Fault Analysis of Frequency Converter Overvoltage

During the operation of the vfd drive, due to different operating conditions, the voltage may fluctuate greatly. When the intermediate DC circuit overvoltage occurs, it will have a great impact on the normal operation of the vfd drive, and even damage the equipment. , Causing casualties, the overvoltage phenomenon generated by the vfd drive usually refers to this type of overvoltage. First, the overvoltage of the frequency converter will have a great impact on the magnetic circuit of the motor. When the frequency converter is overvoltage, the magnetic circuit of the motor will be in a saturated state, thereby increasing the magnetic flux of the motor core. Under this condition, the excitation Both the current and the temperature of the motor will increase significantly. Second, the overvoltage of the vfd drive will affect the insulation performance of the motor. When the overvoltage of the vfd inverter occurs, the output voltage of the vfd inverter cannot be maintained, and there will usually be a large fluctuation. At this time, the frequently changing pulse voltage The insulation layer of the motor is seriously damaged, which affects the insulation life of the insulation layer. Third, the overvoltage of the frequency converter will also reduce the service life of the filter capacitor, and the overvoltage of the intermediate DC circuit may also cause the capacitor to burst when the overvoltage is serious. In view of this, most vfd inverter manufacturers will limit the overvoltage value of the intermediate DC circuit to ensure safe use. When the voltage value of the variable frequency converter exceeds the specified upper limit, the variable frequency converter will take automatic trip protection measures. The overvoltage of the frequency converter will cause the above-mentioned hazards during use, which poses a huge threat to equipment safety and personal safety.

Causes of variable frequency converter overvoltage
1. The cause of overvoltage on the input side
The voltage can be maintained at 380 volts under normal use conditions, and the fluctuation of the voltage due to different loads usually does not exceed 40 volts. When the voltage abnormally increases to the voltage in the power line exceeding 440 volts, the highest intermediate DC voltage value can only reach 636V, in which case the
The voltage value of 636V is not very high, so it can be seen that it is unreasonable to explain this overvoltage phenomenon with the fluctuation of the voltage value on the vfd drive and the line, and the fluctuation of the voltage value is not the only reason for this phenomenon. A large number of such phenomena have been studied and found that: this phenomenon produces
The reason may be that the voltage on the input side cannot be maintained. This phenomenon will occur when the input side is impacted by ultra-high voltage, such as the overvoltage generated when the input side of the power supply is struck by lightning. This impulse overvoltage has two notable features, one is the large range of voltage changes, the other is
The rate of change of voltage is high.

2. Causes of overvoltage on the load side
The overvoltage on the load side is mainly caused by the motor being in the state of regenerative power generation. When the actual operating speed of the motor is higher than the synchronous speed of the motor, the motor can convert the mechanical energy stored in the load transmission-end into electrical energy, which can be used for intermediate DC The loop is fed back, and the feedback process is completed by means of the diode in the variable frequency inverter. The remaining energy causes the voltage of the capacitor to change, and the overvoltage phenomenon occurs.

3. Process requirements lead to overvoltage
According to the technological requirements of the equipment, the technological process limits the shutdown time of the load. In this case, the overvoltage fault cannot be avoided. Some factories have tried to adjust the technological parameters many times, but they still cannot avoid this kind of problem. In essence, the overvoltage fault caused by process requirements can be attributed to the fact that the system cannot consume excess energy, which causes the voltage to reach the limit value and a trip fault occurs.

4. Inverter load dump
When the load changes, because the power of the equipment is not adjusted in time, the speed of the equipment will change significantly. At this time, the motor of the equipment will enter the process of power generation due to the abnormal increase of the speed. This process will make the total energy exceed the loop. Tolerance range leads to failure phenomenon.

5. Motor no load distribution
Generally, when one motor is driving one load, the possibility of overvoltage of the variable frequency converter is small. When multiple motors are driving one load at the same time, the variable frequency converter may have an overvoltage fault. Due to distribution, when two motors drive a load at the same time, there will be a motor with high speed and a motor with low speed. According to the high and low speed, the two motors with high speed and low speed are the prime mover and the generator At this time, the overvoltage fault of the variable frequency converter is likely to occur.

6. Decrease in capacitance capacity
After years of use, the capacitor performance of the intermediate DC circuit of the variable frequency converter will inevitably decline, which will lead to a decline in the ability of the intermediate DC circuit to adjust the voltage. At this time, it is necessary to conduct a comprehensive inspection of the operating status of the capacitor involved in the fault. Especially the capacity retention of the capacitor.