A method of propelling a mobile machine is disclosed. The mobile machine may have a plurality of DC traction motors, including a first DC traction motor and a second DC traction motor. The method may include driving a first traction device with the first DC traction motor, the first DC traction motor including a first field coil and a first armature electrically connected in series. The method may also include driving a second DC traction device with the second DC traction motor electrically connected in series with the first DC traction motor, the second DC traction motor including a second field coil and a second armature electrically connected in series. Additionally, the method may include, in response to slippage of the first traction device, bypassing at least a portion of electric current flowing through the first field coil around the first armature. The method may also include directing at least a portion of the bypassed electric current through the second field coil and the second armature.