Electric motors are widely used in various fields, and the electricity consumption of electric motors occupies a considerable proportion in the total electricity consumption of all countries in the world. In China, the electricity consumption of electric motors has accounted for more than 60% of the total electricity consumption of the society.

In order to meet the needs of transmission and power of different mechanical equipment, the variety of small motor products in my country has developed to more than 140 series, more than 600 varieties, and more than 5,000 specifications; it is widely used in small motors in the chemical, petrochemical and coal industries. Explosion-proof motors have been produced in 130 series and more than 2,000 specifications, of which 02 series, Y series, Y2 series and YX, YX2 series are the main components of the Chinese motor application market.

According to the statistics of the Electric Motor Branch of China Electrical Equipment Industry Association, the total market capacity of large, medium and small AC motors in China in 2001 was about 44,000 MW, of which large motors accounted for 7.3%, medium and small motors accounted for 92.7%, and small motors accounted for the total. 71.5% of the total, medium-sized motors accounted for 21.2% of the total. Among these varieties, small electric motors are the leading products in the Chinese electric motor market.

my country already has the technical conditions for producing high-efficiency motors (YX, YX2, Y2E and other high-efficiency series), but due to the immature market conditions, the output and market capacity are small. In 1998, the high-efficiency electric motor market was mainly exported to the United States with NEMA-compliant electric motors, and the output ratio was less than 2%. In 1999, the high-efficiency motor market was 2%; in 2000, it was 4.7%; in 2001, it was only 6.5%. More than 70% of them are exported, and very few products are used in the Chinese market.

The improvement of the energy efficiency level of electric motors is of great significance for energy saving, environmental protection and capital saving. my country's actual power generation in 2001 was 1.5 trillion kWh, of which about 50% of the electrical energy was converted into mechanical energy by the motor. Therefore, the input electrical energy of the motor was 732.5 billion kWh. If the motor efficiency was increased by 2%, 14.6 billion could be saved. The kilowatt-hour electric energy is equivalent to the annual power generation of two 1 million kilowatt power stations, which can greatly reduce the consumption of primary energy and carbon dioxide emissions, and can correspondingly save the investment in power station construction and the electricity expenses of electric motor users. Therefore, the energy efficiency level of electric motors The improvement has important social and economic significance.

Energy saving of high-efficiency motors

Since the electricity consumption of electric motors in all countries in the world accounts for the majority of their national power generation, improving the efficiency of electric motors is of great significance to saving electric energy.

So, what counts as a high-efficiency motor? What is its index? In my country, the high-efficiency motor that people usually refer to is a high-efficiency three-phase asynchronous motor, that is, a motor whose efficiency level reaches or exceeds the energy-saving evaluation value specified by the national standard. Compared with the Y series motor, the total loss is reduced by 20% to 30%, and the efficiency is increased by 2% to 3%.

In the United States, according to the "National Electrical Equipment Manufacturers Association Standard", the efficiency of high-efficiency motors is 2% to 6% higher than that of standard motors, and the loss is reduced by 20% to 30%. In addition, ultra-high-efficiency electric motors have also appeared in the United States, and their efficiency is 0.8% to 4% higher than the above-mentioned standard electric motors.

Asynchronous motors used in enterprises can be divided into three categories according to working conditions: frequent start, intermittent work and continuous work. In order to improve the operating efficiency of the motor, it is necessary to make the energy required by the production machinery in various states equal to the input energy of the motor as much as possible, so as to effectively utilize the electric energy. In the design and manufacturing department, we must try to reduce the power loss inside the motor and improve the efficiency of the motor. High power and power factor are the main features of energy-efficient motors.

Generally, the efficiency curve of a conventional motor is not flat. As the load decreases, the efficiency decreases greatly. The motors in use are all running at a lower efficiency than the rated efficiency. Therefore, high-efficiency and energy-saving motors should meet the following points:

(1) Calculated according to the rated power, the power loss should be reduced by 30%.

(2) The efficiency curve should be as flat as possible.

(3) The center height and rated size of the shaft shall comply with the national standards.

High-efficiency and energy-saving motors use more materials and have higher costs. Therefore, only when the load rate and utilization rate are high, the investment can be recovered in a short period of time.

The use of high-efficiency motors

The best energy-saving effect of high-efficiency motors is to apply them to equipment with continuous work quota, stable load and no special requirements, especially for high load rate (such as more than 70%) and long continuous operation time (such as annual operation). time is more than 3000 hours) on the device.

Mechanical equipment is the main user type of electric motors. High-efficiency motors are mainly distributed in pumps, fans and gas compressors, followed by petrochemical equipment, petroleum equipment, mining machinery and metallurgical machinery.

In addition, the electricity consumption of the nine major industrial industries accounts for 35% of the total electricity consumption of the whole society. Among them, chemical industry, building materials, electric power and metallurgy are the major consumers of electricity, and they are also the markets with the greatest energy saving potential, and the electricity consumption of electric motors accounts for the whole 68.9% of business electricity consumption. Judging from the use of motors in various industries, mainly Y/Y2 series motors, and a considerable part of JO series motors, YX high-efficiency motors are mainly used in petroleum and urban water supply and drainage industries.

From the perspective of industry demand, petroleum, petrochemical, chemical, textile, electric power, water supply and drainage and other industries should have a certain market demand for high-efficiency motors. The higher the energy consumption, the higher the energy saving potential, and the wider the application of high-efficiency motors. Due to the high energy consumption of petrochemical, chemical and textile industries and the long-term continuous operation of electric motors, they are the industries that use the most high-efficiency electric motors; the petroleum industry, electric power industry and urban water supply and drainage are second.

Motor speed regulation and energy saving

Before the 1970s, most of the electric driving equipment used in industrial applications used the constant speed driving of AC motors. Therefore, for a long time, fans and pumps, which are widely used in industrial production, need to be driven by electric power for flow regulation and control. In the system, people have to keep the constant speed drag of the AC motor, and use baffles and valves to adjust the wind speed, flow, pressure, etc. This primitive method not only increases the complexity of the system but also results in a large waste of energy.

With the development of power electronic technology, microelectronic technology and control theory, the frequency conversion speed regulation technology as the center of AC speed regulation has been significantly developed, and has been widely used in various fields of industrial production.

Taking the fan pump as an example, according to the principle of fluid mechanics, the flow rate is proportional to the rotational speed, and the wind pressure or head is proportional to the square of the rotational speed, so the shaft power is proportional to the cube of the rotational speed. Theoretically, if the flow is 75% of the rated flow, the induction motor speed is controlled at 3/4 of the rated speed, and its shaft power is 42% of the rated power, which can be reduced by 58% compared with the use of baffle or valve regulation If the flow rate drops to 50% of the rated flow, the induction motor speed is run at 1/2 of the rated speed, and its shaft power is 1/8 of the rated power, which can be reduced by 7/7 compared with baffle or valve adjustment. 8 power. Since the power loss of the speed control slip and the additional power loss of the control device are much smaller than the power loss reduced by the speed control, the actual power saving effect is quite obvious. Therefore, the application of speed regulation technology to the electric drive of fluid equipment such as fans and pumps with low load rates and large flow changes can achieve significant power-saving benefits, which is why fans and pumps are speed-regulated technology to save electricity The main reason for the focus object of the application is too.

The advantages of variable frequency speed control system

The frequency converter can easily realize the speed control of the existing AC motor. In industrial production, such as power plants, mining and metallurgy, petroleum, chemical industry, machinery, electronics, building materials, textiles, light industry and many other industries, there are a large number of occasions that require motor speed and soft start. According to the statistics released by the third national industrial census, the total installed capacity of general machinery for fans, pumps and compressors in my country is 160 million kilowatts, of which the fan is about 49 million kilowatts, the water pump is about 10 million kilowatts, and the annual power consumption is 320 billion kWh. It accounts for 1/3 of the country's total electricity consumption and 40% of industrial electricity consumption. It plays an important role in the national economy and has great potential for energy saving. In particular, Article 39(2) of Chapter 4 of the "Energy Conservation Law" implemented in my country on January 1, 1998 clearly stipulates: "Gradually realize the economic operation of electric motors, fans, and pump equipment and systems, and develop Faster power electronics...improve the utilization of electrical energy". Moreover, it has been proved by practice that the control of valves and baffles is replaced by frequency conversion speed regulation, and the power saving effect is obvious, especially for fans and pumps driven by large and medium-capacity AC motors, if frequency conversion speed regulation is adopted, the power saving effect is more obvious. , and the payback period is short, generally 1 to 2 years.

Wide and continuous speed range

The frequency conversion speed regulation system realizes the continuous change of the speed by continuously changing the output frequency of the frequency converter, so that the motor works in the range of small slip, the speed regulation range of the motor is wider, and the operating efficiency is also significantly improved. Generally speaking, the speed regulation range of general-purpose inverters can reach more than 1:10, while the speed regulation range of high-performance vector control inverters can reach 1:1000.

It is easy to realize forward and reverse switching and form an automatic control system

When the AC motor running under the grid voltage performs forward and reverse switching, it can be realized only by changing the phase sequence. If the phase sequence switching is performed when the motor is still at a high speed, a large inrush current will be generated in the motor, and there is even a danger of burning the motor. In the frequency conversion speed regulation system, the motor can be reduced to a low speed by changing the output frequency of the frequency converter, and then the phase sequence can be switched. In this way, the switching current can be relatively small, and the power consumption and heat generation of the motor are also greatly reduced. In addition, the frequency converter has an interface to form an automatic control system together with other equipment.

Small starting current, can be used for frequent starting and braking occasions

The starting current of the asynchronous motor directly started is usually 5 to 6 times the rated current, the motor loss is large, and the required power supply capacity is also large, so it is not suitable to start and stop frequently. When the inverter is used to drive the asynchronous motor, the output frequency of the inverter can be reduced to a very low start, and the starting current of the motor is very small, so the power distribution capacity required by the input terminal of the inverter can also be reduced accordingly. In addition, it can also use frequency converter to achieve electrical braking. During braking, the output frequency of the inverter is gradually reduced first, and the mechanical energy stored by the load will be converted into electrical energy and fed back to the inverter. This part of the energy will be consumed in the form of heat energy through a certain braking circuit, or fed back to the grid. Therefore, the inverter-driven AC motor speed control system can work in frequent starting and braking occasions.

Simple structure, safe and reliable operation

The asynchronous motor in the variable frequency speed regulation system is simple in structure, strong and durable, and usually does not need to use other speed change devices such as gearboxes. Maintenance and repair are relatively simple. Different asynchronous motors can be selected according to different working environments, while frequency converters usually No need to change. Therefore, the variable frequency speed control system can be used in harsh environments such as flammable, explosive, and corrosive.

In view of some of the advantages of variable frequency speed regulation listed above, in the speed regulation technology of AC motors, variable frequency speed regulation has the largest application area and the highest efficiency. AC variable frequency speed regulation is a high-tech that integrates various advanced technologies such as contemporary power electronics, microelectronics, automatic control, sensors, and motors.

Nearly 20 years of theoretical development and application practice show that it has good speed regulation performance and obvious energy saving, which is the development direction of electric drive; its wide application range can save energy and reduce consumption for enterprises, improve product quality and production efficiency, and ultimately improve Economic benefits provide technical and material means. Therefore, all industrialized countries take the development of AC motor frequency conversion speed regulation as a major measure for technological progress, improving efficiency and saving energy consumption. Units that have applied this technology have achieved good power-saving benefits.

Investment cost and profit of variable frequency speed control system

Whether investing in energy-saving projects is beneficial or not, and whether the payback period is long or short, is a matter of great concern to all investors. The answer to this question is yes. However, in view of the experience and lessons of popularizing and applying frequency converters in the past 20 years, some issues still need to be discussed. The solution of these problems is related to the amount of investment, and actually directly involves the cost, profit and payback period of the investment. Energy saving and power saving are long-term, and the benefits after investment recovery are also absolute. The key is how to determine a reasonable recovery period. Investors should have confidence and should know where the risks of the project lie.

(1) The simpler the system, the less equipment investment, not the more advanced the technology, the better.

For ordinary fans, water pumps and other systems driven by medium and small AC motors, it is not necessary to precisely adjust the speed. If the distance between the inverter and the motor is very close, the low-voltage inverter can work normally.

If the output does not need to be adjusted but starts frequently, a soft starter can be purchased to reduce the energy consumption of the starting and braking process, so that the investment is relatively small. The most reasonable speed regulation scheme and the most economical system equipment should be selected according to the actual working conditions of the system.

(2) The variable frequency speed control system is a nonlinear and non-sinusoidal working condition with many harmonics, which will cause electrical stress on the motor, resulting in a decrease in the working life of the motor. In some important applications, it is necessary to use a special motor for variable frequency speed regulation, especially in the voltage of 1 to 10 kV. In the case of high power, or when the feeder cable between the inverter and the motor is very long, a dedicated motor should be selected, or a dedicated inverter with a sine wave output should be selected. This increase in cost is necessary.

(3) The harmonics in the operation of the frequency converter also cause pollution to the power grid, especially the high-power motor speed control system, which lightly damages the power quality of the power grid, and seriously causes the power supply transformer or power capacitor to burn out, or even explode. Moreover, the distortion and reactive power caused by harmonics will increase, and the power factor of the system will decrease, and will bear a large fine according to the regulations of the power grid. For low-voltage medium and low-power inverters, it is also necessary to use active power factor correction. People can't consume a lot of reactive power when they celebrate the saving of active power, and they can't destroy the safety of the power system when they successfully save energy. Be sure to consider the right governance measures.

(4) The motor speed of many household appliances is only tens of watts or hundreds of watts, and the largest single machine is 1 to 2 kilowatts. Under such circumstances, the use of conventional three-phase AC asynchronous motors to implement variable frequency speed regulation has a high cost and is not necessarily the best energy-saving speed regulation method. Here, a permanent magnet brushless motor or an improved switched reluctance motor with a relatively low speed regulation cost can come in handy.

(5) After adopting the AC motor variable frequency speed regulation system, not only energy saving benefits can be achieved, but also comprehensive effects such as increased output, improved quality, increased labor productivity, and improved environment. Therefore, when calculating the profit of the system, we must also look at its comprehensive benefits. As a user, it is often because of the existence of greater non-energy-saving benefits that we have greater determination and motivation to implement the technical transformation of frequency conversion speed regulation. Users should respect the objective laws at this time, and at the same time, they should also manage power pollution such as harmonics.

All in all, whether the use of the AC motor variable frequency speed regulation energy-saving system can recover the investment in a short period of time depends on the overall consideration of the comprehensive investment and the comprehensive benefit (profit). The price of the frequency converter is roughly 800 to 1500 yuan per kilowatt.

In order to make such a high-efficiency and energy-saving technology operate for a long time, some supporting technologies and products should be included in the cost. Save where you should save, and spend decisively where you shouldn't. Correct "technical-economic" comprehensive decision-making is exactly the difficulty in popularizing and applying AC motor variable frequency speed regulation and energy-saving technology. Let us learn from each other's practice, constantly sum up experience and lessons, enhance the in-depth understanding of this technology, so that it can be quickly implemented to the forefront of energy conservation.

Motor frequency conversion speed regulation technology

In order to achieve the purpose of improving production efficiency and saving energy, the system configuration must be correctly selected, especially the selection of the motor and frequency converter in this system, which involves three factors of reliability, performance and price.

The frequency conversion speed regulation system mainly includes asynchronous motor, frequency converter, control link, load and transmission mechanism.

When selecting a motor, not only the motor capacity required to drive the mechanical load and accelerate it should be considered, but also the corresponding motor protection level should be selected according to the production environment. In addition, since the motor is not powered by the power grid at this time, but by the inverter (that is, when the frequency conversion speed regulation is running, the motor is not working in the rated working conditions for which the motor is designed and manufactured most of the time), it will bring harmonics. , Electromagnetic interference, there may be local overvoltage, overcurrent and other problems. At the same time, it should also be considered to minimize the influence of harmonics and electromagnetic interference.

Correct use of the motor

Reasonable selection and use of motors, and determining the best operation mode to reduce the energy loss of the motor are the most basic methods to improve the operation efficiency of the motor. When selecting a motor, the form and power of the motor and various technical parameters should be selected first, so that it has the load characteristics suitable for the production machinery it drives, and can work stably in various states.

Choice of motor power

In theory, the rated output power of the motor is usually selected according to the maximum load, but in fact, the output power of some motors changes periodically. Most of the power loss of the motor is copper loss, and the copper loss is proportional to the square of the load current. When the power factor is constant, it is proportional to the square of the output power. So, to know the motor output power including copper loss and load requirements, simply calculate the root mean square root of the load.

For motors with a load rate lower than 50%, the motor power should be selected according to the principle of economical operation. If the motor with a load rate lower than 50% is changed to a smaller size without analysis and calculation, it may reduce the efficiency of the motor and increase the power loss.

When determining the output power required by the motor, the following two issues should be noted:

(1) For the relevant parameter values (such as friction factor, load, speed, air volume, air pressure, etc.) required for calculating power, the reserved capacity should not be too large.

(2) The selected motor should meet the starting torque, maximum torque and maximum load required by the load.

For running motors, the load factor should be determined. Generally, the rated efficiency and power factor of asynchronous motors are considered within the range of 75% to 100% according to the load rate.

For low-voltage and high-voltage medium-sized motor series, there is still a problem of reasonable selection of voltage level. As long as the selection is appropriate, the effect of energy saving and loss saving can be achieved on the premise of ensuring the performance of the motor.

The commonly used voltage levels of three-phase asynchronous motors in my country are: 220 volts, 380 volts, 3000 volts and 6000 volts. Below 500 volts is called low voltage, and above 500 volts is called high voltage. , For low-voltage and high-voltage motors, where the power supply line is short, the grid capacity is allowable, and the requirements for starting torque and overload capacity are not high, it is appropriate to use low-voltage asynchronous motors. Because this kind of motor has high power and performance index, it is beneficial to save electricity; it is cheap, reduces one-time investment, and is easy to maintain, and general low-voltage electrical appliances can be used.

Of course, for those occasions where the power supply line is long, the power grid capacity is limited, the starting torque is high, or the overload capacity is required, it is appropriate to use a high-voltage motor.

Selection of Motor Load Characteristics

Asynchronous motors are widely used, and there are many types of loads it drives. According to the load characteristics, the rational selection of the motor has practical significance for improving the safety, reliability and energy saving of the equipment during operation.

The operating characteristics of an electric motor are affected by the characteristics of the mechanical load it drives. Some machines, such as most fans, blowers, centrifuges, compressors, etc., require a small starting torque, but the required drag torque increases with the increase of the speed after starting, so motors with general mechanical characteristics are usually selected. .

Other machines, such as reciprocating air compressors, conveyors with loads, etc., require a large starting torque, so motors with high slip mechanical characteristics are often selected. This motor is also suitable for shock loads or loads requiring frequent starts, such as punch presses, oil well pumps and hoisting machinery. Therefore, in order to meet the standards of energy saving and safe operation, only the mechanical characteristics of the motor must be reasonably matched with the load characteristics it supports.