Abstract: This paper designs a voltage/frequency ratio (i.e., flux adjustment) energy-saving device and a three-phase AC control circuit using high-power power electronic devices (IPMs) as switching components, solving the overcurrent and overvoltage protection problems of the IPMs. The control device is based on a dual-CPU TMS320LF2407 and AT89C51, and its main function is to realize energy-saving control and operation status display of variable frequency speed-regulating asynchronous motors. Keywords: Motor; Optimal flux; Energy saving; SVPWM; IPM [align=center]System of Energy Saving control of VVVF Induction Motor Base on Double CPU＊[/align] Yu Bin1 Jia Yaqiong1,2 [align=center]（1．Human Institute of Technology, Hengyang 421008，China； 2．Guilin University of Electronic Technology 541004，China）[/align] Abstract: This paper designs a device that can supervise the running of motors and adjust the voltage to frequency offered to them. The computer is the DSP that is used to control the motor. In addition, we design a circuit whose main components are IPM. It is this circuit that is used to adjust the voltage to frequency by using the SVPWM wave. Keywords: Motor; Adjusting Ratio of Voltage To Frequency; SVPWM; Economizing on Electricity; IPM 0 Introduction Motors are the main power machinery for realizing national electrification, and asynchronous motors are the most widely used type of motor in industrial and agricultural production. For example, most small and medium-sized steel processing equipment, mining machinery, cranes, blowers, water pumps, and agricultural and sideline product processing machinery such as threshing and grinding machines are driven by asynchronous motors. The application of asynchronous motors in daily life and medical equipment is also increasing. According to incomplete statistics, by 2000, small and medium-sized three-phase asynchronous motors accounted for 85% of the total power grid load in my country, with an annual electricity consumption exceeding 375 billion kWh, accounting for 65% of all industrial electricity consumption. The reason for the widespread application of asynchronous motors is that, compared with other motors, they have advantages such as simple structure, reliable operation, high efficiency, easy manufacturing, low cost, and durability. According to incomplete statistics, among the motors currently in use in my country, there are still 400 million kWh of high-energy-consuming motors. The losses of these motors account for 10% to 23% of their rated output, indicating a huge potential for energy saving and consumption reduction. For example, the electricity consumption of fans and water pumps in China accounts for 31% of the total electricity consumption in the country and 50% of the total electricity consumption in industry. However, their operating efficiency is below 70%, which is generally 5% to 10% lower than the world average. The potential for saving electricity in this area alone is about 30 billion kWh per year. Power shortage is a serious problem that is prevalent in most parts of China. According to statistics from the power sector, the annual power shortage in China is about 15% to 20%. In 1998, due to power shortage, nearly 30% of the country 's industrial production capacity was forced to stop, and the annual industrial output value decreased by 547.3 billion yuan. The load data from the power sector shows that the motor load accounts for more than 80% of the total load. Therefore, the study of energy saving of asynchronous motors is very important. 1 Selection of energy-saving schemes for asynchronous motors In terms of energy-saving technology for motor applications, although there are various methods depending on the type, purpose or system of the motor, the following aspects are relevant to the use of motors as power sources in modern enterprises: (1) Replace general high-energy-consuming motors with high-efficiency series motors. However, the cost of this type of motor is relatively high, and the economic effect depends greatly on the load. That is, the energy-saving effect is best for applications that operate continuously at close to the rated load (more than 70%) for a long time. (2) Use power factor controller (i.e., asynchronous motor energy saver) and brake. (3) Speed ​​regulation and energy saving of motor. ① High-performance AC motor variable frequency speed controller. Using microcomputer-controlled high-power transistor inverter technology, using AC-DC-AC mode, it is used for speed regulation of AC asynchronous motor, and the energy saving effect can be as high as 55% or more. ② Rotor series resistance speed regulation. (4) Energy-saving transformation of old high-energy-consuming asynchronous motors. (5) Development of permanent magnet motors. (6) Reasonable selection of asynchronous motors. Among them, the energy-saving method of using frequency converter can achieve obvious energy-saving effect and is easy to carry out technical transformation of old equipment. It is a key energy-saving project promoted by the state. 2 Principle of frequency conversion energy saving The operating efficiency of three-phase asynchronous motor is represented by its power-energy index. The so-called power-energy index refers to the efficiency and power factor of the motor. The efficiency formula for an asynchronous motor is: The above formula shows that the efficiency of an asynchronous motor is related to its total losses, and the only way to improve efficiency is to reduce total losses. The total losses of an AC motor are: Where: Pcu — the sum of stator and rotor copper losses; Pfe — stator core losses; Pm — mechanical losses; Pz — stray losses. Generally, copper losses account for 56% of the total motor losses, and iron losses account for 30%. These two phases are the main factors determining motor efficiency. From the equivalent circuit, the stator and rotor copper losses of the motor are: The core losses of the motor are: The electromagnetic torque of the motor is: The total losses of the motor are: Where: PM is the electromagnetic power; Ω1 is the mechanical synchronous angular velocity; Np is the number of pole pairs. Due to the synchronous speed, the power factor of the motor rotor itself is very high when the motor is running in steady state. It can be approximated that under this condition, the equation is: Taking the stator frequency f[sub]1[/sub] and λ as independent variables, the deflection is calculated and set to zero. The stator frequency with the minimum loss can be obtained as: Where: Therefore: From equation (1), it can be seen that the stator frequency f[sub]1min[/sub] with the minimum loss when the variable frequency speed control AC motor is running is only related to the speed and not to the load. Under the condition of given speed and fixed load, equation (1) and equation (2) constitute the energy-saving control algorithm when the voltage/frequency ratio of the frequency converter is controlled. As long as the required speed is known, the energy-saving frequency can be known. Then, the energy-saving frequency is applied to the motor and the voltage is adjusted to the given speed to find the energy-saving point when the variable frequency speed control AC motor is running, thereby achieving optimal control. 3 System Hardware Design In order to realize the energy-saving control of the variable frequency speed control of the three-phase asynchronous motor, this paper designs an energy-saving control device. The overall control concept of the system is to search for the energy-saving operating point, that is, to find the energy-saving stator frequency for a given torque and speed at the steady-state point according to the energy-saving algorithm, and then use this frequency as the input frequency to adjust the input voltage to control the speed to the required value. Therefore, the system has a main circuit and a control voltage-frequency ratio circuit. The acquisition circuit includes the acquisition of voltage signals, current signals, and speed signals. The system principle block diagram is shown in Figure 1. Figure 1 System Overall Block Diagram 3.1 Main Circuit The main circuit includes rectification, filtering, and inversion. The system power conversion stage adopts an AC/DC rectifier circuit and an IGBT inverter circuit. The three-phase AC voltage is rectified by a diode rectifier module, filtered by a large capacitor, and then sent to the three-phase inverter module. In this paper, the three-phase inverter module adopts an IGBT intelligent power module (IPM). The IGBT intelligent power module (IPM) is a hybrid circuit device that integrates several high-speed, low-power IGBTs and gate drive circuits. It achieves efficient self-protection function by using advanced current-sensing IGBTs and matching gate control circuits. The use of intelligent power modules can shorten the system design time, improve system reliability, and simplify and compact the system hardware circuit, reducing system size. To allow the CPU more resources to handle the main control functions (detecting voltage and current, and generating SVPWM waveforms, etc.), this paper uses a TMS320F2407 as the main processor and an 89C51 (hereinafter referred to as C51) microcontroller as the auxiliary processor, as shown in Figure 2. Figure 2 System Main Control Board Structure Diagram 3.2 Intelligent Power Module (IPM) Since the DC voltage of this system is 537V, 5KVA, and the rated current at full load is 9.3A, considering a 20% overload margin and a 20% current spike factor, the maximum current is 20.46A. This paper selects the PM25RSB120 IPM module manufactured by Mitsubishi Corporation of Japan, with a voltage of 1200V and a current of 25A. The IPM integrates the best IGBT drive circuit and self-protection circuit, but it must be provided with an isolated drive power supply and matched control signals. Therefore, an interface circuit board needs to be designed, which will be directly mounted on the IPM module, as shown in Figure 3. The interface circuit board must meet the following requirements: provide four isolated power supplies with an insulation voltage at least twice the module's rated voltage of 1.5V ± 10% (three for the upper bridge arm, and one for the lower bridge arm and braking). The power supply must be able to provide the maximum current required at the operating switching frequency; it must have signal input isolation function with a transmission delay time not exceeding 0.8µs, a sufficiently large isolation voltage, and fault signal output isolation function; the layout of the interface circuit board must be reasonably designed to minimize noise coupling to the control circuit. Figure 3: IPM Interface Circuit 3.3 Interface Circuit between Microcontroller and LCD Display The display uses a graphic LCD module M-240128T with a built-in T6963C controller. The T6963C is a dot-matrix LCD graphic display controller that can directly interface with 51 or 96 series microprocessors. The LCD module with the built-in T6963C controller has implemented the interface between the T6963C and the row and column drivers and the display buffer RAM. The structure of the LCD screen, data transmission method, and the length and width of the display window have also been configured in hardware. 240128 indicates that the module's length and width are 240x128 points, and its structural block diagram is shown in Figure 4. Figure 4 Internal structural block diagram of the LCD module 3.4 Measurement Circuit Design The raw signals to be measured are high-voltage, high-current sinusoidal AC or high-voltage DC with spikes. They must be processed as analog signals before being sent to the DSP. (a) The AC voltage is converted into an isolated current signal of about 1mA by the voltage transformer PT (essentially a small special transformer), and then formed into a well-formed sine wave by the current-to-voltage conversion circuit and the filter circuit. After rectification, it is sent to the main control board for A/D conversion to obtain the voltage amplitude; (b) The AC current is also converted into a current signal of about 1mA by the current transformer CT, and then also sent to the current-to-voltage conversion circuit and the filter circuit. 4. Software System Design The DSP main control program mainly includes data acquisition, data processing, energy-saving algorithms, and SVPWM waveform generation. It also includes subroutines for self-testing, initialization, communication with C51, and fault handling. The DSP main program flowchart is shown in Figure 5. After system startup, it first performs self-testing and initialization, then waits for the motor start command. Upon receiving the start command, the start module is executed. When the motor reaches the required speed and steady state, the system begins searching for the energy-saving operating point. The energy-saving module is a loop program that runs in this loop when the CPU does not execute any interrupt service routines. In addition, it contains several relatively independent loops as subroutines. Besides the waveform generation loop module, there is also an independent loop module consisting of the AD conversion interrupt service routine. Information transmission between these relatively independent loop modules is accomplished by modifying flag variables. The module issuing the control modifies the corresponding flag variables, while the module receiving the control responds to the changes in flag variables by polling. Figure 5 DSP Main Program Flowchart 5. Result Analysis Due to the adoption of SVPWM modulation, the DC voltage utilization rate is improved and the switching losses are reduced, but the output waveform is still similar to that of SPWM modulation. Figure 6 compares the experimental waveforms of SPWM and SVPWM (both waveforms are observed using a digital oscilloscope after resistor voltage division). Figure 6 shows that under SVPWM modulation, the IGBT is always in the on or off state for one-third of the time within a cycle, meaning that switching losses are reduced by 33%. SPWM modulation mode IGBT waveform SVPWM modulation mode IGBT waveform Figure 6 Experimental waveform comparison References: [1] Chen Wangzhang et al. Electric motor energy saving technology [M]. Beijing: Science Press, 2002 [2] Qi Yilu et al. Energy saving and loss reduction technology manual [M]. Beijing: China Electric Power Press, 1995 [3] Gao Jingde et al. Analysis of AC motor and its system [M]. Beijing: Tsinghua University Press, 2003 [4] Zhao Yong et al. DSP application system design [M]. Beijing: Electronic Industry Press, 2002 [5] Sun Yunlian et al. PWM harmonic elimination three-phase AC voltage regulation and circuit implementation using dual CPU method [J]. Journal of Wuhan University of Water Resources and Electric Power, 2000.2 [6] Fang Xiaocui et al. Practical system design technology of single-chip microcomputer [M]. Beijing: National Defense Industry Press, 1996 Author Introduction: Yu Bin (1979-), male, from Yangzhou, Jiangsu, lecturer, main research direction: information processing and DSP. Email: [email protected] Tel: 0734-7134306/8434290 Address: Department of Electrical and Information Engineering, Building 14, Leigongtang, Hengyang City, Hunan Province, China Postcode: 421008