Abstract This paper analyzes the causes of energy waste in the chilled water, cooling water circulation system, and fan of a central air conditioning system, and explains the working principle of the central air conditioning system. Utilizing the built-in PID function of the frequency converter, a closed-loop automatic control system for the chilled water pump, cooling water pump, and fan is constructed, allowing the motor output power to be automatically adjusted as needed. This not only increases the stability and reliability of the system operation but also saves energy by more than 30%, resulting in significant economic benefits. In a central air conditioning system, the capacity of the chilled water pump and cooling water pump is selected based on the maximum design heat load of the air conditioning system, with a certain design margin. Without speed regulation, the pumps operate at full load, requiring throttling or recirculation to adjust the flow rate, resulting in significant throttling or recirculation waste and thus substantial energy waste from the pump motors. Due to continuous changes in outside temperature, the central air conditioning system operates at low load for most of the time. Statistics show that central air conditioning accounts for over 70% of the total electricity consumption of various buildings. Among this, the power consumption of central air conditioning water pumps accounts for approximately 20-40% of the total power consumption of the air conditioning system. Practice has proven that using variable frequency technology to adjust the speed of cooling pump and chilled water pump motors to regulate flow and pressure changes, instead of using valves to control flow, not only increases the stability and reliability of system operation but also saves energy by over 30%, bringing significant economic benefits. The working principle of a central air conditioning system: Cooling water circulation system The main structure of a central air conditioning system is shown in Figure 1. It consists of chilled water pumps, variable frequency drives, a chilled water (hot water) circulation system, a cooling water pump, variable frequency drives, a cooling water circulation system, and the air conditioning unit. The chilled water pump motors and cooling water pump motors are controlled by variable frequency drives, and the air conditioning unit completes the heat exchange tasks of the chilled water circulation system and the cooling water circulation system. The working principle of a central air conditioning system: 1. Control of the chilled water circulation system. This system consists of chilled water pipes and heat exchangers. The chilled water pump pressurizes the chilled water, causing it to circulate repeatedly within the chilled water pipes. Chilled water flowing from the chiller unit is pressurized by a chilled water pump and sent into the chilled water pipes. In each room, fans deliver clean air, which comes into full contact with the heat exchangers, into the room for heat exchange, removing heat and thus regulating room temperature. 2. Control of the cooling water circulation system. This system consists of cooling water pipes and a cooling tower. A cooling pump pressurizes the cooling water, causing it to circulate repeatedly in the pipes and tower. The chilled water circulation system and the cooling water circulation system exchange heat within the air conditioning unit through the evaporator and condenser. Chilled water releases heat, lowering its temperature, and is then pressurized and circulated by the chilled water pump; cooling water absorbs heat, raising its temperature, and is then pumped into the cooling tower for heat exchange with the atmosphere. The cooled water is then returned to the air conditioning unit for further heat exchange. This continuous circulation removes heat released by the chiller unit. Control circuit for variable frequency energy-saving retrofit of the central air conditioning system The specific control method for the variable frequency closed-loop control energy-saving retrofit of the chilled water circulation system is shown in Figure 2. For chilled water systems, the outlet water temperature depends on the evaporator setpoint in the main unit, while the return water temperature depends on the heat received by the evaporator. The maximum design temperature difference between the chilled water outlet and return water temperatures in a central air conditioning system is 6℃ (e.g., outlet water 8℃, return water 14℃). Currently, temperature sensors are installed on the outlet and return water pipes of the evaporator to detect the temperature, transmitting the data to temperature sensors. The temperature sensors convert the temperature difference between the outlet and return water pipes into a 4-20mA current signal, which is fed back to terminals 4 and 5 of the frequency converter. This 4-20mA current signal controls the output frequency of the frequency converter, which in turn controls the speed of the water pump motor. The temperature sensors and the built-in PID function of the frequency converter constitute a closed-loop automatic control system for the chilled water circulation system. When the temperature difference increases and deviates from the setpoint, the frequency converter pump automatically accelerates to bring the outlet/return water temperature difference closer to the setpoint; conversely, when the outlet/return water temperature difference decreases, the frequency converter pump automatically decelerates. This system achieves energy savings by using a frequency converter to automatically adjust the speed based on temperature difference, replacing the valve-based throttling method for flow regulation. For the cooling water circulation system, whose main function is to dissipate heat from the cooling water and lower its temperature, a closed-loop control system using temperature sensors and the built-in PID function of the frequency converter can still be employed. The specific control method is shown in Figure 3. Temperature probes are installed on the condenser's outlet and return pipes to detect their temperature, transmitting the data to the temperature sensors. The temperature sensors convert the temperature difference between the outlet and return pipes into a 4-20mA current signal, which is fed back to terminals 4 and 5 of the frequency converter. This 4-20mA current signal controls the frequency converter's output frequency, which in turn controls the speed of the water pump motor. The temperature sensor and the built-in PID function of the frequency converter constitute a closed-loop automatic control system for the cooling water circulation system. A large temperature difference indicates that the cooling unit generates a lot of heat, and the speed of the cooling pump should be increased to increase the circulation speed of the cooling water; conversely, the speed should be reduced so that the speed of the cooling water pump changes accordingly to the change in heat load, while the temperature difference of the cooling water remains constant at the set value. This achieves the goal of regulating the flow rate by automatically adjusting the speed of the frequency converter based on the temperature difference instead of using valve throttling, thereby achieving energy saving. Energy-saving retrofit of the frequency converter closed-loop control of the central air conditioning terminal fan: 1. Adjusting the air volume. In a central air conditioning system, the heat transfer medium is usually water. At the terminal, clean air that has sufficient contact with the heat exchanger is directly delivered into the room by the fan to regulate the room temperature. When the temperature of the transfer medium (water) is constant, the cooling capacity brought into the room is changed by changing the air volume, thus conveniently regulating the indoor temperature. Using a frequency converter to achieve stepless speed regulation of the fan to change the air volume replaces the use of baffles to regulate the air volume, which not only saves energy but also reduces system noise and increases indoor comfort. 2. Selection of control mode. (1) Manual adjustment control mode. The specific control mode is shown in Figure 4. When the switch SB is closed, the frequency converter controls the fan motor to start running. The output frequency of the frequency converter is controlled by the manual potentiometer WR, thereby controlling the operation of the fan motor. Through the stepless speed regulation of the fan by the frequency converter, the air volume can be adjusted as needed, thereby achieving the purpose of adjusting the room temperature at will. (2) Automatic constant temperature operation mode. The specific control mode is shown in Figure 5. When the outdoor temperature changes, or the temperature of the cold water conveying medium changes, the room temperature may change accordingly. The frequency converter with built-in PID software module is used to automatically adjust the speed of the fan to achieve automatic constant room temperature. The control terminal mode is the same as the manual mode. The potentiometer WK is used to set the temperature (instead of adjusting the frequency). The frequency converter collects the temperature measurement value from the feedback terminal VPF/IPF, compares it with the given value, and sends it to the PID module for calculation. It automatically changes the output frequency of the frequency converter, thereby adjusting the air volume of the fan to achieve the purpose of automatic constant temperature operation. The distribution of air supply fans may not be uniform. For larger indoor spaces, a "regional temperature averaging method" strategy can be used to adjust the air supply volume to meet specific needs. The frequency converter energy-saving retrofit scheme for chilled water pumps, cooling water pumps, and air supply fan control involves adding the converter while retaining the original power frequency system, and interlocking must be installed between the converter and the original power frequency system to ensure system safety. Several issues to note when setting parameters when using frequency converters: Because the frequency conversion retrofit is based on the original system, the motor is still a standard three-phase asynchronous motor, not a frequency converter-specific motor. Therefore, the upper limit frequency setting of all frequency converters should not exceed 50Hz. When setting the upper and lower limit frequencies of the fan frequency converters, sufficient heat exchange between the air and chilled water pipes and indoor comfort should also be considered. Therefore, appropriate upper and lower limit frequencies should be selected according to the actual site conditions. Generally, the lower limit frequency should not be less than 15Hz, and the upper limit frequency should not exceed 50Hz. To avoid resonance, the motor's resonance speed point should be tested first. Then, all frequency converters should be set with avoidance frequencies and their width values ​​to prevent resonance and ensure reliable and safe system operation. Setting the carrier frequency: Appropriately increasing the frequency converter's carrier frequency can reduce motor operating noise and improve environmental quality. When multiple machines are running in parallel, if the motor is far from the frequency converter, the carrier frequency needs to be adjusted to avoid motor current oscillation. Setting the torque curve (V/F): Water pumps and fans are square-law torque loads, so it is better to choose dedicated frequency converters for water pumps and fans (also known as energy-saving frequency converters) and set their torque curve (V/F) to "square torque" to achieve better energy-saving effects. Performance of central air conditioning systems after frequency conversion retrofit: 1. The frequency converter has multiple protection functions for the motor, such as overvoltage, undervoltage, overcurrent, and short circuit, greatly improving the stability and reliability of the system operation. 2. Due to soft start, soft stop, and reduced speed operation, vibration, noise, and wear are reduced, extending equipment maintenance cycles and service life, increasing the equipment's MTBF (Mean Time Between Failures) value, and reducing the impact on the power grid, thus improving system reliability and operating efficiency. 3. The main circuit of the variable frequency drive system is connected in parallel with the main circuit of the original water pump, and the control circuit of the variable frequency system is interlocked with the original water pump's power frequency control circuit; the parallel connection of the variable frequency system does not affect the normal operation of the original system. If the variable frequency system needs maintenance, it can be immediately switched back to the original power frequency operation. 4. Using variable frequency drive closed-loop control, software configuration and temperature setting for PID regulation can be performed as needed to achieve constant temperature difference control; and the motor output power changes with the heat load, achieving maximum energy saving while meeting usage requirements. The water pumps and fans of the central air conditioning system are retrofitted with variable frequency drive, using temperature sensors and the PID regulation function of the variable frequency drive to form a constant temperature difference closed-loop control system. This not only improves the stability and reliability of the central air conditioning system, but also achieves highly automated regulation, improves the cooling quality and effect of the air conditioning, and achieves an energy saving effect of about 30%. References [1] Li Zuozhou, Wei Hongyi, Cen Minglun. Principles of Refrigeration and Air Conditioning Equipment. Beijing: Higher Education Press, 1994 [2] Wu Zhongzhi, Wu Jialin. Inverter Application Manual (2nd Edition) [M]. Beijing: Machinery Industry Press, 2002