Mastering Variable – Frequency Drive Overvoltage: In – depth Correlation Analysis and Solutions

Crucial Correlation Analysis of Variable – Frequency Drive Overvoltage

一. Introduction

         In the field of industrial automation, the inverter (also known as variable frequency drive inverter) is a key device used to control the speed and torque of the motor to achieve multiple functions such as energy saving and precise control. Understanding the working principle of variable frequency drive is essential because the overvoltage problem of the inverter is one of the more common failures during its operation and can lead to serious consequences, which will affect the normal operation and service life of the equipment. It is of great practical significance to deeply analyze the source of the problem of overvoltage of inverter and discuss effective solutions.

二. the source analysis of the overvoltage problem of the variable frequency drive vfd

(1) Overvoltage on the input side of the power supply

1. Grid Voltage Fluctuations

        In some areas, the quality of power supply from the grid may be unstable. When there is a voltage samp in the power grid, such as the instantaneous increase in the grid voltage caused by the start of a large motor, lightning strike, etc., which exceeds the rated input voltage range of the inverter, it will lead to the overvoltage of the inverter. This voltage fluctuation can be a brief spike or a voltage spike that lasts a few seconds or even longer.
        The power supply system of the grid has significant voltage changes during peak and trough periods. During low power periods, grid voltages can rise, and without proper voltage regulation, drives are susceptible to overvoltage.

2. Power Switching Issues

        In some systems with standby power supply or dual power supply, when the power supply is switched, if the switching process is not properly controlled, such as the phase difference and voltage difference at the moment of switching, excessive voltage shock may be generated at the power input of the inverter. This is especially likely to happen when switching between an uninterruptible power supply (UPS) and the mains, or between two mains.

(2) The regenerative energy of the vfd variable frequency drive itself causes overvoltage

1. Motor Fast Braking Process

        When the motor needs to be stopped quickly, for example, in some production equipment that needs to start and stop frequently, the motor will continue to rotate due to inertia, and the motor is in a state of power generation, and the generated electric energy will be fed back to the inverter. If the drive does not have an efficient energy processing mechanism, this regenerative energy can cause the DC bus voltage to rise. In the case of some large inertial loads, such as the crane’s lifting mechanism braking quickly when the heavy object is lowered, the energy fed back by the motor may be very large, which is easy to cause overvoltage.
         For some high-precision speed control applications, such as the spindle motor of CNC machine tools, when the motor decelerates, the rapid change of its speed will produce a large amount of regenerative electric energy, which will cause the DC bus voltage of the inverter to rise sharply if this electric energy cannot be consumed or fed back in time.

2. Energy Feedback Caused by Sudden Load Changes

         In some occasions where the load changes frequently, such as the uneven thickness of the steel encountered by the rolling mill during the rolling process, the load torque will suddenly decrease, the motor speed will rise instantaneously, and the motor will enter the state of regenerative power generation, and the generated regenerative energy will be fed back to the inverter, which may lead to overvoltage. The energy generated by this sudden load change is rapid and random, which poses a challenge to the voltage stability of the inverter.

三.the solution to the problem of overvoltage of the inverter

(1) Measures against overvoltage on the input side of the power supply

1. Install Voltage Stabilizer

         Installing a voltage regulator at the power input of the drive is an effective method. The automatic voltage regulator (AVR) can monitor the input voltage in real time and stabilize the input voltage within the rated voltage range of the inverter by automatically adjusting the turns ratio of the transformer. For some occasions with high requirements for voltage stability, high-precision servo regulators can be used, which can control the output voltage more accurately and effectively cope with the fluctuation of grid voltage.
         Surge protective devices (SPDs) are used to suppress voltage spikes in the grid. The SPD can direct excessive voltage pulses to the ground in a very short time, protecting the inverter from instantaneous high voltage shocks caused by lightning strikes, etc. When choosing the right SPD, consider its through-flow capacity, response time and other parameters to ensure its effective protection of the inverter.

2. Optimize Power Switching Strategy

         In systems with dual or standby power supplies, synchronous switching devices should be employed. This device can detect and adjust the voltage, phase and other parameters of the two power supplies before the power switching, so as to ensure that the voltage difference and phase difference at the moment of switching are within the allowable range. For example, a switch with simultaneous capture can be used to achieve shock-free switching through a precise control algorithm, reducing the voltage shock to the inverter during power switching.
         For the switching between UPS and mains, the switching circuit of UPS should be reasonably designed and parameter optimized. The buffer circuit can increase the switching transition time, reduce the voltage mutation at the switching moment, and ensure the output voltage stability of the UPS, so that it will not have an overvoltage effect on the inverter during the switching process.

(2) Measures for overvoltage caused by the regenerative energy of the inverter itself

1. Increase Braking Resistance

         Connecting a braking resistor to the DC bus end of the drive is a common way to handle regenerative energy. When the motor is in a state of regenerative power generation, the generated electrical energy can be dissipated in the form of heat energy through the braking resistor. The resistance value of the braking resistor should be reasonably selected according to the power of the inverter, the inertia of the motor and other factors. For the inverter system with high power and large inertia load, it is necessary to select a braking resistor with a small resistance value and a large power to ensure that the regenerative energy can be consumed in a timely and effective manner to prevent the DC bus voltage from being too high.
         Some advanced inverters are equipped with an intelligent braking unit that can automatically control the input and removal of braking resistors based on real-time changes in DC bus voltage. When the DC bus voltage rises to the set threshold, the intelligent braking unit will automatically connect the braking resistor to the circuit for energy consumption. When the voltage drops to a safe range, the braking resistor is automatically removed to achieve dynamic control of the regenerative energy.

2. Adopt energy feedback device

         For applications that frequently generate large amounts of regenerative energy, energy feedback devices can be installed. The energy feedback device can feed back the electric energy generated by the motor regeneration power generation to the power grid to realize the reuse of energy. For example, in some cranes, elevators and other equipment with heavy objects falling or frequent braking of motors, the active energy feedback unit is adopted, which inverts the regenerated electric energy of the DC bus into alternating current at the same frequency and phase as the power grid through advanced power electronics technology, and feeds back to the power grid, which not only solves the problem of overvoltage, but also improves energy utilization.
         In some occasions with high requirements for the quality of the power grid, the energy feedback device can also control the quality of the power return, such as harmonic suppression, etc., to reduce the negative impact on the power grid.

四. Conclusion

        The problem of inverter overvoltage is a complex and comprehensive problem, and its source involves many aspects such as power input, self-energy handling and internal circuit faults. The problem of overvoltage of the inverter can be effectively prevented and solved by taking corresponding measures, such as installing a voltage regulator on the input side of the power supply and optimizing the switching strategy, adding a braking resistance or energy feedback device for regenerative energy, and carrying out regular maintenance and fault diagnosis and repair of internal faults of the inverter. This can not only ensure the stable operation of the inverter and prolong its service life, but also improve the reliability and production efficiency of the entire industrial automation system, and reduce the production loss caused by the failure of the inverter. In practical applications, it is necessary to comprehensively consider and select appropriate solutions according to factors such as specific inverter models, load characteristics, and operating environment to achieve the best overvoltage protection effect.