A ring-shaped permanent magnet is fixed inside the DC motor, and the current passes through the coil on the rotor to generate ampere force. When the coil on the rotor is parallel to the magnetic field, the direction of the magnetic field will change when the coil on the rotor continues to rotate. Therefore, the brush heel at the end of the rotor changes. The plates are alternately contacted, so that the direction of the current on the coil also changes, and the direction of the Lorentz force generated does not change, so the motor can keep rotating in one direction.
The working principle of a DC generator is to convert the alternating electromotive force induced in the armature coil into a DC electromotive force when it is drawn from the brush end by the commutator and the commutation action of the brush.
The direction of the induced electromotive force is determined according to the right-hand rule (the magnetic line of induction 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 conductor's force is determined by the left-hand rule. This pair of electromagnetic forces forms a moment that acts on the armature. This moment is called electromagnetic torque in a rotating electrical machine. The direction of the torque is counterclockwise in an attempt to make the armature rotate counterclockwise. If the electromagnetic torque can overcome the resistance torque on the armature (such as resistance torque caused by friction and other load torques), the armature can rotate in a counterclockwise direction.