direct current braking of a three-phase synchronous machine having first, second, and third Y-connected stator windings adapted to be connected to a three phase power supply by causing direct current to flow in the first and second windings when braking and, at the same time, short-circuiting the third winding, the circuit comprising: a parallel circuit coupling the first stator winding to a first phase of the power supply, the parallel circuit comprising a normally closed first switch and a rectifier valve connected in series with a normally open second switch, a normally closed fourth switch coupling the third stator winding to a third phase of the power supply, and a normally open third switch coupled across the third stator winding, the first, second, third, and fourth switches each having an operating input, the improvement comprising: a current sensor coupled between the third switch and the third stator winding and having an output; a limit indicator having an input coupled to the output of the current sensor and having an output; a tripping switch having an output coupled to the operating input of the first, third and fourth switches and to the input of the pulse generator; a pulse generator having an input and an output; and a digital logic member having OR logic, the logic member having an input coupled to the output of the limit indicator, an input coupled to the output of the pulse generator, and an output coupled to the operating input of the second switch.