Abstract This paper analyzes the harmonic problems, load matching problems and heat generation problems existing in the operation of frequency converters and proposes solutions. Keywords Frequency converter, harmonics, load matching 1. Introduction At present, general-purpose frequency converters are increasingly favored by people due to their advantages such as intelligence, digitalization and networking. With the expansion of the application scope of general-purpose frequency converters, more and more problems have been exposed, mainly in the following aspects: (1) Harmonic problems (2) Frequency converter load matching problems (3) Heat generation problems These problems have attracted the attention of relevant management departments and factories and mines, and relevant technical standards have been formulated. 2. Harmonic problems and countermeasures The main circuit of a general-purpose frequency converter generally consists of three parts: rectification section, inverter section and filtering section. The rectification section is a three-phase bridge uncontrolled rectifier, and the inverter section is an IGBT three-phase bridge inverter, and the output is a PWM waveform. For bipolar modulation inverters, the output voltage waveform expansion is: n - harmonic order n=1,3,5......; al - switching angle, i=1,2,3......N/2; Ed - inverter DC side voltage; N - carrier ratio. It can be seen that the output voltage of general inverters does contain harmonics other than the fundamental wave. Lower harmonics usually have a greater impact on motor load, causing torque pulsation, while higher harmonics increase the leakage current of the inverter output cable, resulting in insufficient motor output. Therefore, both high and low order harmonics of the inverter output must be suppressed. The following countermeasures can be adopted: (1) Increase the internal impedance of the inverter power supply. Under normal circumstances, the internal impedance of the power supply equipment can buffer the reactive power of the inverter DC filter capacitor. This internal impedance is the short-circuit impedance of the transformer. When the power supply capacity is smaller than the inverter capacity, the internal impedance value is relatively larger and the harmonic content is smaller; when the power supply capacity is larger than the inverter capacity, the internal impedance value is relatively larger and the harmonic content is larger. For Mitsubishi FR F540 series inverters, when the internal resistance of the power supply is 4%, it can play a good role in harmonic suppression. Therefore, when selecting the power supply transformer for the inverter, it is best to choose a transformer with a large short-circuit impedance. (2) Install reactors. Installing reactors actually increases the internal impedance of the power supply of the inverter from the outside. Install AC reactors on the AC side of the inverter or DC reactors on the DC side of the inverter, or both, to suppress harmonic currents. (3) Multi-phase operation of transformers. The rectifier section of general inverters is a six-pulse rectifier, so the generated harmonics are relatively large. If multi-phase operation of transformers is applied, so that the two transformers with a phase angle difference of 300° plus the Y, L, Δ-Δ combination form an effect equivalent to 12 pulses, the low-order harmonic current can be reduced by 28%, which plays a good role in harmonic suppression. (4) Adjust the carrier ratio of the inverter. From equations (1), (2), and (3), it can be seen that as long as the carrier ratio is large enough, lower harmonics can be effectively suppressed. In particular, when the reference amplitude and carrier amplitude are less than 1, odd harmonics below the 13th order no longer appear. (5) Dedicated filter. This dedicated filter is used to detect the amplitude and phase of the harmonic current of the inverter and generate a current with the same amplitude and opposite phase as the harmonic current. When passed into the inverter, it can absorb the harmonic current very effectively. 3. Load matching problem and its countermeasures There are many types of production machinery with different performance and process requirements. Their torque characteristics are complex and can be roughly divided into three types: constant torque load, fan and pump load and constant power load. Different types of inverters should be selected for different load types. 3.1 Constant torque load A constant torque load is one in which the load torque is independent of the speed. At any speed, the torque remains constant. Constant torque loads are further divided into friction loads and potential energy loads. Friction-type loads generally require a starting torque of approximately 150% of the rated torque and a braking torque of approximately 100% of the rated torque. Therefore, frequency converters should be selected that have constant torque characteristics, large starting and braking torques, long overload time, and large overload capacity. Potential energy loads generally require large starting torque and energy feedback functions, and can quickly achieve forward and reverse rotation. Frequency converters with four-phase limit operation capabilities should be selected. 3.2 Fan and Pump Loads Fan and pump loads are currently the most widely used equipment in industrial sites. Although pumps and fans have various characteristics, centrifugal pumps and centrifugal fans are the main applications, and general-purpose frequency converters are most commonly used for these types of loads. When designing a frequency converter, the calculation results above should be appropriately modified to select the shortest time without overcurrent tripping during frequency converter startup or overvoltage tripping during frequency converter deceleration. 3.3 Constant Power Loads Constant power loads refer to loads whose torque is roughly inversely proportional to their speed, such as winding machines and uncoilers. When using a frequency converter to drive a constant power load, it should be within a certain speed range. Usually, constant torque speed regulation is used below a certain speed point, while constant power speed regulation is used above that speed point. We usually call this speed point the base frequency, and the voltage corresponding to this point is the rated output voltage of the frequency converter. 4. Heat generation problem and countermeasures The heat generation of the frequency converter is caused by internal losses. Among the various losses in the frequency converter, the main circuit accounts for about 98%, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, it is necessary to heat the frequency converter. The following methods are usually used: (1) Use fan cooling: The internal fan of the frequency converter can remove the heat from the inside of the frequency converter. If the fan does not work properly, the frequency converter should be stopped immediately. (2) Reduce the installation environment temperature: The frequency converter contains electronic components, electrolytic capacitors, etc., and the temperature has a significant impact on its life. The ambient operating temperature of general-purpose frequency converters is generally required to be between 10°C and 50°C. By taking measures to reduce the operating temperature of the frequency converter as much as possible, the service life of the frequency converter will be extended and the performance will be more stable.