Compared to the three-phase AC asynchronous motor, electric motor dc has a complicated structure and is inconvenient for maintenance. However, it has better speed performance and bigger starting torque. Therefore, the production machinery with higher speed or bigger starting torque is often driven by electric motor dc. There are three main advantages of DC motor. First, electric motor dc with good speed performance, wide speed range is easy to adjust smoothly. Second, electric motor dc with big starting and braking torque is easy to start and stop quickly. Third, electric motor dc is easier to be controlled. Because of these advantages, electric motor dc is mainly used in steel rolling machine, electric locomotive, in large gantry planer, mine shaft hoist, heavy equipment and large equipment with large speed range.