Introduction At present, the application of frequency converters in China is rapidly increasing, but many people often encounter confusion in application and need a detailed guide. This article attempts to systematically explain common technical issues from the application perspective, so as to help those who are not familiar with frequency converter applications. Considering the target audience, there is no advanced mathematics in the article, but the basic principles and rich years of practical experience will surely benefit the readers. Basic Principles of General Frequency Converters The general frequency converters mentioned in this document refer to frequency converters that are suitable for industrial general motors and general variable frequency motors, and are powered by general power grid (single-phase 220V, three-phase 380V 50Hz) for speed control. Due to the widespread use in the industrial field, this type of frequency converter has become the mainstream of frequency converters. The basic principle of speed regulation is based on the following formula: In formula (1): n1—synchronous speed (r/min); f1—stator power supply frequency (Hz); p—number of magnetic pole pairs. Generally, there is a slip relationship between the speed n of an asynchronous motor and the synchronous speed n1. In equation (2): n—asynchronous motor speed (r/min); s—asynchronous motor slip rate. As can be seen from equation (2), the speed regulation method can be achieved by changing any one of f1, p, or s. The best method for asynchronous motors is to change the frequency f1 to achieve speed regulation control. According to motor theory, the effective value of the electromotive force of each phase of a three-phase asynchronous motor is related to the following equation. In equation (3): e1—effective value of the electromotive force of each phase of the stator (v); f1—frequency of the stator power supply (Hz); n1—effective number of turns of the stator winding; фm—stator flux (wb). Equation (3) can be divided into two cases for analysis: (1) when the frequency is lower than the rated power supply frequency, it belongs to constant torque speed regulation. In order to maintain the constant output torque of the motor, the air gap flux фm of each pole must be kept constant when designing the frequency converter. As can be seen from equation (3), that is, e1/f1 should be made constant. If the stator leakage impedance voltage drop is ignored, it can be assumed that the voltage u1 supplied to the motor changes proportionally with the frequency f1, i.e., u1/f1 = constant. However, at lower frequencies, the stator leakage impedance voltage drop can no longer be ignored. Therefore, the stator voltage must be artificially increased to compensate for the leakage impedance voltage drop and maintain e1/f1≈constant. At this time, the output u1/f1 relationship of the frequency converter is curve 2 in Figure 1, instead of curve 1. Figure 1 u/f relationship. Most frequency converters output voltage u1 and frequency f1 similar to curve 2 in Figure 1 when the frequency is lower than the rated frequency of the motor. Moreover, the shape of the compensation curve can be changed with different settings. Users need to adjust according to the actual motor operation. (2) When the frequency is higher than the rated power supply frequency of the stator, it belongs to constant power speed regulation. At this time, the output frequency f1 of the frequency converter increases, but the power supply voltage of the frequency converter is determined by the grid voltage and cannot be increased further. According to formula (3), e1 cannot change, and increasing f1 will inevitably cause фm to decrease. Since фm is proportional to current or torque, it will also cause the torque to decrease. Although the torque decreases, the product of the two remains unchanged because the speed increases. The product of torque and speed represents power. Therefore, the motor is operating in a constant power output state at this time. The characteristics of the constant torque and constant power regions of the asynchronous motor variable frequency speed regulation are shown in Figure 2. Figure 2 Output characteristics of asynchronous motor speed regulation From the above analysis, it can be seen that when the general frequency converter regulates the speed of the asynchronous motor, the output frequency and voltage change according to a certain law. Below the rated frequency, the output voltage of the frequency converter increases with the increase of the output frequency, which is the so-called variable voltage variable frequency speed regulation (vvvf). Above the rated frequency, the voltage does not change, only the frequency changes. In fact, most variable frequency speed regulation applications are used below the rated frequency. The compensation used at low frequencies is to solve the decrease of low frequency torque. There are various methods used, such as vector control technology, direct torque control technology, and pseudo-superconducting technology (a patented technology of Senlan frequency converter), etc. Its function is nothing more than dynamically changing the output voltage, output phase or output frequency of the inverter at low frequencies. That is, by using circuit and computer technology, the shape of curve 1 in Figure 2 is changed in real time rather than fixedly to achieve torque increase at low speeds and stable operation without causing malfunctions due to excessive current. Figure 3 shows the basic circuit of a general-purpose frequency converter. It consists of four main parts: 1—Rectifier section, which converts AC voltage to DC voltage; 2—Filter section, which filters the pulsating AC power into smoother DC power; 3—Inverter section, which converts the DC power back into three-phase AC power. This inverter circuit typically uses power switching elements driven by the control circuit to output a PWM wave or a sinusoidal pulse width modulated SPWM wave. When this waveform voltage is applied to the load, the load inductance makes the current continuous, becoming a current waveform close to a sine wave; 4—Control circuit, which generates the various drive signals required for the output inverter bridge. These signals are determined by external commands and include frequency, frequency rise and fall rates, external on/off control, and comprehensive control of various protection and feedback signals within the frequency converter. It should be noted that the output waveform of the general frequency converter to the load is a bipolar SWPM wave. This waveform can greatly improve the efficiency of the frequency converter, but at the same time, this waveform makes the output of the frequency converter different from the normal sine wave, resulting in many special features of the frequency converter, which users need to pay attention to. The bipolar SWPM wave is shown in Figure 4. Figure 4(a) shows the comparison between the triangular carrier wave and the sinusoidal signal, and Figure 4(b) shows the SWPM waveform obtained after the comparison. Figure 4 Bipolar SWPM modulator 3 Introduction to the basic series of Senlan frequency converters The basic series, power and characteristics of Senlan frequency converters are shown in Table 1. For details, please refer to the "User Manual" of each series. Since each series of Senlan frequency converters has its own characteristics, it is necessary to select the appropriate one according to the purpose before use. (1) The BT40 series has three-phase 380V and single-phase 220V power supply, which is suitable for general industrial control speed regulation. The control mode of v/f=constant has a torque boosting function, which can be adjusted by the user according to the needs. It is relatively convenient to use and operate. (2) The BT12 series is a frequency converter specifically designed for fan and pump loads. Using this series is beneficial for the design and simplification of fan and pump speed control systems. The product has functions such as PID, multi-pump switching, pump replacement, sleep wake-up, fire control, water level control, and timed start-up and shutdown. (3) The SB60 series is a fully functional series called "Senlan All-Round King". It can adapt to high-requirement occasions. The product not only has V/F open-loop and closed-loop modes, but also sensorless vector control mode and PG speed sensor vector control mode. It can also communicate with the host computer using the RS-485 interface. The shell is made of plastic, which is beautiful and elegant. The power is 11-15kw. The SB60 series has good safety. The protection level is one level higher than BT40 and BT12, namely IP20, and has obtained the European Community CE certification. (4) The SB61 series has the same functions as the SB60 series, but with larger power, ranging from 15 to 315kw. It has a metal powder-coated shell and IP20 protection level. (5) The SB20 series is a low-power economic series. Its functions are simplified compared to the SB60. It is suitable for general low-power motor speed regulation. It is compact, economical and practical. (6) The SB40 series is an improved version of the BT40. Its appearance, performance characteristics, protection level and reliability have been improved. It uses surface-mount components to improve electrical performance and reduce interference. It uses temperature-controlled fans to extend the fan life. There are also rotary frequency adjustment models available. (7) The SB12 series is an improved version of the BT12. It has a similar range of improvements as the SB40. (8) The SB80 series frequency converter adopts the latest 32-bit embedded high-speed motor control dedicated digital processor. It uses the model reference adaptive method to solve the problem of online identification of motor resistance parameters and realizes a high-performance sensorless true vector control algorithm based on instantaneous rotor magnetic field orientation, which is difficult for ordinary frequency converters to achieve. The SB80 series utilizes the latest model adaptive technology and excitation current sinusoidal injection detection technology, enabling simultaneous identification of three parameters: speed, stator resistance, and rotor resistance. This allows for accurate identification of parameter changes during motor operation. Combined with our patented technology, "a current sampling resistor" (patent number: 01206891.8), the detection and observation accuracy of stator and rotor resistance are significantly improved. This not only eliminates the influence of initial parameter errors but also automatically adapts to parameter changes caused by motor temperature variations, ensuring accurate flux observation and speed identification. The use of these advanced technologies marks the first time true dynamic current vector control based on instantaneous rotor magnetic field orientation and precise flux observation has been achieved. The stability of its parameter identification and the speed of its current control provide excellent solutions for power outage restarts, rotary starts, rapid acceleration and deceleration, and trip-free control under sudden load changes. 4. Selection Principles of Frequency Converters 4.1 The output power and current of the frequency converter must be equal to or greater than the power and current of the driven asynchronous motor. Since the overload capacity of the frequency converter is not as strong as that of the motor, if the motor is overloaded, the frequency converter will be damaged first (if the frequency converter's protection function is inadequate). Also, if the selected motor power is greater than the actual mechanical load power, but the user may adjust the mechanical power to reach the motor's output power, the frequency converter must be able to handle this. In other words, the power of the frequency converter must be equal to or greater than the motor power. Some motors have special rated current values, not close to the commonly used standard specifications, and some motors have low rated voltage and high rated current. In these cases, the rated current of the frequency converter must be equal to or greater than the rated current of the motor. 4.2 It is essential to recognize the fundamental difference between frequency converter speed regulation and mechanical speed regulation. Never blindly convert a motor using mechanical speed regulation to a frequency converter with the same power. Because power is the product of torque and speed: In mechanical speed changes (such as gearboxes and belt drives), if the gear ratio is k, and the motor power remains constant, ignoring the transmission efficiency, a k-fold decrease in speed will result in a k-fold increase in torque. This is a constant power load, as shown by curve 1 in Figure 5. Figure 5 shows the mechanical characteristics of different loads. The torque-speed curve of the frequency converter is shown by curve 3 in Figure 2. Below the rated frequency, it operates with constant torque, and the motor cannot increase its output torque. Above the rated frequency, the speed increases while the torque decreases. Figure 5 illustrates the mechanical characteristics of common loads. In Figure 5, 3 represents a square-law load (such as a fan or water pump), and 2 represents a constant torque load (such as a conveyor belt). For these two types of loads, the load torque does not increase when the motor operates below the rated frequency. Therefore, when the frequency is below the rated frequency, the frequency converter power can be configured according to the motor power. In Figure 5, 1 is a constant power load (such as a cutting machine tool). The torque increases at low speeds. However, when the frequency converter and motor are below the rated frequency, the current is limited and the torque cannot increase. Therefore, if the frequency converter reduces the motor speed, the motor may not be able to drive the load. When selecting, the motor and frequency converter with a larger power than the original motor should be selected according to the proportion of torque increase caused by deceleration. For example, the original 1.5kw motor has a load torque of 1kgm and a speed of 1460r/min. After mechanical speed change, the speed drops to 720r/min and the torque can reach 2kgm. However, the original motor and frequency converter cannot output a torque of 2kgm. Therefore, if the motor and frequency converter are both 1.5×2=3kw, a standard power of 3.7 or 4kw motor and frequency converter should be selected. 4.3 The selection model of the frequency converter should be carefully considered according to the usage requirements. (1) The basic considerations are the usage environment conditions, grid voltage, load size and nature. (2) When the ambient temperature is high for a long time and the frequency converter is installed in a poorly ventilated and cooled cabinet, the life of the frequency converter will be shortened. Electronic components, especially electrolytic capacitors, will have their lifespan halved for every 10°C increase above their rated temperature. Therefore, the ambient temperature should be kept low. In addition to setting up a complete ventilation and cooling system to ensure the normal operation of the inverter, it is absolutely necessary to increase the capacity level to reduce the temperature rise during rated operation. (3) Abnormal grid voltage will be harmful to the inverter. If the voltage is too high, such as if the line voltage of 380V rises to 450V, it will cause damage. Therefore, if the grid voltage exceeds the range specified in the user manual, a transformer should be used to adjust it to ensure the safety of the inverter. (4) In high-altitude areas, the air density is reduced, and the heat sink cannot achieve the rated heat sink effect. Generally, above 1000m, the capacity decreases by 10% for every 100m increase. If necessary, the capacity level can be increased to prevent the inverter from overheating. (5) When using it for different purposes, the selection of the inverter series model should be analyzed. For general purpose inverters, the v/f=constant control method is sufficient. For applications with a large load variation range and high operating accuracy requirements, especially when stable speed and load capacity are required at low speeds, inverters with vector control or other methods should be selected. For precision transmissions such as CNC machine tools, closed-loop control and speed sensor methods should be used. Corresponding inverters should also have these compatible interfaces. Comprehensive consideration is required when selecting. (6) The protection level of the inverter should be selected for different places of use. To prevent rodents, foreign objects, etc. from entering, protection should be selected. Common IP10, IP20, IP30, and IP40 levels can prevent the entry of solid objects of φ50, φ12, φ2.5, and φ1, respectively. (7) When the inverter modulates the frequency higher than the factory setting frequency to reduce motor noise, it will cause the inverter loss to increase. The higher the frequency, the greater the loss, so the load should be reduced appropriately. The corresponding load reduction curves for different modulation frequencies and load rates are shown. Different companies and different series will have different curves, but the trend is similar. Many users do not understand this point and increase the modulation frequency indiscriminately, causing the inverter to overheat and be damaged, or the inverter to fail to output the rated power. (8) Vector control mode can only correspond to one inverter driving one motor, and the rated current of the inverter should be equal to or greater than the rated current of the motor. The actual operating current of the motor should not be too small than the rated current (not less than 1/8 of the rated current of the inverter). In order to use vector control correctly, the inverter needs to input or automatically identify the cold state parameters of the motor before driving. (9) When one inverter drives multiple motors, the inverter capacity should be larger than the sum of the capacities of multiple motors, and only the v/f control mode can be selected, not the vector control mode. (10) When multiple inverters share a single rectifier/feedback unit, i.e., using a common DC bus, it is beneficial for the storage and utilization of braking energy of multiple inverters. At this time, the capacity of the rectifier/feedback unit must be large enough, and there must be measures to prevent overload damage to the rectifier bridge of the low-power inverter. Multiple motors cannot be braked simultaneously during use. (11) For fan and pump loads (i.e., square-law loads), if the flow rate was originally regulated by valves or dampers, switching to inverter speed control will result in significant energy savings. However, for friction loads (constant torque loads), the energy-saving effect of using inverter speed control is basically not reflected. In cases where mechanical speed change is used to increase torque, the inverter may not be able to drive the load. This must be carefully considered when selecting an inverter! In these cases, the purpose of using an inverter is to adjust the speed of the working machinery.