There is a ring permanent magnet fixed in the DC motor, […]
There is a ring permanent magnet fixed in the DC motor, and the current produces ampere force through the coil on the rotor. When the coil on the rotor is parallel to the magnetic field, the direction of the magnetic field will change, so the brush at the end of the rotor will contact with the converter alternately, so the direction of the current on the coil will also change and the direction of the resulting Lorentz force will remain unchanged, so the motor can keep the same direction.
The working principle of the direct current generator is to apply the alternating electromotive force induced in the armature coil, and the commutator is matched with the reversing action of the electric brush so as to change into the principle of the direct current electromotive force when the electric brush is led out from the brush end.
The direction of the induced electromotive force is determined by the right hand (the magnetic induction line points to the palm of the hand, the thumb points to the direction of movement of the conductor, and the other four fingers point to the direction of the induced electromotive force in the conductor).
The direction of the force of the conductor is determined by the left hand. This pair of electromagnetic forces forms a moment acting on the armature, which is referred to as an electromagnetic torque in the rotating electric machine, and the direction of the torque is a counter-clockwise direction in an attempt to rotate the armature in a counter-clockwise direction. The armature can be rotated in a counter-clockwise direction if the electromagnetic torque is able to overcome the resistance torque on the armature (e. g., the resistance torque due to friction and other load torque).