Motor protection as the most important electromechanical energy conversion device, its scope of application has been all over the national economy in various fields and people's daily lives. Whether it is in the industrial and agricultural production, transportation, defense, aerospace, medical and health, business and office equipment, or in daily life of household appliances and consumer electronics products (such as refrigerators, air conditioners, DVD, etc.) With a variety of motors. According to statistics, in all the power resources, more than 90 percent from the motor. Similarly, 60 percent of the electricity produced in our country is used for motors. Motor and human life are closely related, inseparable. Electric era, the motor speed control is generally used simulation method, the motor is relatively simple control applications. Simple control refers to the start of the motor, braking, forward and reverse control and sequence control. This type of control can be achieved by relays, programmable controllers and switching elements. There is also a class of control called complex control, refers to the motor speed, corner, torque, voltage, current, power and other physical control.

DC motor control the history of the development of commonly used control of DC motors are the following: First, the initial DC speed control system is a constant DC voltage to the DC motor armature power supply, by changing the resistance in the armature circuit to achieve speed The This method is simple and easy to manufacture equipment, cheap. But the disadvantage is low efficiency, mechanical characteristics of soft, can not be a wide range of smooth speed, so rarely used.

In the late thirties, there was a generator-motor (also known as a rotary variable flow group). With the use of magnetic amplifiers, motor amplifiers, thyristors and other control devices, excellent speed performance was obtained. A wide speed range (ten to one to ten), a smaller speed change rate and smooth speed, etc., especially when the motor deceleration, the motor can be very easy on the motor shaft flywheel inertia feedback To the grid, so that on the one hand can be smooth braking characteristics, on the other hand can reduce the energy loss, improve efficiency. However, generators, motor speed control system, the main drawback is the need to increase the two with the speed motor equivalent of rotating motor and some auxiliary excitation equipment, and thus large size, maintenance difficulties.

With the advent of microcomputers, ultra-large scale integrated circuits, new electronic power switching devices and new sensors, as well as automatic control theory, power electronics technology, computer control technology in-depth development, DC motor control devices are constantly moving forward. Microcomputer application of the DC electrical drive control system tends to digital, intelligent, greatly promoted the development of electrical transmission. In recent years, some advanced countries have launched and used a large number of computer-controlled DC power transmission devices, such as Siemens SIMOREG K 6RA24, ABB's PAD / PSD and so on. With the accelerated pace of modernization, people's living standards continue to improve, the demand for automation is also getting higher and higher, DC motor applications are also expanding.