PURPOSE: To reduce the induced troubles without changing main circuits, by varying a control lead angle step by step without varying continuously with the induced troubles being limited at small angles.

CONSTITUTION: The field-current value which is detected by DCCT11 for detecting the field current is supplied to a function generator 12, and a magnetic flux is calculated. The armature current value which is detected by DCCT13 for detecting the armature current and the magnetic flux are supplied to a multiplier 14, and a braking force is calculated. The difference between the calculated braking value and the command value of the braking force which is the output of a braking pattern generator 15 is amplified in a comparating amplifier 16. A gate signal is formed through a phase shifter 17 for a field control bridge circuit 8, and the phase angle of the bridge circuit 8 is controlled by the gate circuit 18. When the maximum value of the phase angle is detected by a detector 19, the command signal which varies a phase-lead angle γ in a bridge circuit 4 step by step is supplied to a phase shifter 20, and γ is varied through a gate circuit 21. In this method, induced troubles can be reduced.