Abstract: This paper introduces typical application examples of vector frequency converters in constant pressure water supply automatic control systems, textile machinery, lifting equipment, cement machinery, logistics machinery, and other related fields. Keywords: Frequency converter, vector control , frequency converter application examples I. Application of Frequency Converters in Constant Pressure Water Supply Automatic Control Systems Variable frequency speed regulation constant pressure water supply equipment, with its advantages of energy saving, safety, and high-quality water supply, has enabled the water supply industry to experience a leap forward in technological equipment since the early 1990s. The constant pressure water supply speed regulation system realizes stepless speed regulation of the water pump motor, automatically adjusts the system's operating parameters according to changes in water consumption, and maintains constant water pressure to meet water requirements when water consumption changes. It is currently the most advanced and reasonable energy-saving water supply system. The working principle of the variable frequency constant pressure water supply automatic control system is as follows: During normal operation, pressure sensors on the user's water supply network sample the water pressure data and convert the pressure signal into an electrical signal, which is transmitted to the PID controller. The controller then compares and calculates the pressure value set by the user, converting the result into a frequency adjustment signal and a pump start count signal, which are sent to the frequency converter and programmable logic controller (PLC) respectively. The frequency converter adjusts the power frequency of the pump motor, thereby adjusting the pump speed. The PLC controls the pump operation based on the pump start count signal transmitted from the PID controller. By adjusting the number of pumps started and stopped, and the speed of the variable frequency pump, the water pressure in the user's network is kept constant at the user's pre-designed pressure value, ensuring that the water volume "lifted" by the pump group matches the constantly changing water consumption of the user's network, achieving the goal of "variable constant pressure water supply." The following application system of a vector frequency converter in a municipal water supply project consists of a programmable logic controller, a frequency converter, and a motor. The PLC controls the variable frequency speed controller, which has the function of controlling the pumps for constant pressure water supply. A pressure sensor installed on the pipeline converts water pressure into a 4-20mA analog signal. This signal is then used by a PID controller built into the PLC to adjust the speed of the electric water pump. When water consumption increases and the pipeline pressure falls below the set pressure, the variable frequency drive increases its output frequency, increasing the pump speed and water supply. Once the set pressure is reached, the pump speed remains constant, maintaining the pipeline pressure at the set level; conversely, the same applies when the pressure drops. This closed-loop PID control achieves constant pressure water supply. If a motor malfunctions (e.g., overvoltage, overcurrent, overload, or motor overheating protection), the system automatically stops. Once the system recovers, the operation can be resumed following the established steps. II. Application of Frequency Converters in Textile Machinery Most cotton textile equipment utilizes frequency conversion speed control technology and programmable logic controller (PLC) technology. A significant number of products also employ industrial control computers, microcontrollers, AC servo systems, touchscreen human-machine interfaces, and fieldbus technology, achieving mechatronics integration in textile machinery. Frequency converters are widely used in textile equipment, from opening and carding machines, drawing frames, roving frames, spinning frames, winding frames, warping frames, sizing frames, and shuttleless looms. Based on usage, they can be categorized into three types: Type 1: One frequency converter controls one motor for one main machine, such as drawing frames, roving frames, and spinning frames. Type 2: One frequency converter controls multiple motors for one main machine, such as drawing frames and air-jet spinning machines (single-spindle, single-motor drive type). The third category: Multiple frequency converters control multiple motors from a single main unit, with computer-controlled coordination and synchronization of these motors to achieve winding and forming functions. Examples include new roving frames that eliminate the conical wheel, new sizing frames that eliminate long-side shaft drives and continuously variable transmissions, and slitting and warping machines. The capacity range of frequency converters used in textile equipment is 0.37KW to 500KW, with over 90% being 0.37KW to 37KW. The application of vector frequency converters in carding machines: An old carding machine used in a cotton mill inevitably had some design flaws due to limitations imposed by the technology, manufacturing costs, and market demands at the time. For example, the electromagnetic clutch in the transmission system of the A186D carding machine had a high failure rate, frequently causing shutdowns and occasional fire alarms, resulting in losses in production efficiency and product quality. Maintaining and repairing it required significant manpower and resources. The inertia wheel electromagnetic clutch was abandoned. This resulted in thin slivers during the transition from slow to fast speeds, and in severe cases, edge breakage and web tearing, affecting production quality. To avoid this phenomenon, operators sometimes use improper operating methods to compensate for the above-mentioned equipment defects, but this results in a large amount of waste sliver, which is also undesirable. The process of improving the carding machine's operating status using vector frequency converters: The A186D carding machine, to achieve smooth doffer speed increases and decreases, incorporates a dual-speed motor, inertia wheel, and electromagnetic clutch in its mechanical transmission, using a combination of electrical and mechanical means. The A186E, A186F, and FA201 carding machines further improve the frequency of speed increases and decreases by adding a star-delta conversion control element to the motor. The FA201B and FA212 carding machines utilize Weike vector frequency converters for speed regulation, enabling arbitrary adjustment of the doffer speed increase slope and variable doffer process speed. This provides a good example for the retrofitting of older machines. Retrofitting the A186D machine with vector frequency converter speed regulation not only improves equipment performance and reduces downtime due to malfunctions, but also increases production efficiency and quality. III. Application of Frequency Converters in Lifting Equipment Frequency converters (FDs) can achieve start/stop, forward/reverse rotation, S-curve acceleration/deceleration, and multi-segment speed control through their external control terminals. Some parameters of the motor itself used in vector control calculations can be automatically measured by the FD. Furthermore, the FD has protection functions against overcurrent, overload, motor overheating, overvoltage and undervoltage, overspeed, and stall. The FD can also provide stop signals, zero-speed signals, speed arrival signals, and running preparation signals. The programmable logic controller (PLC) integrates external signals and control signals from the FD, analyzes and performs logic calculations, and then issues control commands to external equipment and the FD. The key to elevator speed control is acceleration and deceleration leveling. The following points should be mastered when controlling it: 1. Start-up control To ensure that the elevator starts smoothly without impact or reverse slippage, the start-up control should be in the following order: (1) First, send a pre-excitation command to the frequency converter to establish a magnetic field for the motor (at this time, the speed is given as zero); (2) After the first delay, send an open brake command; (3) After the second delay, confirm that the brake is open and then give a speed command. 2. Deceleration leveling control The elevator decelerates directly to the floor level according to the distance, that is, the deceleration distance of each floor should be completely consistent. There is no crawling process when decelerating to the floor level, and the speed is directly reduced from the running speed to zero. To ensure the comfort when stopping, the brake should be closed only when the elevator reaches zero speed (this signal is given by the frequency converter), and after a first delay, a stop excitation command should be given. If the elevator does not level after stopping, re-leveling control should be performed. 3. Re-leveling Control: If the elevator does not level accurately after stopping, or if the car shifts due to deformation of the wire rope after leveling, re-leveling should be performed. Re-leveling should be carried out at a lower speed (usually 1% of the operating speed) and with the elevator doors open and the elevator within the leveling zone. The frequency converter has excellent low-frequency torque; our tests show that the elevator can reliably re-level even at 110% of its rated load. Because the control system uses pulse code positioning control technology, the deceleration sensor in the shaft is omitted, leaving only a leveling sensor on the car top, which also has a re-leveling function. Practice has proven that the modified elevator has good running comfort; the comfort of starting, decelerating, and leveling does not change with changes in car load, achieving satisfactory results. Several Issues to Note During Modification: There is no fixed model for elevator technical modification; everything should be determined based on the actual site conditions. However, in the process of converting old AC dual-speed elevators and voltage-regulating speed-regulating elevators into frequency converter speed-regulating elevators, we found that the following commonly encountered issues should be given special attention to ensure the safe use of the modified elevator. 1. When converting freight elevators to fully automatic collective control, safety contact plates or photoelectric protection devices should be installed. If a weighing device is missing, it should be installed. 2. Maintain the original rated load capacity, rated speed, and original wire rope traction ratio. 3. Use circuitry or software to ensure that the car and machine room cannot move when the car top is traveling at low speed, ensuring the safety of operators on the car top. 4. For landing doors without automatic closing devices, a reliable automatic closing device should be added to each landing door. 5. When landing door panels are connected by ropes or chains, the passive door panels should be equipped with electrical safety devices. 6. Inspect and test the speed governor, safety brake, and their linkage. Any unqualified speed governors must be replaced. 7. Brakes should be completely disassembled. No oil residue is allowed on the brake band. The movable iron core of the electromagnet and the copper sleeve must be clean and lubricated with graphite powder. IV. Application of Frequency Converters in Cement Machinery The application of frequency converter technology in China's cement industry is becoming increasingly widespread. In many production processes requiring speed regulation, such as rotary kilns, single-coolers, feeders, batching systems, fans, and water pumps, AC variable frequency speed control has become an inevitable trend, replacing voltage regulation, slip-ring speed control, and DC speed control. In cement grinding, the particle size of the material entering the ball mill significantly impacts its hourly output. Pre-crushing, as an important way to increase mill hourly output and reduce grinding power consumption, has attracted the attention of many cement companies. According to process requirements, cement vertical kilns discharge material for 2-3 minutes at intervals of 2-3 minutes each time. However, currently, almost all cement companies operate crushers at a constant speed using the industrial frequency, running continuously for 24 hours, resulting in huge energy waste and affecting the service life of the motor and crusher. Furthermore, due to the large inertia of the crusher, frequent start-stop is difficult. Therefore, even with a frequency converter, it is difficult to solve the problem of the pump voltage rise during system braking causing the protection circuit to activate, preventing the system from operating normally. To address the above characteristics of the system, a series of frequency converters are used to achieve variable frequency speed regulation and soft start of the crusher; a regenerative energy feedback unit is used to overcome the excessively high pump voltage generated during crusher braking; and a PLC is used to implement closed-loop logic control of the system, synchronizing the operation of the crusher with the discharge of the vertical kiln, achieving intermittent operation. This improves process control quality while maximizing energy savings and reducing production costs. On-site commissioning and operation results show that the system is reliable, with an energy saving rate of over 60%. The above system has been put into actual operation in a cement plant. The system automatically starts and brakes based on the feeding signal, and the crusher's operating speed is continuously adjustable. The motor can achieve frequent soft starts with virtually no starting current surge and sufficient starting torque. Under variable frequency operation conditions, if the frequency converter suddenly fails, it automatically switches to "power frequency" mode to continue operation, while simultaneously issuing an audible and visual alarm signal (internal option). Based on the needs of the site conditions, the frequency setpoint for variable frequency operation when there is a discharge signal is set to 43 Hz, the system operating current is 27 A, and the operating voltage is 280 V. The modified system consumes an average of 57,000 kWh of electricity per year. According to on-site records, before the upgrade, the system operated at a frequency of 50 Hz, with a current of 32 A and a voltage of 400 V, consuming an average of 194,200 kWh per year. The energy saving rate after the upgrade was 70.6%. The system's key advantages are as follows: 1. The drive system of the cement clinker crusher was upgraded using variable frequency speed control technology, meeting the crusher's low-speed, intermittent operation characteristics, ensuring process control quality, achieving significant energy savings, and extending the service life of the crusher and motor. 2. Energy feedback control technology was used to overcome the pump voltage rise caused by the crusher's large inertia, effectively ensuring the safe operation of the frequency converter. In addition to the more than 20 protection functions and fault self-diagnosis functions of the frequency converter and energy feedback device, the system also added motor overheat protection, control circuit protection, and alarms. 3. The use of a programmable logic controller (PLC) implemented various logic controls, automatic control of frequency converter start-stop, manual/automatic operation, power frequency/frequency conversion, and fault self-switching functions, making the system flexible, convenient, and fully functional. V. Application of Variable Frequency Drives in Logistics Machinery A speed-regulating belt scale is a real-time continuous metering device used to measure and control the speed and material flow of a belt conveyor. It is widely used for the metering control and conveying of bulk solid raw materials. When the motor drives the belt, the material is output along with the belt movement. The load sensor W detects the material and converts it into an electrical signal, which is then sent to the controller. Simultaneously, the speed sensor also sends the detected motor speed signal to the controller. The speed and load signals are transformed and processed by the controller to calculate the instantaneous and cumulative flow of the material. After comparing this with the set value, the controller adjusts the output control signal using methods such as PID control to control the motor speed and stabilize the material flow at the set value. Since the belt scale is an integrated device combining control, metering, and conveying, using a variable frequency drive ensures stable and reliable operation in industrial environments. Practice has shown that in harsh industrial environments, when using a slip-ring motor for speed regulation, the slip-ring clutch is prone to blockage by dust or foreign objects due to poor sealing, leading to runaway (loss of control) phenomena. The low-speed performance of slip-ring speed-regulating motors is poor. When the belt scale operates at low speed, the belt conveyor speed is often unstable, which can seriously affect the normal operation and metering accuracy of the belt scale. Furthermore, when the setpoint of the belt scale needs to vary significantly, the speed regulation range of slip-ring speed-regulating motors is insufficient. Using a frequency converter (VFD) not only effectively solves the above problems but also leverages the VFD's characteristics of stiff mechanical characteristics and low slip rate during speed regulation. By detecting the load signal of the belt scale and employing a control method combining preset control and PID control, the system's response time is greatly improved. This is crucial for ensuring the control and metering accuracy of the belt scale when the material on it changes drastically. The application of VFDs also shows significant energy-saving effects. Its application in water pumps and fans can save approximately 40% of electricity compared to traditional valves and baffles. Taking a 100kW fan as an example, based on 8000 hours of operation per year, it can save 320,000 kWh annually. VI. Other Application Examples of Frequency Converters 1. Central Air Conditioning Variable Frequency Speed ​​Control: Using frequency converters to regulate the speed of chilled water pumps and cooling water pumps in a central air conditioning system allows one frequency converter to control multiple pumps simultaneously, achieving high efficiency and energy saving, avoiding the "overpowered" phenomenon, with energy savings of 30%–60%. It also enables multi-point temperature and humidity detection and centralized monitoring, achieving optimal comfort control. 2. Injection Molding Machine Variable Frequency Speed ​​Control: The injection molding process generally consists of the following steps: mold clamping → injection and pressure holding → molten plastic feeding → cooling and shaping → mold opening and ejector pins. Each step has a different load. Using a frequency converter to control the oil pumps allows for corresponding power output for each step, significantly saving energy with a savings rate of 30%–60%. 3. Crane Motor Variable Frequency Speed ​​Control: Cranes typically have multiple motors, controlling the trolley, crane, and hook movement. These motors can all be modified using frequency converters. The following are the obvious advantages after the modification: (1) The motor starting current is small and the torque is large, avoiding the impact of large current and saving electricity significantly. (2) Spare parts are saved and there is no need to replace low-voltage electrical appliances such as contactors. (3) No manual maintenance is required and the reliability is extremely high. 4. Variable frequency speed regulation of fans and water pumps Traditional fans and water pumps adjust the flow rate by dampers or valves. Since the flow rate is proportional to the speed and the power is proportional to the cube of the speed, the power (power consumption) will decrease by the cube of the speed by using a frequency converter to adjust the flow rate. The energy saving effect is very obvious and the power saving rate can reach 30-70%. 5. The application of frequency converters in winding machines and wire drawing machines has the characteristics of smooth start-up, large starting torque and stepless speed regulation, which can improve output and reduce failure rate. 6. Variable frequency speed regulation of boiler fans Boiler fans include induced draft fans and blowers. Generally, the air volume is changed by adjusting the damper. After using a frequency converter, the damper can be adjusted to the maximum and the speed can be adjusted by the frequency converter. Generally, the energy saving rate is over 40%. 7. Variable frequency speed control of air compressors can control multiple air compressors simultaneously through one frequency converter, avoiding energy consumption due to motor idling, eliminating the need for dedicated personnel, and automatically achieving constant pressure air supply, resulting in high efficiency and energy saving.