Application of Danfoss VLT series VFD (Variable-frequency Drive) in variable air volume air conditioning system

Safety, high efficiency, comfort, health, flexibility, and convenience are people's comprehensive requirements for modern buildings. To meet the above requirements, it is inseparable from the support of modern technologies such as building automation, office automation, network technology, intelligent communication, intelligent fire prevention, intelligent security, and integrated wiring. The VAV air-conditioning system is the latest air-conditioning technology that combines comfort, health, high efficiency, and energy saving. It is not only talked about by construction experts, but also favored by the majority of users. The number of modern buildings using VAV air-conditioning systems is increasing day by day, like mushrooms after rain. While bringing people a comfortable and healthy environment, it also enables users to obtain huge economic benefits.

The International Liaison Department Building of the Central Committee of the Communist Party of China is located in the south of Muxidi Bridge on Chang'an Street, Beijing. The building is equipped with a variable air volume air conditioning system with excellent performance and complete functions. Completely eliminate the various disadvantages of fan coil units and decentralized heat exchange. This modern building with a total height of 17 floors and a total area of 68,000 square meters was put into operation in June 2004, and the effect is good. VAVBOX is widely used in this system, which greatly increases the flexibility of the system and the comfort of the room. It is said that this is the first case in the country. The ventilation system that uses the frequency converter to adjust the air volume greatly improves the efficiency of the air conditioning system. The building uses 61 sets of Danfoss VLT6000 series frequency conversion speed regulators, which are used to regulate the flow of fans on each floor. The VLT VFD (Variable-frequency Drive) with excellent performance has laid a solid foundation for the stable operation of the control system. In particular, the excellent electromagnetic compatibility conditions of VLT VFD (Variable-frequency Drive) provide the necessary conditions for the safe operation of the entire building's control system, communication system, computer system and security monitoring system.

In this building, except for the refrigeration station, all building electrical equipment is controlled by the central control station according to a schedule (a clock control method). Fully automatic operation and partial manual intervention are allowed. Its communication control system is a secondary network plus DDC control, which is composed of Andover products in the United States, as shown in Figure 1.

In Figure 1, the central control station consists of three primary stations and five secondary stations. DDC controllers are distributed in various sites (rooms and other controlled points) as three-level stations. The first-level station and the second-level station are connected by Ethernet (ETHERNET): the connection between the second-level station and the third-level station is composed of Andover's Infanet network. The two-wire, high-speed (12Mb) Infanet makes the huge control system run flexibly and orderly. The third-level station uses DDC to control the field equipment. Figure 2 shows the schematic diagram of the connection between the DDC controller and the VLT frequency conversion speed regulator ABC device. It can be seen from the figure that the DDC controls the VFD (Variable-frequency Drive) after testing parameters such as the wind pressure of the pipe network according to the bus settings, thereby changing the flow rate of the fan. Installation of transmitter feedback signals for various sensors, heat exchange method, airflow control and airflow direction

See Figure 3, which shows the schematic diagram of the wind cabinet. It can be seen from the figure that this variable air volume control method integrates air volume, air pressure, temperature and humidity, heat exchange, and fresh air control. Compared with the traditional fan coil heat exchange method in the past, it has the advantages of high efficiency, low consumption, good air quality, no customer drip, and convenient control. It is worth pointing out that this method of controlling the fan by frequency conversion, adjusting the flow rate and achieving constant pressure and variable flow rate with 4-20mA has a very high energy-saving effect. The speed regulation signal varies from region to region, and its power saving effect can reach 30%-50%. There are 82 above-mentioned wind cabinets in the whole system, which are distributed on each floor. So many variable air volume wind cabinets will bring considerable economic benefits as shown in Figure 2. DDC controller and VLT

It must be pointed out that the frequency converter is the connection device of the high-power switch VFD (Variable-frequency Drive). During its operation, longitudinal interference and radio frequency interference will inevitably occur. This technical instruction, collectively called "electromagnetic compatibility conditions", is of great significance to modern buildings that use frequency converters in large numbers. Its performance is an important factor that directly affects the stable operation of the entire control system. It also has an important impact on the system debugging quality and debugging speed. Therefore, in the variable air volume control system, when designing and selecting the variable frequency speed governor, full attention must be paid to its electromagnetic compatibility conditions to avoid various interference problems caused by it, especially under the condition of a large number of VFD (Variable-frequency Drive) applications, it will be very difficult to deal with the interference caused by poor electromagnetic compatibility conditions.