1. Overview In industrial enterprises, electric motors are among the most widely used and numerous electrical devices. Currently, a large number of AC motors operate at fixed speeds, which is increasingly unable to meet the automation requirements of production processes. Simultaneously, their operation under low power factor and low efficiency conditions results in significant energy waste. Due to increasingly fierce market competition in the petrochemical industry, new requirements have been placed on the models, quality, and quantity of our plant's petrochemical products. To meet these process requirements, a frequency converter system was added to the original drive motors. This allows for smooth changes in material conveying rates, meeting production process requirements while achieving energy savings. The frequency converter system used in our plant's No. 1 and No. 2 motors serves as both a power source and an actuator for changing process parameters. It replaces the original actuator—the regulating valve—transforming the process control of media transmission. 2. Energy-saving principle of variable frequency drive (VFD) speed regulation operation: In actual production processes, the load selection of various pumps is greater than the actual flow required for production. However, in actual operation, the required flow is often much smaller than the designed flow. If the motor used cannot be speed-regulated, the flow can usually only be controlled by adjusting the valve, resulting in significant energy loss in the valve. If the motor is allowed to operate at a variable speed instead of adjusting the valve, the motor speed will decrease when the required flow decreases, and the energy consumption will be significantly reduced. H(n1) and H(n2) represent the Q=f(H) curve when adjusting the speed, and R1 and R2 represent the pipeline resistance curve when adjusting the valve. When controlling the valve, the flow needs to be reduced, the valve is closed, which increases the frictional resistance of the valve. Q2 → Q1, A → B, HA → HB. The power consumption P1 during valve control is represented by 0HBBQ1. When controlling the speed, Q2 → Q1, A → C, HA → HC. The power consumption P2 during speed control is represented by 0HCCQ1. If P1 > P2, it means that the power consumption during speed regulation is less than the power consumption during valve throttling. P=rQH, where Q is the pump shaft power, H is the flow rate, r is the head, and r is the liquid density. At points B and C, PB-PC=Q1(HB-HC)r. This portion represents the saved electrical energy. For pump load, the following expressions apply: Q1/Q2 = n1/n2, H1/H2 = (n1/n2)2, P1/p2 = (n1/n2)3. From the above formula, it can be seen that when the speed decreases by 1/2, the flow rate decreases by 1/2, the pressure decreases by 1/4, and the power decreases by 1/8, meaning the power decreases in a cubic relationship with the speed. If the method of partially closing the valve is not used, but instead the motor speed is reduced, then as the pump's output pressure decreases, the power previously consumed by the valve can be completely avoided while delivering the same flow rate. Without a frequency converter, the pump's outlet flow rate is controlled and regulated by the outlet valve. When the flow rate is low, regulating by partially closing the valve increases the pump pipe pressure difference, causing some energy to be wasted on the outlet valve. Using a frequency converter reduces the pump speed, pump head, and motor output power, thus eliminating the pressure differential previously consumed at the pump outlet valve. 3. Control Scheme of the Frequency Converter System The motors of our factory's constant-duty pump B109 (Line 1) and constant-duty pump B114 have motor powers of 75kW and 55kW respectively, a speed of 2982 rpm, a rated voltage of 380V, rated currents of 132A and 103A respectively, and rated outlet flow rates of 28.520 m³/h and 20 m³/h respectively. Under normal operating load, the motor operates at its rated speed of 2982 rpm, which is not adjustable. To maintain stable flow, control is achieved by controlling the outlet valve. A differential pressure transmitter detects the flow signal and sends it to a PID controller, which then outputs a 4-20mA control signal to control the opening of the outlet regulating valve, thereby controlling the outlet flow and maintaining flow stability. The original system had the following problems during actual operation: (1) The throttling flow was too large, and the throttling flow of the pump outlet valve was close to half of the pump's rated flow, wasting a lot of electrical energy. (2) The control accuracy was low, and the outlet flow fluctuated greatly (about 3%). (3) The motor worked at its rated speed, and the output remained unchanged while consuming electrical energy. (4) The motor was noisy, and the pressure of the pump and pipeline valves was high, which easily caused leakage. Based on the above-mentioned process requirements of the system, we followed the following principles when designing the frequency converter system: a) Maintain stable outlet flow; b) Outlet flow control accuracy of 0.5%; c) The motor speed range should be 0～2982 rpm; d) According to the working characteristics of the pump, the system design should be based on the constant torque principle; e) Energy saving and consumption reduction; f) The system design adopts dual switching of power frequency and frequency conversion to ensure the continuity and reliability of production, and can serve as backups for each other; g) Two DCS output contacts are used, one controlling the original regulating valve and the other controlling the frequency converter. In the event of a frequency converter failure, the DCS can automatically identify the frequency converter fault signal and then switch to the regulating valve to adjust the flow. When the frequency converter is in normal operation, the regulating valve is in the fully open position. Following the above principles, after research and comparison, we selected the Toshiba A5P frequency converter from Japan. This frequency converter features advanced technology, complete functions, compact structure, and high reliability, and is specifically designed for pump and fan loads. FRH: Frequency setting; ACC/DEC: Acceleration/deceleration control circuit; A/D: Analog-to-digital converter; V/F: Voltage-to-frequency converter; BD: Base drive circuit; CPU: Microprocessor; LED: Display circuit. The main circuit of the frequency converter is a typical "AC-DC-AC" SPWM voltage-type main circuit. The frequency setpoint FRH (speed setpoint) is converted into frequency and voltage reference signals by the ACC and DED acceleration/deceleration control circuits. These signals then pass through the A/D conversion circuit and the V/F function generator circuit, respectively, before entering the CPU to form SPWM pulses. These pulses become the control signals for the IGBTs, driving them and thus transforming the constant voltage and frequency AC power into adjustable voltage and frequency AC power after passing through the frequency converter. The structure diagram of the A5P frequency converter is shown in Figure 3. The entire control system uses a microcomputer for sampling, calculation, real-time control, fault alarm, and display. 4. Operation of the frequency converter system In July 1997, after installing Toshiba A5P frequency converters on the motors of the constant-voltage pump and the constant-voltage pump, the operation of the system was compared with that of the power frequency, as shown below: (1) Comparison of motor operating parameters and energy saving power = ((39.16×0.82×380)-(16.3×0.9×168))×1.73=16846 (W) Energy saving rate = ((39.16×0.82×380)-(16.3×0.9×158))/(39.16×0.82×380)=79% Energy saving power = ((44.6×0.8×380)-(20×0.9×121))×1.73=19688 (W) Energy saving rate = ((44.6×0.8×380)-(20×0.9×121))/(44.6×0.8×380)=83% It can be seen from the comparison table that the use of frequency converter can meet the production needs and save a lot of energy. (2) Comparison of control accuracy Under the same process conditions, the pump outlet flow fluctuation curves are shown in Figure 4 above when using power frequency and frequency conversion. Therefore, after the pump adopts frequency conversion speed regulation, the flow control accuracy is very high, and the curve recorded by the recorder is a very stable recording line. 5 Application effect and economic benefit analysis Since the frequency converter was put into operation, it has been reliable, highly automated, and has significant energy saving effect, achieving good economic benefits. (1) Stable process control: Due to the high precision adjustment of the frequency converter, the adjustment signal has high speed transmission, which reduces the lag phenomenon caused by the previous instrument control, thereby improving the system control accuracy, stabilizing the pressure, and improving the product quality. (2) Significant energy saving effect: Based on 8000 hours per year, the annual electricity saving of pump 114/1 is: energy saving rate × motor power × working time = 79% × 21.11 kW × 8000 hours = 133415 kWh. The annual electricity saving of pump 109/2 is: energy saving rate × motor power × working time = 83% × 23.45 kW × 8000 hours = 155708 kWh. The total electricity cost saved by pumps 114/1 and 109/2 is: electricity saving × electricity price = 289123 × 0.50 = 144561 yuan. The cost of frequency converter modification is 150,000 yuan, so the investment can be recovered in about one year. (3) Reduced maintenance: Due to the full opening of the outlet valve, the motor runs at a reduced speed, which reduces the pipeline pressure, reduces leakage of process equipment, reduces wear of pumps and motors, reduces the temperature rise of motors, and extends the equipment maintenance cycle. Since the frequency converter replaces the regulating valve, it solves the problem of the impact on production caused by the high failure rate of the regulating valve and reduces the maintenance of the instrument. (4) The system realizes soft start: Since the frequency converter has a soft start function, it reduces the impact on the power grid. 6 Issues to be noted when using frequency converter (1) When using frequency converter, it is necessary to meet the process requirements. In a certain specific environment, the pumps of the old equipment are limited by the head and flow rate, and the frequency converter may not be suitable. In addition, pumps that do not operate under variable conditions should not be used. It is not possible to copy it indiscriminately, but to start from the process conditions and the parameters of the pump itself. (2) When the frequency converter is used for speed adjustment, it is necessary for the electrical, instrumentation, process and equipment professionals to work closely together to ensure the safe operation of the frequency converter. Engineering technicians should train the relevant professionals before installation and commissioning. (3) Most production equipment instrument control valves are mostly air-closing valves. After the frequency converter is used, the air-closing valve is changed to air-opening regulation. Attention should be paid to avoid accidents.