The development of the motor can be divided into four stages in general, 1 DC motor, 2 AC motor, 3 control motor, 4 special motor.
As the most important electromechanical energy conversion device, the motor has been used in all fields of national economy and people's daily life. Both in industrial and agricultural production, transportation, defense, aerospace, medical, business and office equipment, or in the daily life of household appliances and consumer electronics products (such as refrigerators, air conditioners, DVD etc.), motor are widely used in a variety of. According to the data, more than ninety percent of all power resources come from motors. Similarly, sixty percent of the electrical energy produced in our country is used for motors. Electric motors are closely related to people's lives. In the electric age, the motor speed control adopts analog method, and the simple control of the motor is applied more. Simple control refers to the start, braking, positive and reverse control and sequence control of the motor. This type of control can be accomplished by relays, programmable controllers, and switching elements. Another type of control is called complex control, which refers to the motor speed, angle, torque, voltage, current, power and other physical quantities to control.