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Application of frequency converter in fans, pumps and compressors

Nov 27, 2020

Application of frequency converter in fans, pumps and compressors

The various advantages of frequency converters promote the wide use of frequency converters in various industrial scenarios including heating, ventilation and air conditioning (Heating, Ventilation and Air Conditioning, HVAC for short).

Although the installation of a frequency converter (VFD) in a new system or an existing system requires additional capital investment, the frequency converter reduces production costs, equipment maintenance costs, and energy costs, so these inputs can usually be recovered within 3 years.

In the heating, ventilation and air conditioning system of the inverter, the equipment with the best effect is the fan, pump, and compressor.

Inverter fan and inverter pump

The running speed of the fan and pump will change with the change of the power supply frequency, so they are the ideal partner of the inverter. After the inverter is connected, we call it the inverter fan and the inverter pump. According to the Affinity laws of the fan and pump, the power of the fan or pump is proportional to the speed of the motor. Therefore, reducing the operating speed of the motor can directly reduce the operating power of the motor and save a lot of power.

In a typical application scenario, the impeller diameter (D) of a fan or pump is constant, and the air or water flow (Q), and the power (W) of the fan or pump only depends on the speed (N). These relationships are represented by the following equations (where equations (1) and (2) represent the basic working laws):

Formula (3) clearly shows how speed changes affect power changes. In theory, a 50% speed reduction will result in a power reduction of 87.5%. Therefore, the inverter can effectively save energy after automatically adjusting the motor speed.

In actual use, the system may be water or air, and the flow of air and water systems are different. At the same time, the type of pipe and area also affect the control strategy of the inverter, such as a single-pipe variable air volume system, double or multiple pipes. Variable air volume system, single zone system, multi zone system, primary and/or secondary cooling water system and hot water system.

Each application scenario needs to formulate a specific control strategy based on the actual situation, instead of applying it directly, otherwise it will only get half the effort. After the inverter is connected to the IoT cloud control platform, the background cloud server can be based on the on-site speed sensor and temperature. Sensors, flow sensors, water level sensors, etc. precisely control the running speed of the motor.

Inverter compressor

In the heating, ventilation and air conditioning industry, there are several common compressor types for refrigeration systems: reciprocating, screw, scroll and centrifugal. These types are all well coordinated with inverters. Commonly used working media in compressors include air and refrigerants. Air compressors are often used as the power source of pneumatic control systems in industrial processes. Refrigeration compressors are usually used for air conditioning, refrigerators, cold storage, and integrated air in residential and commercial buildings. Processing unit (AHU) or cooling radiator.

Among a wide variety of compressors, the energy performance of centrifugal compressors is similar to that of centrifugal fans and pumps. Scroll compressors are particularly suitable for connecting frequency converters, scroll compressors and reciprocating compressors due to their inherent structural design. It is the most commonly used type on the compression unit and heat pump unit of rooftop air conditioning units.

Unlike fans and pumps, scroll compressors and reciprocating compressors usually have no proportional relationship between power and frequency. The above figure is based on the approximate linear relationship between the power and frequency of the scroll compressor based on experiments. This relationship clearly shows how the compressor power changes with frequency. By understanding this relationship, new and simpler compressor inverter control models can be developed.

Most fixed-speed compressors on the market can be retrofitted with frequency converters to control the working power of the compressor.