Guide: DC motor is a motor that transfers direct current electrical energy to mechanical energy. Because of its good speed performance and are widely used in electric drive.
As shown below poles DC motor working principle diagram. The structure of DC motor is the same as DC generator. Except that the brush A, B external connect direct power current. As shown below the instantaneous current flowing illustration + → A commutator segments → 1 → a → b → c → d → the commutator segments 2 → B →-. According to the law of electromagnetic force, current-carrying conductor ab, cd will be subject to the electromagnetic force f. Its size is:
F=BxLi（N ), i is the current flowing through the conductor.
Direction of the conductor suffered electromagnetic force is determined by the left hand, in this transient, ab located under the N pole, the force direction from right to left, cd situated under S pole, the force direction from left to right, the electromagnetic force to the shaft will form an electromagnetic torque T. Under the electromagnetic torque T, armature is counterclockwise rotating.
When the armature turns to 90 °, the brush is not in contact with the commutator segments, but in contact with the insulating sheet between commutation sheet, in which case no current flows through the coil, i = 0, so the electromagnetic torque T = 0. However, due to the mechanical inertia, the armature still turned at an angle, brush A, B respectively connect with commutator segment 2,1. Current i flows in coils. At this time, in the conductor ab, cd , current change the direction, that is, b → a, d → c , and conductor ab run to S-pole , ab suffered direction of the electromagnetic force f is from left to right, cd run to the next N pole, cd electromagnetic force suffered the direction from right to left. Therefore, the coil is still under the effect of anti-clockwise torque, the armature remains in the same direction to rotate.
In DC motor, although the DC power is supplied to brush sides, under the effect of the brush and commutator, coil inside has turned to be alternating current, resulting in the electromagnetic torque in a single direction , the drive motor continues to rotate. At the same time, in the rotating coil , will generate a inductive potential, a direction opposite to the direction of the current in the coil, so called back EMF. To maintain the DC motor rotating, the applied voltage must be higher than the back EMF to continue to overcome the back EMF and inflow current , it is this constantly overcome to achieve the electrical energy is converted into mechanical energy.
Thus, the DC motor is reversible, that is, either a DC motor working as a generator or a motor. When the input mechanical torque convert mechanical energy into electrical energy, the motor working as a generator to run; when the input DC current electromagnetic torque, convert electrical energy into mechanical energy, the motor working as the electromotor motor, For example, when the electric locomotive traction conditions, traction motor as the motor is running, resulting in traction; in the braking condition, traction motor working as a generator to run the locomotives and train kinetic energy is converted into electrical energy, generating braking force to electrically brake the locomotive system.