With the development of power electronics, microelectronics and AC motor speed control technology, many DC generator has been replaced by a variety of thyristor rectifier power supply. DC motors and DC drive systems are also certain areas, especially in the field of large motor is running, maintenance is relatively easy AC drive motor and the AC drive system replaced, the scope of application was reduced trend.

However, DC motors and DC transmission speed can be decoupled through long-term development, we have accumulated rich operation and maintenance experience. Taking into account the AC variable speed motors and AC drive system. Within a certain range, and some capacity operating mode, there is also the system of technical, economic rationality problem, certain operational requirements and operating environment is also inseparable from the DC system. Thus, in a period of time, DC motors and DC drive systems as well as the development of space, it can not or should not be fully substituted.