I. Working Principles of Variable Frequency Speed ​​Control and Hydraulic Coupling Speed ​​Control When a motor uses variable frequency speed control, the motor shaft is directly connected to the load. However, the motor is no longer directly powered by the power grid but by the frequency converter. The frequency converter changes the motor speed by altering the power supply frequency, thus achieving stepless speed regulation over a fairly wide frequency range, and exhibiting excellent efficiency and power factor characteristics across the entire range. With variable frequency speed control, the asynchronous motor speed is n = 60f(1-s)/p, where f is the frequency converter output frequency, s is the asynchronous motor slip, and p is the number of pole pairs. The hydraulic coupling transfers motor energy and changes the output speed by controlling the change in the angular momentum of the working oil in the working chamber. The motor drives its driving impeller through the input shaft of the hydraulic coupling, accelerating the working oil. The accelerated working oil then drives the driven turbine of the hydraulic coupling, transferring energy to the output shaft and the load. Therefore, by controlling the amount of working oil participating in energy transfer within the working chamber, the torque of the output shaft can be controlled, achieving the purpose of controlling the load speed. Therefore, hydraulic couplings can also achieve stepless speed regulation of the load. If a hydraulic coupling is used for speed regulation, the motor shaft is connected to the hydraulic coupling, and the load is also connected to the hydraulic coupling. The motor is still powered by the grid and runs at full speed. II. Energy Saving Comparison between Variable Frequency Speed ​​Regulation and Hydraulic Coupling Speed ​​Regulation 1. Reasons for Power Loss Besides the power loss of the motor itself, both variable frequency speed regulation and hydraulic coupling speed regulation involve additional power losses. The hydraulic coupling obtains mechanical energy from the motor output shaft and sends it to the load after hydraulic speed change; its efficiency cannot be 1. Similarly, the frequency converter obtains electrical energy from the grid and sends it to the motor armature after inversion; its efficiency also cannot be 1. Moreover, the efficiency curves of the two methods are different across the entire speed range. Figure 1, "Efficiency Curves of Two Speed ​​Control Methods," shows typical efficiency-speed curves for a hydraulic coupling and a frequency converter (high-speed frequency converter). As the output speed decreases, the efficiency of the hydraulic coupling decreases almost proportionally (e.g., 0.95 at rated speed, approximately 0.72 at 75% speed, and approximately 0.19 at 20% speed). In contrast, the efficiency of the frequency converter remains relatively high even as the output speed decreases (e.g., 0.97 at rated speed, greater than 0.95 at speeds above 75%, and greater than 0.9 at speeds above 20%). The curve data shows that when the output speed decreases, the efficiency of the hydraulic coupling decreases much faster than that of the frequency converter. Therefore, the low-speed characteristics of the frequency converter are better than those of the hydraulic coupling. Of course, we should recognize that when used for fan and pump loads, since the shaft power is proportional to the cube of the speed, although the efficiency of the hydraulic coupling decreases proportionally as the speed decreases, the overall shaft power of the motor still decreases proportionally to the square of the speed decrease, thus achieving energy saving. Variable frequency drive (VFD) changes the voltage and frequency of the motor armature through power electronic rectification and pulse width modulation (PWM) inverter technology. Except for a small portion of energy consumption required for its own control that remains constant, the losses of the power electronic devices are basically proportional to the output power. Therefore, VFD can maintain high efficiency across the entire speed range. In contrast, hydraulic couplings rely on pumps and turbines to transmit energy. At low speeds, the efficiency of both the pump and turbine decreases, thus the overall efficiency decreases as the speed decreases. 2. Theoretical Energy Saving Comparison: Theoretical calculations are performed, illustrated with the following example: If the airflow of a 1000kW fan is reduced from 100% to 70%, since flow rate is directly proportional to the first power of speed, the speed can be reduced by 70%, theoretically lowering the load power to 34.3%. If direct high-frequency variable speed control is used, with an efficiency of 0.95, and considering the decrease in motor efficiency at low power and the decrease in pipeline system efficiency, the total power input to the grid is approximately 34.3%/0.95/0.85/0.95 = 44.71%, or 447.1kW, resulting in an energy saving of 55.29%. Assuming 300 days per year, this translates to an annual energy saving of 3.98 million kWh. If a hydraulic coupling is used, with an efficiency of 0.665, the total power input to the grid is approximately 34.3%/0.665/0.85/0.95 = 63.87%, or 638.7kW, resulting in an energy saving of 36.13%, or an annual energy saving of 2.6 million kWh. Therefore, variable frequency speed control saves an additional 1.38 million kWh of electricity annually. The following table lists the energy savings: [align=center]Comparison of energy saving between hydraulic coupling and variable frequency speed control when the speed of a 1000kW high-pressure fan motor is reduced by 70%[/align] 3. Actual Energy Saving Comparison Taking a 200MW unit induced draft fan modified with a hydraulic coupling and variable frequency speed control by a power design institute as an example: The asynchronous motor has a rated value of 1250kW, 6KV, 142A, rated efficiency of 95%, rated speed of 742RPM, and rated power factor of 0.85. [align=center]The input current of the three adjustment methods under different generator loads is as follows: The comprehensive input power of the motors under the three adjustment methods is as follows: The estimated daily power consumption of the three adjustment methods is as follows:[/align] Based on 300 days and 7200 hours of unit operation per year, using variable frequency speed control saves 3.85 million kWh of electricity annually, while using a hydraulic coupling saves 2.68 million kWh annually. Although the motor power is different, the measured energy saving ratio is basically consistent with the theoretical calculation value. III. Comparison of Other Performance Aspects of Variable Frequency Speed ​​Control and Hydraulic Coupling Speed ​​Control Besides differences in energy saving, variable frequency speed control and hydraulic coupling speed control also differ significantly in power factor, starting performance, operational reliability, operation and maintenance, regulation and control characteristics, investment, and return on investment. 1. Power Factor: Variable frequency speed control can maintain a high power factor over a wide speed range (e.g., power factor greater than 0.95% at speeds above 20%), while the power factor of a hydraulic coupling is lower than the rated power factor of the motor at low speeds. Below 70% of the rated speed, the power factor will be lower than 0.7. If a hydraulic coupling is used and an improvement in power factor is required, a power factor compensation device must be added. 2. Starting Performance: When using variable frequency speed control, if the motor starts at rated torque, the starting current input from the power grid is less than 10% of the motor's rated current. For loads such as fans and pumps, the starting current is even smaller. Furthermore, the entire starting process is controllable, and the starting point and ramp-up time can be set. Hydraulic couplings cannot directly improve starting performance; the starting current reaches 5-7 times the rated current. Even with wound-rotor rotors, improving starting performance using rotor series resistance requires adding a starting device, but the starting current will still be more than twice the rated current, and more than 20 times that of frequency converter starting. The impact of starting on the motor and power grid is considerable. For the motor, it can cause rotor squirrel cage bar breakage and stator winding weld failure. Statistics show that approximately 15% of motor failures are caused by direct starting. For the power grid, direct starting causes a short-term voltage drop, interfering with the operation of other equipment. 3. Operational Reliability and Maintenance: The mechanical structure and piping system of hydraulic couplings are complex. Long-term reliable operation increases the workload of system maintenance. If a fault occurs, direct constant-speed operation is not possible, and shutdown for repair is necessary. High-voltage frequency converters have complex electronic circuits, but the technology is now mature. In particular, high-voltage frequency converters using a multi-level, series-connected configuration feature automatic unit switching and redundant operation. They can operate continuously without interruption in case of unit failure, ensuring reliability. Maintenance is also quite easy, requiring only periodic replacement of the inlet air filter. 4. Adjustment and Control Characteristics: Hydraulic couplings rely on adjusting the oil volume in the working chamber to change the output speed, resulting in a slow response that may not keep up with control requirements. In contrast, frequency converters change frequency very quickly, allowing adjustment at the system's maximum permissible speed. Hydraulic couplings have lower speed adjustment accuracy, while frequency converters, being digital control systems, achieve a frequency stabilization accuracy of over 0.1%, thus enabling precise control. 5. Investment and Return: Currently, the initial investment for hydraulic couplings is lower than that for variable frequency drives (VFDs). However, VFDs offer significantly better energy savings and other advantages compared to hydraulic couplings. As illustrated in the previous example, a 1000kW motor using VFDs saves 1.38 million kWh more per year compared to using a hydraulic coupling. If VFDs require an additional investment of 600,000 yuan, the return can be achieved in just over a year. Furthermore, VFDs save hundreds of thousands of yuan annually compared to hydraulic couplings, resulting in a better overall return on investment.