Abstract: This paper comprehensively analyzes the overvoltage and overcurrent situations of frequency converters and proposes targeted measures to avoid overvoltage and overcurrent. These measures have achieved good results in practical applications. Keywords : Frequency converter; overvoltage; overcurrent; causes; measures 0 Introduction Overvoltage or overcurrent situations are common in the commissioning and use of frequency converters. Once this happens, at best, the protection will activate, causing the frequency converter and the entire speed control system to stop operating, affecting production; at worst, it will damage the frequency converter and system equipment. In long-term practical work, we have summarized various situations of overvoltage and overcurrent in frequency converters and proposed corresponding measures through analysis. These measures have achieved good results in practical applications and are introduced below for your reference. 1 Overvoltage of Frequency Converters 1.1 Analysis of Overvoltage Situations Overvoltage of frequency converters refers to the voltage of the frequency converter exceeding the rated value due to certain reasons. There are two aspects to the cause of overvoltage: 1.1.1 Problems with the frequency converter itself. Poor operating environment, circuit board corrosion, damage to voltage control channels; loose or unreliable connectors causing poor contact in the voltage feedback line; poor inverter grounding, etc., can all lead to inverter overvoltage. 1.1.2 Load causes regenerative braking of the motor. In actual operation, this type of overvoltage is mainly reflected on the DC bus of the inverter's rectified output, manifesting as an increase in DC voltage. This is the most common type of overvoltage. Once this type of overvoltage occurs in the inverter, its protection function will activate to prevent damage to the internal circuitry. Although damage to the inverter itself is relatively rare, it causes the inverter and the entire speed control system to stop operating, affecting production. The reason for the increase in DC bus voltage is regenerative voltage. When a large flywheel torque (GD) load decelerates or stops, the inverter's set time is too short; or the potential energy load of a crane is lowered; or the fan or stretcher is affected by external forces, etc., causing the actual speed of the motor to be higher than the inverter's command speed. The motor is in a regenerative braking state, and the kinetic energy of the load is converted into electrical energy, which charges the energy storage capacitor on the inverter's DC bus through the freewheeling diode of the inverter circuit, causing the DC bus voltage to rise and resulting in regenerative overvoltage. Taking a commonly used low-voltage inverter as an example, under normal operation, the DC voltage after rectification by the inverter's rectifier circuit is the average value of the three-phase full-wave rectification, i.e., U[sub]d[/sub]=1.35U[sub]L[/sub]=1.35×380=513V. When regenerative overvoltage occurs, the energy storage capacitor is charged. When the voltage rises to about 760V, the inverter's overvoltage protection will activate. 1.2 Measures Taken 1.2.1 Overvoltage caused by problems with the inverter itself. This mainly occurs in situations where frequency converters have been used for a long time and in harsh environments. Two measures are taken: First, ensure proper dust, drip, and moisture prevention. For example, install the frequency converter internally to prevent dust and drips, and keep the upper and lower ventilation holes in the electrical cabinet unobstructed (if the ventilation holes are fitted with metal mesh, install an exhaust fan at the upper exhaust hole); to prevent moisture, install a dehumidifier or air conditioner inside the cabinet; for harsh working environments, the frequency converter can be installed in a closed system, using an external air conditioner for ventilation, etc., to protect the frequency converter circuitry from environmental corrosion. Second, strengthen routine inspection and maintenance, such as whether the temperature, humidity, vibration, and other environmental conditions during frequency converter operation meet requirements; whether the frequency converter's operating parameters are within the specified range; whether the frequency converter, motor, transformer, and reactor are overheating, discolored, or have an unusual odor, etc. Additionally, ensure the frequency converter is properly grounded and that the connections are securely fastened. 1.2.2 Overvoltage caused by the load during regenerative braking of the motor. The measures taken are to consume or absorb the regenerated electrical energy (or to avoid this braking altogether), and the specific methods depend on the specific circumstances. For overvoltage generated during stopping, if there are no special requirements for stopping time or position, extending the stopping time set by the inverter can solve the problem; if there are specific requirements for stopping time or position, regenerative braking can be used. In this case, DC current is applied to the motor stator windings to generate a fixed magnetic field, and the rotor windings cut the magnetic field to generate electricity, forming a braking torque to stop the motor. Adjusting the DC current can change the magnitude of the braking torque, achieving accurate stopping. During braking, the kinetic energy of the load is converted into electrical energy and consumed in the rotor circuit, avoiding overvoltage. However, since the electrical energy consumed in the rotor during braking can cause the motor to overheat, the braking time cannot be too long. For overvoltage caused by excessive load GD during deceleration, the deceleration time set by the inverter can be appropriately extended to solve the problem; for external forces causing the motor to be in a regenerative braking state, this braking is necessary, and the resulting overvoltage is within the normal operating condition. To avoid excessive DC bus voltage, the following methods can be used: a. A. A braking resistor is connected in parallel in the DC circuit of the frequency converter. When the DC bus voltage rises to the set value (around 700 V for commonly used low-voltage frequency converters), the control power transistor turns on, and the regenerative energy is consumed through the braking resistor, thus preventing the DC voltage from rising. Since the energy is consumed in the braking resistor outside the motor, the motor will not overheat, and therefore can operate for a longer period of time or more frequently; b. For multi-motor drive systems, multiple frequency converters share an internal rectifier, and all inverters are connected in parallel to the common bus of the rectifier output. When a motor is operating in regenerative braking mode, the generated regenerative energy is absorbed by other motors operating in motor mode through the parallel DC bus. If it cannot be completely absorbed, it is consumed through the shared braking resistor. In this case, the regenerative energy is partially or completely absorbed, improving the efficiency of the entire drive system; c. For applications with frequent regenerative braking, energy feedback type frequency converters can be used. The inverter on the grid side of this type of frequency converter is reversible. When regenerative braking occurs, the regenerated electrical energy is fed back to the grid through the reversible inverter, allowing the energy to be fully utilized. This method makes the speed control system more efficient, but it has high requirements for power supply stability. If there is a sudden power outage during braking, the inverter will be overturned. 2 Overcurrent of Frequency Converter 2.1 Analysis of Overcurrent Situations Overcurrent of frequency converter refers to the current of the frequency converter exceeding the rated value due to certain reasons. There are two aspects to the occurrence of overcurrent: 2.1.1 External reasons of frequency converter. Specifically, there are the following situations: a. The load driven by the motor changes suddenly, resulting in an excessive inrush current; b. The insulation between the motor and the power cable is damaged, resulting in a short circuit between turns or between phases to ground, forming a short circuit current; c. The frequency converter output side is equipped with power capacitors or surge absorption devices, which can easily cause current surges at the output end; d. The speed feedback signal is lost or abnormal (especially when a speed encoder is installed), which also causes overcurrent. 2.1.2 Overcurrent caused by problems with the frequency converter itself. The following are some common issues: a. The motor acceleration time is set too short, or the proportional (P) and integral (I) time parameters in the PID controller are unreasonable, resulting in excessive overshoot and oscillation of the inverter output current; b. The current transformer is damaged, in which case current is displayed even when the main circuit is powered on but the inverter has not started; c. The current detection channel on the main circuit interface board is damaged, resulting in overcurrent; d. Poor contact in the current feedback signal line will cause intermittent overcurrent. 2.2 Measures Taken 2.2.1 For overcurrent caused by external factors of the inverter, the following measures are taken: a. When the load is unstable, DTC mode can be used, which has a fast control speed (generating a set of accurate actual values ​​of torque and flux every 25 m, which are then optimized by the torque comparator and flux comparator to determine the optimal switching position of the inverter), effectively suppressing overcurrent; b. Remove the power capacitor or surge absorption device on the output side of the inverter. To improve the power factor, the user can connect an appropriate capacitor in parallel at the input; c. Strengthen inspection and maintenance work to ensure the insulation of the motor and power cable is intact; ensure the integrity of the speed feedback channel and feedback signal. 2.2.2 For overcurrent caused by the problem of the frequency converter itself, the following measures are taken: a. Appropriately extend the set acceleration time of the frequency converter and adjust the time parameters of the PID regulator; b. Strengthen inspection and maintenance work to ensure the normal operation of the current transformer; pay attention to dust prevention and moisture prevention, maintain the working environment, and prevent the circuit interface board from being corroded and damaged; tighten the connecting plugs to ensure good contact of the current feedback signal line, etc. References : [1] Zhang Yanbin, et al. Practical Variable Frequency Speed ​​Regulation Technology Training Tutorial [M]. Beijing: Machinery Industry Press, 2003. [2] Hu Chongyue. Modern AC Speed ​​Regulation Technology [M]. Beijing: Machinery Industry Press, 2003.