Abstract : This paper analyzes the causes of problems such as overvoltage, overcurrent, high harmonics, vibration and noise, and heat generation that occur in frequency converters during operation, and proposes corresponding countermeasures or treatment methods. Keywords : frequency converter; common problems; cause analysis; treatment methods With the improvement of frequency conversion technology, the application range of frequency converters is becoming wider and wider. The problems that occur during operation are also increasing. The main problems are: overvoltage, overcurrent, high harmonics, vibration and noise, heat generation, etc. This paper analyzes the causes of the above common problems and proposes corresponding treatment methods. [b]1 Causes and treatment methods of overvoltage 1.1 Causes of overvoltage[/b] (1) Overvoltage caused by transformer disconnection According to the theory of overvoltage formation, when the transformer is disconnected, the current in the transformer inductance cannot change abruptly. The magnetic field energy stored therein forms an oscillation between the transformer excitation inductance and the capacitance to ground, thus causing overvoltage. (2) Overvoltage generated by transformer closing under load: In actual tests, overvoltages several times higher than the power supply voltage were detected when closing an unloaded transformer. The physical principle is as follows: An unloaded transformer can still be equivalent to a parallel connection of an excitation inductance and the transformer's own equivalent capacitance. If the transformer's neutral point is not grounded and the switch is closed aperiodically (one or two phases close first), the feeder capacitance, transformer-to-ground capacitance, longitudinal capacitance, and transformer inductance will oscillate, resulting in a high overvoltage, especially a high overvoltage at the transformer's neutral point. Although transformers are basically closed under load, overvoltage will still be generated when closing a transformer with a load, although it will be smaller than when unloaded. In a real load, there is a relatively large capacitance. Since the energy stored in the capacitor does not increase suddenly, and the transmission cable has distributed capacitance to ground when transmitting high-frequency oscillating voltage, these capacitances have an absorption effect on overvoltage. The combined effect of these two factors suppresses the overvoltage during the transformer closing process, but sometimes the value is still very high, and may even exceed the withstand voltage of the components, which is very dangerous. (3) Commutation overvoltage of rectifier components When rectifier components are commutated, the commutation overvoltage is also very high. This will not only damage the components, but also generate electromagnetic interference. 1.2 Overvoltage handling methods (1) For the breaking overvoltage of the phase-shifting transformer of the frequency converter, the overvoltage absorption circuit is composed of RC absorption network and zinc oxide surge arrester, which achieves good results. (2) For the overvoltage generated by the transformer closing under load, a switch with good periodic performance can be selected (the switch will be out of period after long-term operation); a good RC absorption circuit or active suppressor technology can be used; a transformer with electrostatic shielding measures can also effectively suppress the closing overvoltage. However, the difficulty of making electrostatic shielding layer for high-power transformers will be quite large. (3) For the overvoltage generated by the commutation of rectifier components, the points to note are: the reverse withstand voltage of the rectifier components must be sufficient, and the absorption circuit and freewheeling circuit must be properly implemented. Otherwise, the rectifier device may be broken down by the overvoltage. (4) Since the overvoltage of the frequency converter is basically generated when the transformer is switched on and off, we should start from the transformer to find a way to suppress the overvoltage of the frequency converter. We can use: ① Increase the excitation inductance and ground capacitance of the transformer. Increasing the excitation inductance reduces the no-load current, which will increase the cost of the transformer. ② Increase the ground capacitance of the transformer: It is easy to analyze in principle, but in reality, due to the limitations of the transformer's structure and materials, it is unlikely to make a transformer with any insulation method or high insulation level. Therefore, it is also quite difficult to increase the ground capacitance C of the transformer. [b]2 Causes and treatment methods of overcurrent 2.1 Causes of overcurrent[/b] (1) Overcurrent during operation means that the drive system has overcurrent during operation. The causes are roughly as follows: ① The motor encounters an impact load, or the transmission mechanism is "stuck", causing a sudden increase in the motor current. ② Short circuit on the output side of the frequency converter, such as mutual short circuit between the connection line between the output end and the motor, or short circuit inside the motor. ③ Abnormal operation of the inverter itself, such as abnormal operation of two inverter devices in the same bridge arm of the inverter bridge during continuous alternating operation. For example, due to excessively high ambient temperature or aging of the inverter devices themselves, the parameters of the inverter devices change, resulting in one device being turned on while the other device has not yet been turned off during the alternation process, causing "straight-through" between the upper and lower devices in the same bridge arm, resulting in a short circuit between the positive and negative poles of the DC voltage. (2) Overcurrent during speed increase When the load inertia is large and the speed increase time is set too short, it means that during the speed increase process, the inverter's working efficiency increases too quickly, the synchronous speed of the motor increases rapidly, and the speed of the motor rotor cannot keep up due to the large load inertia, resulting in excessive speed increase current. (3) Overcurrent during speed reduction When the load inertia is large and the speed reduction time is set too short, it will also cause overcurrent. Because the deceleration time is too short, the synchronous speed drops rapidly, while the motor rotor, due to the large inertia of the load, still maintains a high speed. At this time, the speed at which the rotor winding cuts the magnetic lines of force is too high, resulting in overcurrent. 2.2 Overcurrent handling methods (1) Tripping immediately upon acceleration during startup is a very serious overcurrent phenomenon. The main checks are: whether the working machinery is stuck; whether there is a short circuit on the load side, and whether there is a short circuit to ground using a megohmmeter; whether the inverter power module is damaged; and whether the starting torque of the motor is too small, causing the drive system to not turn. (2) Tripping not immediately upon startup, but during operation, the main checks are: whether the acceleration time is set too short, and the acceleration time is increased; whether the deceleration time is set too short, and the deceleration time is increased; whether the torque compensation (U/f ratio) is set too high, causing excessive no-load current at low frequencies; and whether the electronic thermal relay is improperly set, with the operating current set too small, causing the inverter to malfunction. [b]3 Causes and Treatment of Harmonic Generation 3.1 Causes of Harmonic Generation[/b] Structurally, frequency converters can be divided into two main categories: direct frequency converters and indirect frequency converters. Indirect frequency converters convert the power frequency current into DC through a rectifier, and then convert the DC into AC with a controllable frequency through an inverter. Direct frequency converters convert the power frequency AC into AC with a controllable frequency without intermediate links. Each phase of it is a reversible circuit with two-phase thyristor rectifier devices connected in antiparallel. The positive and negative groups switch with each other at a certain period, and an alternating output voltage is obtained on the load. Its amplitude is determined by the control angle of each rectifier device, and the frequency is determined by the switching frequency of the two-phase rectifier devices. Currently, indirect frequency converters are more commonly used. Indirect frequency converters have three different structural forms: (1) Using a controllable rectifier to transform the voltage and using an inverter to convert the frequency. Voltage and frequency regulation are carried out in two separate links, and the two need to be coordinated in the control circuit. (2) Using an uncontrolled rectifier to rectify and a chopper to transform voltage, and an inverter to convert frequency, this type of inverter uses a chopper in the rectification stage and pulse width modulation (PWM) for voltage regulation. (3) Using a down-controlled rectifier for rectification, and a PWM inverter to convert frequency simultaneously, this type of inverter can only output a very realistic sine wave if it uses fully controlled components with controllable shutdown (such as insulated gate bipolar transistors, IGBTs, etc.). Regardless of the type of inverter, a large number of nonlinear power electronic components such as thyristors are used. No matter which rectification method is used, the way the inverter draws energy from the grid is not a continuous sine wave, but rather a pulsating discontinuous current from the grid. This pulsating current and the impedance of the grid together form a pulsating voltage drop superimposed on the grid voltage, causing voltage distortion. According to Fourier series analysis, this non-periodic sine wave current is composed of a fundamental wave with the same frequency and harmonics with a frequency greater than the fundamental wave frequency. 3.2 Harmonic Reduction Methods To eliminate harmonics, the following countermeasures are mainly adopted: (1) Under normal circumstances, the internal impedance of the power supply equipment can buffer the reactive power of the DC filter capacitor of the frequency converter. This internal impedance is the short-circuit impedance of the transformer. When the power supply capacity is smaller than the frequency converter capacity, the internal impedance value is relatively larger and the harmonic content is smaller; when the power supply capacity is larger than the frequency converter capacity, the internal impedance value is relatively smaller and the harmonic content is larger. Therefore, when selecting the power supply transformer for the frequency converter, it is best to choose a transformer with a large short-circuit impedance. (2) Installing reactors actually increases the internal impedance of the frequency converter power supply from the outside. Installing reactors on the AC side or DC side of the frequency converter, or installing them simultaneously, can suppress harmonic currents. (3) Multiphase operation of transformers. Usually, the rectifier section of the frequency converter is a 6-pulse rectifier, so the generated harmonics are relatively large. Multiphase operation of the transformer can be applied, such as making the phase angles differ by 30°. Two transformers in the Y-Δ and Δ-Δ combination form a 12-pulse rectifier, which can reduce harmonic current and play a role in harmonic suppression. (4) Increasing the carrier ratio of the frequency converter can effectively suppress low-order harmonics. (5) The filter can detect the amplitude and phase of the harmonic current of the frequency converter and generate a current with the same amplitude and opposite phase as the harmonic current, thereby effectively absorbing and eliminating the harmonic current. [b]4 Causes and Treatment of Vibration and Noise 4.1 Causes of Vibration and Noise[/b] When the frequency converter is working, the magnetic field caused by the high-order harmonics in the output waveform generates electromagnetic driving force on many mechanical parts. The frequency of the driving force can always be close to or coincide with the natural frequency of some mechanical parts, resulting in resonance. The high-order harmonics that have a great impact on vibration are mainly the lower-order harmonic components, which have a greater impact in PAM (Pulse Amplitude Modulation) mode and square wave PWM mode. However, when using sinusoidal wave PWM mode, the low-order harmonic components are small and the impact is smaller. When a frequency converter drives a motor, the output voltage and current contain high-order harmonic components, increasing the high-order harmonic flux in the air gap and thus increasing noise. Electromagnetic noise has the following characteristics: Low-order harmonic components in the frequency converter output resonate with the rotor's natural mechanical frequency, increasing noise near the rotor's natural frequency. High-order harmonic components in the frequency converter output resonate with the core, housing, bearings, etc., increasing noise near the natural frequencies of these components. The noise generated by the frequency converter driving the motor, especially the harsh noise, is related to the switching frequency of the PWM control, and is particularly significant in the low-frequency region. Using a frequency converter for speed regulation will generate noise and vibration; this is due to the high-order harmonic components in the frequency converter's output waveform. With changes in operating frequency, the fundamental and high-order harmonic components vary over a wide range, potentially causing resonance with various parts of the motor. 4.2 Vibration and Noise Treatment Methods To reduce or eliminate vibration, an AC reactor can be connected to the frequency converter output side to absorb the high-order harmonic current components in the frequency converter's output current. When using PAM or square wave PWM inverters, sinusoidal wave PWM inverters can be used instead to reduce pulsating torque. To prevent vibration of the mechanical system formed by the motor and load, the entire system must not resonate with the electromagnetic force generated by the motor. The following measures are generally used to suppress and reduce noise: connect an AC reactor to the output side of the inverter. If there is a margin in the electromagnetic torque, U/f can be set smaller. If a special motor is used and the noise volume is serious at lower frequencies, the resonance with the natural frequency of the shaft system (including the load) should be checked. [b]5 Causes and solutions for inverter overheating 5.1 Causes of inverter overheating[/b] Inverter overheating is caused by internal losses, mainly in the main circuit, accounting for about 98%, and the control circuit accounts for 2%. 5.2 Solutions for inverter overheating (1) Use a fan for cooling The fan inside the inverter can remove the heat inside the inverter housing. (2) Reduce the ambient temperature The inverter is an electronic device containing electronic components, electrolytic capacitors, etc., so temperature has a significant impact on its lifespan. The ambient operating temperature requirement for general-purpose frequency converters is generally -10℃ to +50℃. If the operating temperature of the frequency converter can be reduced, the service life of the frequency converter will be extended and the performance will be more stable.