To enhance reliability of braking operation when a switching is made from motor mode to braking mode by providing a main operation transformer having a secondary winding being connected in a braking circuit.

A transformer 26 is connected permanently, on the primary side 28 thereof, with two poles 21, 23 of a power supply 22. The other end of the secondary winding 30 of the transformer 26 is connected with the contact 54 of a second switch S2 being opened in motor mode and with contacts 50, 51 in braking mode. Since connection between the contacts 50, 52 is separated in braking mode, the secondary winding 30 of the transformer 26 is connected with a field winding 16 and connected, at the other end thereof, with an armature winding 12. More specifically, a closed braking circuit 68 comprising the two field windings 14, 16, the secondary winding 30 of the transformer 26, the armature winding 12, and two commutator windings 18, 20 is connected in series.