To make the armature get a changeless direction electromagnetic torque, the key is: how to change the current's direction which flowing in the coil in time when the coil side is in differential magnetic polarity, which means the process of so-called direction commutation . So it must add a device called commutator. The commutator can coordinate with electric brush to guarantee that the coil side current in every polarity has always the same direction, which could make the electric motor to rotate continuously. Thisis the working principle of DC electric motor.
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