The principle of frequency conversion speed regulation and energy saving reform of motor system

The motor is a high-energy-consuming power device with a large application range and a wide range of applications. According to statistics, China's total installed capacity is about 400 million kilowatts, and annual electricity consumption is about 600 billion kwh, accounting for 70-80% of industrial electricity. In China, small and medium-sized motors are mainly used, accounting for about 80%, while small and medium-sized motors consume 90% of the total loss. In the practical application of China, the difference between the motor and the foreign country is very large. The efficiency of the unit is 75%, which is 10% lower than that of foreign countries. The operating efficiency of the system is 30-40%, which is 20-30% lower than the international advanced level. Therefore, small and medium-sized motors in China have great potential for energy saving, and it is imperative to promote motor energy conservation.

Because the asynchronous motor has the advantages of simple structure, convenient manufacture, low price, sturdy and durable, reliable operation, and can be used in harsh environments, it has been widely used in industrial and agricultural production. In particular, the pumping of various types of industries and the dragging of fans are not the only ones. Therefore, the energy-saving work of motors that drive pumps and fans has received much attention.

With the rapid development of science and technology, especially the high development and application of power electronics technology, microelectronic technology, and automatic control technology make the energy-saving effect of the inverter more significant. It not only realizes stepless speed regulation, but also operates efficiently at different loads, has good dynamic characteristics, and can realize high-performance, high-reliability, high-precision automatic control. Compared with other speed control modes (such as: step-down speed regulation, variable pole speed regulation, slip speed regulation, AC cascade speed regulation, etc.), the frequency conversion speed regulation performance is stable, the speed regulation range is wide, and the efficiency is high, with modern control. With the development of theory and power electronics technology, AC frequency conversion speed control technology is improving day by day, and it has become the latest trend of AC motor speed regulation. Frequency converters (inverters) have been widely used in the industrial field.

The frequency converter uses the speed regulation signal transmission fast, the control system has small time lag, sensitive response, high control system precision, easy to use, is conducive to improving output, ensuring quality and reducing production cost. Therefore, the use of frequency converter is energy saving for factories and mining enterprises. The preferred product.

Inverter Motor Power Saver is a revolutionary new generation of motor-specific control products. Based on microprocessor digital control technology, it dynamically adjusts the voltage and current in motor operation engineering through its built-in dedicated power-saving optimization control software. Under the condition of motor speed, the output torque of the motor is precisely matched with the load demand, thus effectively avoiding the waste of electric energy caused by excessive output of the motor.

AC motor is the most widely used motor, accounting for 85% of the total number of motors. It has the advantages of simple structure, low cost and no maintenance. However, its weakness is that it is difficult to adjust the speed, so it is limited in many applications. Or use mechanical means to achieve speed regulation.

The frequency converter has two typical applications in terms of load type: 1. Constant torque application; 2. Variable torque application. For the purpose of application, there are mainly: 1. The main purpose of improving the process is to ensure the optimal speed in the process, the optimal speed under different loads, and accurate positioning. With its excellent speed control performance, it can improve productivity, improve product quality, improve comfort, rationalize equipment, adapt to or improve the environment. 2, the main purpose of energy saving - the speed control of the fan or pump machinery that needs to be adjusted by flow or pressure to achieve energy-saving retrofit effect is very significant.

Second, the principle of frequency control

In the power-consuming equipment used by enterprises, the load of motors such as fans, pumps, air compressors, hydraulic oil pumps, and circulating pumps accounts for the majority. Limited by technical conditions, the flow, pressure or air volume control system of this type of load is almost entirely a valve control system, that is, the motor is driven by the rated speed, and the flow, pressure or air volume provided by the system is constant. When the working demand of the equipment changes, The load flow, pressure or air volume is adjusted by the overflow, overflow valve or proportional adjustment at the outlet end to meet the changing conditions of the equipment. After the overflow overflow valve or the proportional control valve overflows, a large amount of energy is released. This part of the dissipated energy is actually a part of the energy absorbed by the motor from the grid, resulting in great waste of electrical energy. From the working characteristics of this type of load, the motor power is proportional to the speed cube, and the speed is proportional to the frequency. If we change the working mode of the motor so that it does not always run at the rated working frequency, but instead starts and stops the control and adjusts the operation by the variable frequency adjustment control system, the speed can be in the range of 0~2900r/min. Continuously adjustable, that is, the output flow, pressure or air volume can be continuously adjusted within the range of 0~100%, so that it can accurately match the work requirements of the load, thereby achieving the purpose of energy saving.

The AC motor speed is as follows: n=60f(1-s)/p

Where: n = motor speed

f=power frequency

p=the number of pole pairs of the motor

s = slip rate

It can be seen from the formula that the synchronous speed n of the AC motor is proportional to the power frequency f, so changing the power frequency can change the motor speed, thereby achieving the purpose of speed regulation.

Third, the principle of frequency conversion and power saving

Frequency control and power saving, as the name suggests, frequency control can save electricity. The following is mainly to analyze the power saving principle of two types of typical load applications.

(1) Constant torque load application

The constant torque load is the load torque constant regardless of the speed.

The following formula: P = K * T * N

K = coefficient

P = shaft power

T = load torque

N=speed

It can be seen from the above formula that the shaft power is proportional to the rotational speed of the motor. When the motor speed is adjusted due to the needs of the process, the corresponding proportion of power saving can naturally be achieved.

(2), variable torque load application

Centrifugal fans and pumps are typical variable torque loads. Their working characteristics are: most of them are long-term continuous operation. Since the load torque is proportional to the square of the speed, once the speed exceeds the rated speed, the motor will be seriously Overload, so fans and pumps generally do not exceed the rated frequency.

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