1. Load Characteristics of the Back Drive Device in a Horizontal Screw Centrifuge The speed control device installed on the small shaft end of the differential in a horizontal screw centrifuge is called the back drive device. These devices include: eddy current brakes, asynchronous motors, hydraulic motors, and mechanical overload protection devices (small shaft speed is zero). When the screw lags behind the drum, these devices exert braking torque on the small shaft at the cost of consuming the centrifuge's kinetic energy, thereby achieving the purpose of adjusting the differential speed. For the small shaft, the back drive device is a negative load. In general-purpose frequency converter speed control systems, the motor connected to the small shaft of the differential is in a regenerative state for a long time, operating in the fourth quadrant, receiving mechanical energy from the centrifuge, and feeding the regenerative braking energy back to the DC bus of the frequency converter, where it is then consumed by the braking resistor. How to recover this energy is a topic of great concern to centrifuge manufacturers both domestically and internationally. Using a specially designed four-quadrant frequency converter (such as ABB's Acs611 frequency converter), the regenerative energy can be directly fed back to the power grid, but frequency converters are expensive and rarely used in China except by steel rolling mills. Alra_Laval's DS706 large-scale wastewater treatment centrifuge, manufactured in recent years, utilizes a dual-frequency conversion energy feedback energy-saving frequency control system (using Acs800 series frequency converters) and is currently operating at the Stonecutters Island Wastewater Treatment Plant in Hong Kong. Domestic manufacturers have also applied common DC bus AC frequency conversion technology to horizontal screw centrifuges using domestically produced frequency converters, enabling the recovery of most of this energy and achieving significant social and economic benefits. The promotion and application of this technology is undoubtedly of great significance. 2. Structure and Characteristics of the Common DC Bus AC Frequency Conversion Speed ​​Control System 2.1 Structure As shown in Figure 1, the centrifuge 3 is driven by the main motor 2, with the differential shaft and auxiliary motor 5 coaxially connected. The speeds of the main and auxiliary motors are controlled by frequency converters 1 and 6, respectively. Their DC buses are connected in parallel, and three-phase power is input to the main frequency converter 1. 2.2 Characteristics 1) Excellent Energy-Saving Performance When the screw lags, the regenerated energy is sent to the DC bus of the auxiliary frequency converter. Since the DC buses of the main and auxiliary frequency converters are connected in parallel, this energy is utilized by the main motor through the main frequency converter. For simplicity, assume the centrifuge operates at constant torque and constant differential speed in steady state (ignoring the effects of acceleration and deceleration torque during speed regulation). The recovered energy is then P = 0.8Mn/9550, where P is power (kW); M is the small shaft torque (Nm); and n is the small shaft speed (r/min). The 0.8 times before M is because during regenerative braking, even without a discharge braking resistor, 20% of the copper losses inside the motor are converted into braking torque. 2) Fast dynamic response: Some PID control systems often exhibit overshoot and long transition times, such as eddy current brake speed control systems, where the stabilization period can sometimes last for several minutes. Variable frequency speed control systems have a torque response time of only 150–200 ms, significantly improving dynamic characteristics. 3) It easily handles material accumulation in the drum caused by sudden events. When the auxiliary motor reverses and operates in the first quadrant (motor state), the differential speed is very large: An = (Ni + n) / I (Ni is the drum speed in r/min; I is the differential speed ratio). Since the frequency converter has a static starting torque of twice the rated torque, the material accumulated in the drum is easily discharged. 4) It is beneficial for achieving constant torque control. For some materials, such as urban sewage, which contains 60% to 70% organic matter, the sludge is compressible, and the solids content changes constantly. This causes the screw conveyor torque to change with the feed flow rate and solids content. The electrical system needs to control the feed rate or differential speed in a timely manner according to the torque changes; otherwise, material blockage is easy to occur. The key to constant torque control is to continuously measure the screw conveyor torque in real time, and the torque sensing element must be selected appropriately. In hydraulic motor speed control systems, hydraulic oil pressure transmitters are used; in eddy current brake speed control systems, resistance strain gauge torque sensors are used; in the frequency converter speed control system described in this article, the torque and current analog quantities output by the frequency converter can be used directly, eliminating the need for a separate sensor. For example, the Emerson TD3000 frequency converter has two operating modes: torque control and speed control. When torque control is selected, the frequency converter output frequency will automatically adjust according to the output torque signal. When the screw conveyor torque increases, the output frequency decreases, the differential speed increases, and the sludge is quickly pushed out of the drum; conversely, the output frequency increases, the differential speed decreases, and the torque increases. Ultimately, the screw conveyor torque is stabilized near the set value. 2.3 Speed ​​Control System Design Taking the LW430W centrifuge as an example, the operating speed ni = 2200 r/min, the rated output torque of the differential is 4000～5000Nm, the speed ratio i = 91, the differential adjustment range An = 2～20 r/min (normal operation 10～12r/min); the auxiliary motor and the differential shaft are directly connected (as shown in Figure 1), the differential speed is calculated according to An = (ni-n)/i, and the data in Table 2 is obtained, which can completely meet the process requirements. 1) Inverter Selection There are no special requirements for the main inverter, but the auxiliary inverter must be able to shield the input phase loss protection. If the centrifuge requires constant torque control, a vector control inverter should be selected. 2) Power Matching of Main and Auxiliary Inverters Not all inverters with arbitrary power can be connected as shown in Figure 1. When selecting the power of the main inverter, the ability of the auxiliary inverter to draw power from the main inverter when the auxiliary motor is in motor mode must be considered. 3) Auxiliary Motor Selection The rated output torque of the auxiliary motor should be able to meet the requirements of the screw conveyor torque. Since the torque M transmitted by the differential small shaft is one of the screw pushing torque Mz, the rated torque of the auxiliary motor should be greater than M/I. In specific calculations, the differential speed adjustment range and motor connection method should be considered. A common three-phase asynchronous motor is selected, with a speed control accuracy of 0.5% to 0.1%. A variable frequency motor with an encoder is selected; under PG vector control mode, the speed control accuracy can reach 0.1% to 0.05%. A design example is shown in Table 1, which is a power matching table for the main and auxiliary frequency converters and the selection of auxiliary motors for more than ten varieties of urban sewage treatment centrifuges in four series from φ350 to φ720, manufactured by Haishen Electromechanical Plant. The main frequency converter is Emerson TD2000, the auxiliary frequency converter is Emerson TD3000, and all auxiliary motors are 4-pole variable frequency motors, equipped with OMRON E6C2-CWZ6C type 600-line photoelectric encoders. In Table 2, the output torque decreases when the differential speed is below 7.7 r/min because the variable frequency motor operates with constant torque speed regulation below 50Hz and constant power speed regulation above 50Hz. However, the low differential speed only occurs when the feed concentration is particularly low or during the initial feeding stage of the centrifuge, at which point the pushing torque is also relatively small. 2.4 Application Example Figure 2 is a simplified electrical control diagram of the LW520 high-speed centrifuge used for soybean protein slurry separation. The main frequency converter U1 drives the centrifuge, allowing stepless speed regulation from 0 to 3500 r/min. The output frequency of the frequency converter is set by terminals X1 and X2. S1 is the centrifuge operating state selection switch. When S1 is set to position X1, the centrifuge operates at the separation frequency; when S1 is set to position X2, it operates at the rinsing frequency. The separation frequency is factory set to 45 Hz (drum speed 3150 r/min), and the rinsing frequency is factory set to 5 Hz (drum speed 350 r/min). If it is necessary to change the operating frequency, the frequency converter parameter F58 can be adjusted. F59 is used for setting. U2 is the auxiliary frequency converter, used to adjust the speed difference between the centrifuge drum and the screw, i.e., the differential speed. Changing the magnitude of the differential speed can change the sludge pushing speed of the centrifuge and also affect the hourly sludge processing capacity of the centrifuge. The DC buses of the main and auxiliary frequency converters of this machine are directly connected in parallel, which has excellent energy-saving effect. PR is a speed display instrument, used to display the speed of the centrifuge drum and the differential speed. There is a switch inside the tachometer to select the synchronization alarm point, which can be selected from 1 r/min, 5 r/min, and 10 r/min. When the differential speed is less than the alarm point, the normally open contact of the relay installed inside the tachometer closes first, and then the relay K1 is activated, stopping the auxiliary motor. Through the external contact of relay K1, the user can connect an external audible alarm system, or cut off the feed valve when an alarm occurs, or communicate with a remote control system. The time relay KT is used to solve the problem of the differential speed being lower than the alarm point during the centrifuge startup phase. In addition to the simple circuit and convenient operation, the main feature of this design is that the differential speed adjustment is fast and accurate, and the stability can reach ±0.1 r/min. 3. Conclusion The AC variable frequency speed control system with a common DC bus effectively solves the problem of regenerative energy recovery from the back drive motor of the horizontal screw centrifuge, bringing significant benefits to users. Taking the Shanghai Longhua Water Purification Plant as an example, the plant currently treats 100,000 tons of urban sewage annually, using two LW430W centrifuges. Assuming a differential speed of 10 r/min and a small shaft torque of 15 Nm, preliminary calculations show that each centrifuge saves 1.5 kW. Based on 10 hours of operation per day and 300 days of operation per year, this translates to an annual electricity saving of 4,500 kWh. At a price of 0.631 yuan per kWh, this equates to an annual electricity cost saving of 2,839.5 yuan. The two centrifuges combined save a total of 5,679 yuan in electricity costs. This speed control system demonstrates strong viability and is worthy of widespread application.