PURPOSE: To prevent an armature current from abruptly varying when the supplied voltage is instantaneously varied by connecting the primary side coil of a transformer in series with an armature circuit and connecting the secondary side coil of the transformer in series with a separately-excited field coil.

CONSTITUTION: The primary side coil 6a of a transformer 6 is connected in series with the armature circuit of separately-excited motor, and the secondary side coil 6b magnetically coupled to the primary side coil is connected in series with a separately-excited field coil 4. This transformr 6 is constructed that the polarity of the transformer 6 becomes in a direction that the voltage induced at the coil 6b is applied to a separately-excited field exciting power source 2 when an armature current increases. In this manner, since the variation in the counterelectromotive force of the armatue in respense to the armature current can be obtained, the deterioration of rectification of the motor can be prevented by reducing the peak value of the armature current as low as possible.