Mastering Low – voltage Issues in Inverters: Solutions & Troubleshooting

Undervoltage problems of frequency converters

 一、Preface

        In today’s highly automated industrial production field, frequency converters, as crucial power control equipment, are widely used in various scenarios such as motor speed regulation, exerting a significant impact on production efficiency and product quality. However, during the actual operation process, frequency converters may encounter various issues, among which the undervoltage problem is a rather common and troublesome one. It can lead to unstable equipment operation, production interruptions, and even damage to the equipment. Therefore, a thorough understanding of the causes of the undervoltage problem in frequency converters and the mastery of effective solutions are of profound significance for ensuring the continuity and stability of industrial production.

二、Causes of the Undervoltage Problem in Frequency Converters

 (A) Problems on the Power Supply Side

1. Grid Voltage Fluctuations

         The industrial power grid is a complex system, and its voltage is not always stable. During peak electricity consumption periods, a large number of equipment operate simultaneously, causing a sharp increase in the grid load. For example, in large manufacturing factories, when multiple production lines are started simultaneously, and numerous electrical equipment such as motors and heating devices work at the same time, the grid voltage will decline due to the heavy load. This decline may cause the input voltage of the frequency converter to fall below its normal working range, thus triggering the undervoltage problem. Moreover, the startup or shutdown of large equipment in the grid will also produce voltage sags or fluctuations. For instance, the instant startup of a large compressor in a factory will cause obvious voltage fluctuations in the grid, affecting the normal operation of nearby frequency converters.

         In addition, faults or maintenance work on the external power grid can also cause voltage problems. For example, when a substation conducts line inspections, equipment upgrades, or fault repairs, the supply voltage may change abnormally. Such changes may not be notified to users in advance, leaving the frequency converter vulnerable to undervoltage situations without any preparation.

2. Power Line Problems

         If there are faults in the power supply lines from the grid to the frequency converter, it will directly affect the input voltage of the frequency converter. Long-term used cables may experience a decrease in insulation performance and an increase in resistance due to aging, moisture, or mechanical damage. When current passes through these damaged cables, according to Ohm’s law (U = IR), a significant voltage drop will occur on the lines, causing the actual received voltage of the frequency converter to decrease. This voltage drop problem will be more pronounced especially in the case of long-distance power supply or poor cable quality.

         If the connection components such as power switches and fuses have poor contact or malfunction, it will also lead to voltage losses. For example, the contacts of a switch may oxidize and wear out after long-term use, resulting in an increase in contact resistance. When current passes through, an additional voltage drop will be generated at these parts, causing the input voltage of the frequency converter to be insufficient.

 (B) Reasons Related to the Load

1. Motor Startup Impact

         When a motor starts, especially for high-power motors or those with heavy-load startup, the startup current will be extremely large. This is because at the instant of motor startup, the rotor has not yet started to rotate, which is equivalent to a short-circuit state. According to the electromagnetic principle of the motor, the startup current can be several times the rated current. For example, when a large industrial motor starts, the startup current may reach 5 7 times the rated current. When this large current flows through the power supply lines, a significant voltage drop will occur on the internal resistance of the power supply and the line resistance, causing the input voltage of the frequency converter to drop instantaneously. If the voltage adjustment ability of the frequency converter is insufficient or the voltage protection threshold is low, it is easy to trigger the undervoltage alarm.

2. Energy Feedback Caused by Sudden Load Changes

         In some special industrial application scenarios, the load may change suddenly. For example, during the process of a crane lowering a heavy object or a centrifuge decelerating, the motor is in a power generation state and will convert mechanical energy into electrical energy and feed it back to the frequency converter. If the braking system of the frequency converter is not configured properly or cannot effectively handle these feedback energies, the DC bus voltage will rise. To protect the equipment, the protection mechanism inside the frequency converter may take some measures, such as cutting off part of the circuit or adjusting the working state of the power module. However, if not handled properly during this process, the voltage may drop too quickly later, thus triggering the undervoltage problem.

 (C) Environmental Factors

1. Temperature Effects

         Extreme environmental temperatures can affect the power supply system. In a high-temperature environment, the resistance of the power line will increase with the increase in temperature. According to the resistance temperature coefficient formula (R = R_0(1 + αΔT), where R_0 is the initial resistance, α is the resistance temperature coefficient, and ΔT is the temperature change), this will lead to an increase in the voltage drop on the line. Meanwhile, high temperatures may also affect the performance of electronic components in the power supply equipment, such as changes in the capacitance value of capacitors and changes in the conduction characteristics of semiconductor devices, which indirectly affect the supply voltage of the frequency converter.

         In a cold environment, the performance of some battery-powered backup power supply systems may decline. For example, the chemical reaction rate of lead-acid batteries slows down in a low-temperature environment, the battery capacity decreases, and the output voltage will also decrease accordingly. If the backup power supply of the frequency converter is affected by low temperatures, when there is a problem with the main power supply, it may not be able to provide sufficient voltage support, resulting in an undervoltage situation.

2. Electromagnetic Interference

         The industrial environment is full of various sources of electromagnetic interference, such as large motors, transformers, and high-frequency heating equipment. The electromagnetic fields generated by these equipment may couple onto the power supply lines, affecting the transmission of voltage signals. For example, when a nearby high-frequency welding machine is working, the high-frequency electromagnetic field it generates may induce an additional voltage on the power supply line of the frequency converter, interfering with the normal voltage signal or introducing noise into the voltage detection circuit, causing the frequency converter to misjudge the voltage value and wrongly trigger the undervoltage protection.

三、Solutions to the Undervoltage Problem in Frequency Converters

 (A) Solutions for Problems on the Power Supply Side

1. Stabilizing the Power Supply Voltage

         In places where the grid voltage fluctuates frequently, an automatic voltage regulator (AVR) can be installed. The AVR samples the input voltage and then uses components such as transformers and thyristors to automatically adjust the output voltage to keep it within a stable range. For example, for a grid environment with a voltage fluctuation range of ±15%, a suitable AVR can stabilize the output voltage within ±2%, providing a reliable power supply for the frequency converter.

         For key equipment with extremely high requirements for voltage stability, an uninterruptible power supply (UPS) system can be used. The UPS can not only provide a stable voltage output when the grid voltage fluctuates but also continue to supply power to the frequency converter for a period of time when the grid power is cut off, ensuring the equipment to shut down safely and orderly, avoiding equipment damage and production interruption caused by sudden power outages. When choosing a UPS, the capacity and output characteristics should be determined according to the power and load characteristics of the frequency converter.

2. Power Line Maintenance and Optimization

         Regular inspection and maintenance of the power supply lines are important measures to prevent the undervoltage problem. Check the appearance of the cables for any signs of damage or aging, especially at the cable joints, to ensure that they are firmly connected, without looseness, oxidation, etc. For cables that are severely aged or damaged, they should be replaced in time. Meanwhile, the line layout can be optimized to try to shorten the length of the power supply lines, reduce the line resistance, and lower the voltage drop.

         Regularly inspect and maintain the connection components such as power switches and fuses. Clean the contacts of the switches, check their wear conditions, and replace the contacts that are severely worn. Ensure that the rated current and voltage of the fuses meet the requirements of the circuit and that the connections are reliable to reduce unnecessary voltage losses.

 (B) Solutions for Problems Related to the Load

1. Optimizing the Motor Startup Process

         For motors with large startup currents, soft start technology can be adopted. A soft starter controls the conduction angle of the thyristors to gradually increase the terminal voltage of the motor, making the motor start slowly, thereby limiting the startup current. For example, by using the ramp voltage startup method, during the startup process, the motor voltage gradually rises to the rated voltage according to the set ramp time, which can limit the startup current within a reasonable range, effectively reducing the impact on the grid voltage during startup and lowering the risk of undervoltage in the frequency converter.

         In addition, traditional startup methods such as star-delta startup and autotransformer step-down startup can also be considered. The appropriate startup method should be selected according to the specific situation of the motor. Meanwhile, when selecting the motor, the load characteristics should be fully considered, and a motor with suitable power and startup torque should be chosen to avoid excessive impact on the frequency converter voltage during motor startup.

2. Improving Braking and Energy Feedback Handling

         According to the characteristics of the load and the power of the motor, the braking unit and braking resistor should be reasonably configured. In equipment such as cranes and centrifuges where energy feedback exists, accurately calculate the resistance value and power of the braking resistor to ensure that the braking unit can timely and effectively consume the energy fed back by the motor. For example, for a specific power crane motor, through precise calculation, select a braking resistor with a suitable resistance value and power, and regularly check its working state, such as whether the resistance wire is broken or the temperature is abnormal.

         For some large systems with high requirements for energy feedback, active front end (AFE) technology or energy feedback units can be considered. These technologies can feed back the electrical energy fed back by the motor to the grid, realizing the bidirectional flow of energy, not only solving the undervoltage problem but also improving the energy utilization efficiency.

 (C) Measures to Deal with Environmental Factors

1. Temperature Control and Protection

         In a high-temperature environment, effective heat dissipation should be carried out for the power supply equipment and the frequency converter. Air conditioning systems, ventilation equipment, or radiators can be installed to ensure that the equipment works within a suitable temperature range. For cables exposed to a high-temperature environment, high-temperature-resistant cables can be used or the cables can be insulated to reduce the impact of temperature on the cable resistance.

         In a cold environment, warming measures should be taken for the battery-powered backup power supply system. For example, place the storage battery in an insulated box and install a heating device to ensure that the storage battery can maintain good performance in a low-temperature environment and provide stable voltage support for the frequency converter when needed.

2. Electromagnetic Interference Protection

         Shielded cables should be used for the power supply lines of the frequency converter, and the shielding layer should be properly grounded. Shielded cables can effectively reduce the interference of external electromagnetic fields on the voltage signals. Meanwhile, filters can be installed on the power supply lines to filter out high-frequency interference signals. For example, by using a low-pass filter, high-frequency electromagnetic interference can be blocked from entering the power supply circuit of the frequency converter, ensuring the accuracy of voltage detection and reducing the false triggering of undervoltage protection caused by electromagnetic interference.

 四、Summary

        The undervoltage problem of frequency converters is a complex issue involving multiple factors, from voltage fluctuations and line faults on the power supply side, to motor startup impacts and energy feedback problems related to the load, and then to the influences of temperature and electromagnetic interference in the environmental factors. Each of these links may trigger an undervoltage situation. Through in-depth analysis of these causes, we have proposed a series of targeted solutions, including stabilizing the power supply voltage, optimizing the lines, improving motor startup and braking handling, dealing with environmental temperature changes and electromagnetic interference protection, etc. In actual industrial production, technicians need to comprehensively consider various factors, and according to the specific production environment and equipment characteristics, flexibly apply these measures to effectively prevent and solve the undervoltage problem of frequency converters, ensure the stable operation of frequency converters, and thereby ensure the smooth progress of the entire production process, reduce economic losses and production delays caused by equipment failures, and improve the reliability and efficiency of industrial production.