DC motor has good performance of controlling speed. The performance of speed control means that the motor artificially change its speed under certain load conditions based on the need. DC motor can do the stepless speed regulation evenly and smoothly and has wide speed range under the conditions of heavy load. And the other advantage of DC motor speed control is the large starting torque which can control the speed evenly and economically. Therefore, the machine which starts under heavy load or requires uniform speed control, such as large reversible rolling mill, hoist, electric locomotives, trams, is driven by DC motor.

The principle of DC motor is to make the armature to be effected by electromagnetic torque in one direction. The key point is how will the direction of the current flowing through the coil transform in time when the coil side is in the different polarity of the magnetic pole. So we must add a device called commutator with the brush to ensure that each pole current coil side is always in one direction. Then the motor can rotate continuously.