Abstract: A voltage V.sub.ac of a synchronous motor is converted into positive and negative voltages of a predetermined magnitude relative to the ground during the positive and negative half-wave periods, respectively. The voltage is delivered during the positive and negative periods of a current I.sub.b and is used as a signal representing a power factor. The difference between a target value of the power factor and the signal representing the power factor thus delivered is used for compensating for the difference between a target current of a field system and a feed-back signal thereby to control a field current.