The most widely used of all kinds of motors is the AC asynchronous motor (also known as induction motor). It is easy to use, reliable, inexpensive, and has a solid structure, but has a low power factor and difficult speed regulation. Large-capacity, low-speed power machine commonly used synchronous motor (see synchronous motor). Synchronous motor not only high power factor, and its speed and load size has nothing to do with, only determined by the grid frequency. It is more stable. In the requirements of a wide range of speed occasions more often use DC motors. However, it has commutator, complex structure, expensive, difficult to maintain, not suitable for harsh environments.After the 1970s, with the development of power electronics technology, AC motor speed control technology is becoming more and more mature, the price of the equipment is decreasing, and it has begun to be used. The maximum output mechanical power that the motor can bear without overheating is called its rated power under the specified operation system (continuous, short-time operation system, intermittent cycle operation system), and the regulations on the nameplate should be noted when using the motor. When the motor is running, it is necessary to pay attention to match the characteristics of the load with the characteristics of the motor to avoid flying or stopping. Electric motors can provide a wide range of power, from milliwatts to 10,000 kilowatts. The use and control of electric motors are very convenient, with the ability of self-starting, acceleration, braking, reversing, stopping, etc., which can meet a variety of operational requirements; electric motors are more efficient, and there is no smoke, dust, odor, no pollution of the environment, and the noise is also smaller. Because of its series of advantages, it is widely used in industrial and agricultural production, transportation, national defense, commercial and household appliances, medical electrical equipment and other aspects. General motor speed control when its output power will change with the speed.