Abstract: A circuit for changing the poles of a rotor of a synchronous machine having contact brushes in operative connection with first and second slip rings includes a switch for reversibly connecting the contact brushes to an electrical current source, reversible pole coils operatively connected to one of the slip rings and connected through a first diode to a circuit portion which is formed by irreversible pole coils, and by a second diode and a third diode, and which is operatively connected with the other of the slip rings, and a circuit branch including a fourth diode having a polarity reversed with respect to the polarities of the first, second and third diodes. One terminal of the fourth diode is connected between the reversible pole coils and the first diode and the other terminal of the fourth diode is connected between the irreversible pole coils and the second diode.