Working Principle of DC Motor

1. Working principle
of DC generator The working principle of DC generator is to convert the alternating electromotive force induced in the armature coil into DC electromotive force when it is led out from the brush end by the commutation action of the commutator and the brush.
The direction of the induced electromotive force is determined by the right-hand rule (the magnetic flux lines point to the palm of the hand, the thumb points to the direction of the conductor movement, and the directions of the other four fingers are the directions of the induced electromotive force in the conductor.)
At the moment shown in Figure 1.1, the directions of the induced electromotive force of conductors ab and cd are from b to a and from d to c respectively. At this time, brush A is positive and brush B is negative.
Figure 1.1 Principle model of DC generator

When the coil rotates 180° counterclockwise, the conductor cd is located under the N pole, the conductor ab is located under the S pole, and the electromotive force in each conductor changes direction.

Figure 1.2 Principle model of DC generator

As can be seen from the figure, the conductor in contact with brush A is always located under the N pole, and similarly, the conductor in contact with brush B is always located under the S pole. Therefore, brush A always has positive polarity, and brush B always has negative polarity, so the brush end can lead to a pulsating electromotive force with constant direction but varying magnitude. If the number of coils on the armature is increased and they are connected according to a certain rule, the pulsation degree can be reduced, and a DC electromotive force can be obtained. This is the working principle of a DC generator.

2. Working principle of DC motor
The direction of the force on the conductor is determined by the left-hand rule. This pair of electromagnetic forces forms a torque acting on the armature. This torque is called electromagnetic torque in the rotating motor. The direction of the torque is counterclockwise, attempting to make the armature rotate counterclockwise. If this electromagnetic torque can overcome the resistance torque on the armature (such as the resistance torque caused by friction and other load torques), the armature can rotate counterclockwise. Figure 1.3 Principle model of DC motor

When the armature rotates 180°, the conductor cd turns to the N pole, and the conductor ab turns to the S pole. Since the direction of the current supplied by the DC power supply remains unchanged, it still flows in from the brush A, passes through the conductors cd and ab, and flows out from the brush B. At this time, the force direction of the conductor cd changes from right to left, and the force direction of the conductor ab changes from left to right, and the direction of the electromagnetic torque generated is still counterclockwise.

Figure 1.4 DC motor principle model

Therefore, once the armature rotates, due to the commutation of the current by the commutator and the brush, the DC current flows in alternately from the conductors ab and cd, so that as long as the coil side is under the N pole, the direction of the current is always the direction of the brush A, and when it is under the S pole, it is always the direction of the brush B. This ensures that the current in the coil side under each pole is always in one direction, thus forming a torque with a constant direction, so that the motor can rotate continuously. This is the working principle of the DC motor.