1 Introduction With the advancement of science and technology, the rapid development of high-power transistor electronic technology, and the leapfrog progress of large-scale integrated circuits and microcomputer technology, AC motor variable frequency speed control technology has become increasingly sophisticated. Variable frequency drives (VFDs) are used for AC asynchronous motor speed control, and their performance surpasses any previous AC speed control method. Moreover, their simple structure makes them the latest trend in AC motor speed control. Since squirrel-cage motors account for a large proportion of all motors, their speed control methods and control technologies will undoubtedly become key technologies for motor control. The combination of VFDs and squirrel-cage motors is the best choice for AC motor speed control systems. [b]2 Constant Voltage Frequency Ratio AC-DC-AC VFD Control Principle[/b] The speed of an asynchronous motor is mainly determined by the power supply frequency and the number of pole pairs. By changing the power supply (stator) frequency, the motor speed can be adjusted, and sufficient torque can be obtained even during wide-range speed control operation. To prevent the motor from experiencing magnetic saturation due to frequency changes, which would increase the excitation current and reduce the power factor and efficiency, the voltage-frequency ratio of the frequency converter needs to be controlled to maintain a constant ratio—a constant voltage-frequency ratio control—to keep the air gap flux at its rated value. The schematic diagram of the constant voltage-frequency ratio control method using a PWM-controlled AC-DC-AC frequency converter is shown in Figure 1. The speed command serves as both the command value for adjusting the acceleration/deceleration frequency f and, after appropriate voltage division, also as the command value for the stator voltage u1. The ratio of this f command value to the u1 command value determines the U/f ratio. Since the frequency and voltage are controlled by the same command value, the voltage-frequency ratio can be kept constant. To prevent excessive starting current in the motor, a command integrator is added after the command signal to convert the step command signal into a ramp signal ugt that gradually changes according to a set slope, thus allowing the motor voltage and speed to rise and fall smoothly. Furthermore, to enable the motor to rotate in both directions, the command signal can be positive or negative. Since the direction of the motor rotation is determined by the phase sequence of the inverter output voltage, and polarity does not need to be reflected by the frequency and voltage command signals, an absolute value converter is used to change the position of the IGBT and output an absolute signal uabs. After the uabs is processed by the voltage and frequency control circuit, the voltage and frequency command signals are obtained. After the PWM generation circuit, the PWM signal controlling the inverter is formed. Then, the drive circuit controls the on and off of the IGBT in the inverter, so that the inverter outputs the required frequency, phase sequence and magnitude of AC voltage, thereby controlling the speed and direction of the AC motor. [b]3 Practical Application[/b] The fuel used in the heating furnace of the small and medium-sized workshop of Laiwu Steel is a mixture of blast furnace/coke oven gas. The combustion blower mainly provides combustion oxygen for the furnace combustion. The amount of air delivered by the combustion blower is directly related to whether the combustion in the furnace reaches the optimal optimization effect and whether the optimal combustion state is reached. In the original design, after the combustion fan started, it operated at a constant maximum speed to deliver air. The air volume was adjusted by regulating the opening of the baffle at the fan inlet. During normal combustion, inaccurate detection of the baffle opening often resulted in excessive or insufficient air volume, causing over- or under-burning of the steel billets, affecting the normal rolling process in subsequent stages, and even leading to scrap and reduced yield. Furthermore, during roll changing and shutdown periods, the combustion fan continued to operate at a constant maximum speed, resulting in significant energy waste. To improve the reliability of the heating furnace, reduce energy consumption, and optimize gas combustion within the furnace, a differential pressure transmitter was installed inside the furnace to monitor the negative pressure during combustion in real time. This signal was transmitted to a PID controller, and the difference was compared with the set value. The analog value of the difference was then sent to the frequency converter controlling the combustion fan, synchronously adjusting its output frequency. This allowed the combustion fan to automatically adjust the air volume according to combustion requirements, achieving optimal combustion. In this modification, the measurement point of the differential pressure transmitter probe should be selected at the original reserved probe port on the furnace side wall. This is crucial for accurately measuring the negative pressure inside the furnace. Since the measured negative pressure will vary depending on the selected measurement point, careful selection is essential. When the combustion effect inside the furnace is optimal, i.e., when the air volume is optimal, the negative pressure value at that point in the furnace will remain constant, indicating optimal combustion. During on-site commissioning, due to the large rotational inertia of the blower, the blower motor is in a generator state during startup or shutdown, causing the inverter controlling it to trip and shut down. Therefore, the soft-start time (acceleration time) and soft-stop time (deceleration time) of the inverter controlling the motor need to be extended by at least 70 seconds to prevent the inverter from shutting down due to overvoltage protection. [b]4 Application Effect[/b] Since the load torque of fans and pumps is usually proportional to the square of the speed, and the shaft power is proportional to the vertical of the speed, the previous method of adjusting the air and water volume by the fixed speed of the motor and the baffle valve can be changed to adjusting the speed according to the required air and water volume to achieve effective energy saving. Figure 2 shows the relationship between the speed, air volume control and power of the combustion fan. The ideal curve represents the power required for speed control using a speed control device with 100% efficiency. Compared with baffle control, the frequency converter has the highest control efficiency, which is close to the ideal curve. It has significant energy saving effect, high control accuracy and wide speed range, is easy to use and maintain, and is easy to realize automatic control and remote control functions. Therefore, it is widely used. [b]5 Precautions for using frequency converters[/b] (1) Select a cable size in which the voltage drop at the maximum current on the output side is less than 2% of the rated voltage. (2) The wiring of the low-voltage control circuit should be at least 100m away from the power supply line and should never be placed in the same conductor trough; in addition, the control circuit wiring should intersect the main circuit wiring at a right angle. (3) The wiring of the control circuit should use shielded twisted pair cable, and the twist pitch of the twisted pair cable should be less than 15m. (4) In order to prevent mutual interference between multiple signals, the signal lines should be twisted separately. (5) If the operation command comes from a distance and the control circuit wiring needs to be longer, a relay control can be used. (6) In addition to preventing electric shock, the ground wire is also very effective in preventing noise, so it must be grounded. [b]References[/b] [1] Wang Zhaoan, Huang Jun. Power Electronics Technology. Beijing: Machinery Industry Press, 2000, 5 [2] Man Yongkui. General Frequency Converter and Its Application. Beijing: Machinery Industry Press, 1996, 4 About the Author Wang Dahai (1973-) Male engineer, graduated from the Department of Automation of Wuhan University of Science and Technology, engaged in network engineering and software development.