This paper summarizes the great contributions of AC motor variable frequency speed regulation technology to energy saving, environmental protection, manufacturing industry (production and quality improvement), traffic traction and household appliances.

1. Frequency control

AC motor, especially cage asynchronous motor, has a series of advantages, such as small volume, light weight, simple structure, convenient maintenance, low manufacturing cost and operation cost, and reliable operation in harsh environment. However, in the past, due to the difficulty in speed regulation or the inefficiency of some speed regulation methods, the DC speed regulation is still occupied in the field of speed regulation for a long time, and the advantages of AC motor have not been brought into play in the speed control drive. Until the early 1970s, with the rapid development of power electronics, Microelectronics (integrated circuit) and microcomputer technology, the long-term desired variable frequency governor has been industrialized.

1.1 the so-called variable frequency governor -- it transforms three-phase power frequency (50 Hz) AC power supply (or any power supply) into three-phase AC power supply with adjustable voltage and frequency. Sometimes, the variable frequency governor is called VVVF. It is mainly used to adjust the speed of AC motor (asynchronous machine or synchronous machine).

1.2 an AC motor variable frequency speed regulation system is composed of three parts: variable frequency governor driver, AC motor and controller. The key equipment is frequency converter, which can realize the smooth change of motor voltage and frequency.

1.3 variable frequency speed regulation is incomparable with the former voltage regulation speed regulation, pole changing speed regulation, cascade speed regulation, slip speed regulation and hydraulic coupling speed regulation in the aspects of frequency regulation range, static precision, dynamic quality, system efficiency, perfect protection function, easy realization of automatic control and process control. It is recognized as the most ideal and promising speed regulation scheme of AC motor, and represents the development direction of electric drive in the future.

1.4 in the past 30 years, frequency control has been widely used in steel, metallurgy, petroleum, chemical industry, textile, chemical fiber, light industry, papermaking, rubber, plastic, power, water and other industries. The application of variable frequency speed regulation of low voltage motor has been very popular and mature. High voltage motor frequency control is also being concerned and gradually applied. In addition to excellent speed regulation performance, AC motor frequency conversion speed regulation also has significant effect on saving electric energy and protecting environment. It is an ideal speed regulating device for enterprise technical transformation and product upgrading.

2. Frequency control and energy saving of fan and water pump

2.1 there is a large power gap between the contradiction between power supply and demand (the supply exceeds demand), which requires power saving.

According to the statistics of relevant departments, in 2002, China's installed power generation capacity was 31.9 billion kw, and the annual power generation was 134.6 billion kwh. Although the power scale ranks the second in the world, the per capita power consumption is the bottom of the world. Moreover, China's rapid economic development needs more power. If the national economic growth rate is 8%, the power generation capacity of China should be 570-600 million KW by 2010, and the annual power generation capacity will reach 28000-2900 billion kwh. In the summer of 2003, the continuous high temperature caused the shortage of power supply in some provinces and cities, and forced to take measures to limit power supply. Due to the overburden of power network, the instability of local power system is caused.

The above shows that China's power supply and demand imbalance, supply is less than demand. Therefore, it is necessary to save electricity.

2.2 energy saving benefits of frequency conversion speed regulation of fan and water pump

2.2.1 the total installed capacity of motor in China is 450 million KW, which consumes about 65% of the national power generation. Therefore, it is very important to realize the motor power saving. There are two ways to save energy in general motors: one is to improve the efficiency of the motor itself to achieve long-term and efficient operation, which is mainly used for constant speed machinery; the other is to improve the control accuracy of motor speed to make the motor run at the most energy-saving speed.

2.2.2 fans, pumps and compressors are motor driven equipment with large amount and wide range in national economic production. The total motor capacity reaches 150 million, and the power consumption accounts for about 35% of the national power generation. About 20% ~ 30% of the fan water pumps need to be adjusted.

2.2.3 frequency control is the best scheme for energy saving of fan and water pump. According to fluid theory, the shaft power of centrifugal fan pump is a cubic function of rotational speed. When the speed is reduced, the power consumption will be greatly reduced. For example, when the speed is 50%, the mechanical power of the shaft is only 12.5%. Of course, the efficiency of the speed control scheme varies greatly. The efficiency of the hydraulic speed control device after slip is not high, η ≈ (1-s). At 50% speed, η vs ≈ 50%, while the efficiency factor of frequency conversion governor is high, η VVVF ≈ 95% ~ 98%, and it is almost unchanged. Therefore, in many speed regulation schemes, frequency conversion speed regulation has the best energy-saving benefit and should be the first choice.

[example] for the same fan water pump, the shaft power is 100% when the speed is 100% and the shaft power is reduced to 50% when the speed flow is 50%.

If the damper is used and the valve is controlled to 50% flow, 70-80% power still needs to be absorbed from the power grid; if the slip low efficiency speed regulation is adopted to 50%, the power of ≈ 25% needs to be absorbed from the power grid; if the frequency conversion is used to speed up to 50% flow, only 10% power needs to be drawn from the grid.

2.2.4 there are various methods for regulating the flow of fan and water pump, and each has its own characteristics, but it can be divided into three types

(1) The traditional mechanical method is used to adjust the opening degree of fan baffle, deflector and water pump valve;

(2) Electromagnetic slip clutch or hydraulic coupling is used to adjust the speed of fan and water pump (while the motor runs at constant speed);

(3) AC motor frequency control scheme.

The following table shows the comparison of flow regulation methods of three types of fans and water pumps and the conclusions. (see Table 1)

2.2.5 nowadays, high voltage should be widely applied to fans, pumps and compressors driven by high-voltage motors