1. Control Theory and Energy-Saving Technology of Doubly Fed Motors
1.1 Advantages of Doubly Fed Motors
Doubly-fed induction generators (DFIGs) not only belong to the category of asynchronous motors, but can also be called alternating current-excitation motors (ACEMs) or asynchronous synchronous motors (Asynchronous Synchronous Motors) because they contain independent excitation windings. During operation, they can regulate the power factor by increasing the excitation current, just like synchronous motors. Synchronous motors, however, are DC-excited, so they can only regulate the current amplitude, and therefore generally only regulate reactive power. DFIGs, on the other hand, can regulate three quantities: adjustable excitation current amplitude, adjustable excitation frequency, and adjustable phase. This means that DFIGs can change the motor speed by changing the excitation frequency to achieve speed regulation. Similarly, during sudden load changes, the motor speed can be changed by controlling the excitation frequency, fully utilizing the kinetic energy generated by the rotor and absorbing or releasing the load. Compared to synchronous motors, both DFIGs and synchronous motors can regulate the power factor by changing the rotor excitation current amplitude. Therefore, AC excitation can not only adjust reactive power but also active power during use. The main advantage of doubly-fed motors is that they use an adjustable AC excitation power supply.
1.2 Hengshi's Power-Saving Technology
Motor power can be mainly divided into three types: reactive power, active power, and apparent power. The key point of constant apparent power in use is that when active power changes, the apparent power of the motor can be controlled at a fixed value by adjusting the motor's power factor. The advantage of this approach is that when the motor is under light load, the remaining motor capacity can be fed back to the grid as reactive power, while when the motor is under heavy load, the power factor can be adjusted to be close to the unit power value, and reactive power from the grid is not absorbed. Similarly, constant apparent power can also be used in fans and pumps, mainly for the following reasons:
(1) In the operation of power plants and steel plants in my country, the equipment with the highest electricity consumption is mainly air conditioning, fans and water pumps. Therefore, it is very necessary to implement energy-saving measures for fans and pumps, and the energy-saving effect will be very obvious. The state has regulations on the technical equipment used by all enterprises in production, namely reliability and efficiency. However, the waste of resources in the production process has not been given much attention, so it is very necessary to implement energy-saving measures for water pumps and fans.
(2) The efficiency of power consumption in my country is relatively low, mainly because the production process generally suffers from the phenomenon of "underpowered power." Design institutes, in order to minimize their liability, adopt conservative and outdated design schemes, using parameters that exceed actual dimensions. This results in excessive margins, pressure, and flow coefficients. Manufacturers, in order to reduce their own risk, further increase the pressure and flow coefficients, leading to the "underpowered power" phenomenon and causing the product to deviate from its original design, thus reducing efficiency. However, the use of power consumption precisely compensates for this deficiency and also provides reactive power to the factory.
2. Application technologies of PLC and frequency converter
In energy-saving control systems for doubly-fed motors, the selection of controllers and rotor excitation power supplies requires great care and attention, as their quality directly affects the system's reliability, stability, and energy efficiency. Due to their unique advantages, PLCs and general-purpose frequency converters are well-suited for use in doubly-fed motor control systems.
2.1 PLC Selection
When selecting a PLC model, the first step is to analyze and determine the requirements for the PLC's input/output ports and communication ports. Then, based on the specific needs of the doubly-fed motor energy-saving control system, all input/output signals in the system are summarized and categorized. Next, I/O addresses are allocated according to the PLC's input/output point requirements, ensuring that each input/output signal accurately corresponds to the PLC's input/output relays. Finally, the PLC model is confirmed based on the results.
Siemens offers a wide variety of PLC products, such as the S7200, which is a small PLC and a third-generation product. The S7200 has the following features:
(1) Powerful functions. The S7200 has 5 CPU modules, which can be expanded to 7, and can also be expanded to 248 digital I/O points, and has more than 30KB of data storage space. It integrates 6 high-speed counters, pulse generators, and pulse width modulators, which greatly improves working efficiency. It can achieve custom PID parameters.
(2) Advanced program structure. The program structure of S7200 is very simple. In the software used for programming, the main program, subroutines and interrupt routines are stored separately. Subroutines can also use input and output variables as software interfaces, which is conducive to achieving the purpose of structured programming.
(3) Flexible and convenient addressing. All the components of the S7200 can be read and written in terms of bits, bytes, words and double words, such as bit memory, variable memory, input/output structure, etc.
(4) The programming software is powerful and easy to use. The STEP7Micro/WINV4.0 programming software includes three main parts: ladder diagram, statement list, and function block diagram programming language. While the S7200 has powerful instructions, it is also easy to master.
(5) Powerful communication capabilities. The CPU module of the S7200 used for programming or communication has an RS485 interface, and there may be one or two of them.
2.2 Selection of Frequency Converter
For inverters, the Siemens MicroMaster440/420 is recommended. As a classic model based on three-phase AC speed-regulating motors, the MicroMaster440/420 is primarily controlled by a microprocessor, with air filter output mainly relying on bipolar insulated-gate transistors (BITs). It boasts strong overall performance and delivers outstanding reliability to equipment. Furthermore, this inverter features a flexible internal modular structure, comprehensive motor protection, and built-in RS485/232C interfaces and a PI closed-loop controller for process control, meeting user-defined I/O terminal requirements. It also utilizes magnetic flux current to dynamically control frequency conversion characteristics, enabling torque output even at low frequencies and eliminating high-speed current limitations during tripping operation.
The MicroMaster440, equipped with a doubly-fed motor energy-saving control system, can meet various frequency conversion requirements of the system. For example, it can control voltage increase and frequency separately, achieving bidirectional adjustment of power and speed factor, demonstrating excellent performance in various system control applications. With an output power range of 0.74-90kW, the Siemens MicroMaster440 excels in higher-power environments. Combined with sensorless automatic vector control using ECO energy-saving control, it effectively enhances the advanced adsorption and delayed release control functions of mechanical braking, ensuring continuous and stable operation of the elevator. Finally, the MicroMaster440 also optimizes conveyor belt fault detection, ensuring safe production line operation. It utilizes a PID controller with customizable parameters to achieve master-slave control, suitable for multi-machine coaxial drive modes.
3. Design of a doubly-fed motor energy-saving control system based on PLC and frequency converter
The doubly-fed motor system structure diagram (see Figure 1) reveals that the controller system is primarily based on a PLC, controlling the frequency converter and switching circuits, and performing circuit detection before frequency conversion adjustment. A touchscreen provides an intuitive human-machine interface for the control system, allowing for real-time adjustments to system parameters and motor status. Control personnel can observe system operation using the touchscreen. Circuit detection of current frequency, phase, and voltage increase effectively controls rated voltage and power, and provides good control over motor speed. The circuit detection function transmits voltage data to the PLC, which then analyzes and rationally controls the data. The starter acts on the motor; after a soft start, the control circuit switches modes. A successful start is considered only when the system is in doubly-fed mode. The system's energy-saving algorithms and control modes are programmed into the PLC, using parameter adjustments to control the overall motor state, ultimately achieving energy-saving control.
Figure 1. Structure diagram of the doubly-fed engine energy-saving system
3.1 Main Circuit and Control Circuit Design
The main circuit configuration references the circuits of a doubly-fed induction generator (DFIG) and a DC generator. It's important to note that when the DC generator is loaded by the DFIG, the stator winding of the latter needs to be connected to a 50Hz power grid, and the inverter is powered via the rotor winding. The current input phase and frequency are determined by the rotor winding, thus controlling the power and speed of the DFIG. The energy-saving control system design requires a PLC as the controller to receive and verify circuit signals, analyze the current voltage parameters of the motor, and finally, the PLC executes its internal algorithm based on these parameters to control the contactor coils and the inverter's power output, comprehensively displaying the DFIG's operating status.
3.2 Design of Transient Parameter Detection Circuit for Doubly Fed Motor
Speed detection consists of a pulse isolation circuit and a rotary encoder. The encoder collects speed parameters and converts them into pulse signals, which are then analyzed and processed by the pulse isolation circuit to become pulse signals recognizable by the PLC. The PLC processes and optimizes the feedback information and converts it back into pulse signals to set a reasonable speed for the motor. The design must fully consider the importance of power factor in system control, and real-time monitoring of the power factor is necessary. This is because the cosine function is an even function, meaning the power factor will always be positive, regardless of whether it is positive or negative. Therefore, it is difficult for operators to judge the load condition from the power factor alone. In the design, current and voltage can be used as criteria to determine whether the power factor is leading or lagging behind. Furthermore, as the power factor is a nonlinear function, it cannot be simulated using circuit analogy. Therefore, a microprocessor can be designed specifically for the power factor.
3.3 Inverter Capacity Release Circuit Design
The significance of inverter capacity release lies in maintaining the safe operation of the inverter. The design principle should be based on not changing the capacity, utilizing current common inverter conditions to enhance the current output, thereby ensuring the current meets the system control requirements of the doubly-fed motor. In the design, inverter capacity release can be achieved using a voltage reduction method, connecting the rotor to the secondary side of the motor's transformer and the primary side of the transformer. This allows the doubly-fed motor to continuously and stably operate under heavy load within the 1350rpm range. If the motor power factor is 1, then the secondary side of the transformer must be connected to a voltage of at least 13V and a current of at least 9A, resulting in an inverter output power of 39V and 3A. After completing the rotor connection, the inverter's voltage and current are increased to three times, thus achieving a three-fold capacity release. It is important to note at this design stage that, based on the rotor induction and voltage/current characteristics of the doubly-fed motor, the inverter voltage output is in a low-frequency state; therefore, the step-down transformer used in the system needs to be replaced with a dedicated low-frequency transformer.
4. Conclusion
Motor energy-saving technology has achieved significant progress over a long period, with applications spanning a wide range of industries, including petrochemicals, metallurgy, and light industry. The demand for motor energy-saving technology is increasing daily. Through comprehensive optimization of system design and the promotion of variable frequency speed control equipment, various production processes requiring speed and flow regulation can be adapted to. Effective use of motor systems provides strong support for capacity allocation and overall system operation. Further research and expansion of motor capacity will enhance power for manufacturing. The continuous and stable operation of motor control systems relies on sound process control and energy allocation. my country should further promote motor energy-saving technology to help enterprises achieve higher profit margins and production safety.
