Abstract: Inverter overvoltage is mainly caused by excessive output rotational inertia of the inverter, leading the motor to enter a generator state (equivalent to a generator), which in turn causes inverter overvoltage. Since inverters and motors are used in different applications, the causes of overvoltage also differ. This article describes the fault analysis and handling of an inverter in a steel pipe transmission line.
Keywords: frequency converter; overvoltage alarm; steel pipe conveyor line
1. Introduction to Steel Pipe Transmission Line
Our company has a corrosion protection branch factory specializing in the internal and external corrosion protection of steel pipelines used for oil and gas transportation. The workshop has multiple transmission lines for transporting steel pipes. The main transmission method involves two rows of solid tires that propel the pipes forward in a spiral motion. One row acts as the driving wheel, driven by a three-phase asynchronous motor, while the other row acts as the driven wheel. See Figure 1:
Figure 1
1. Losses caused by overvoltage alarm of frequency converter in steel pipe transmission line
Inverter overvoltage mainly refers to overvoltage in the intermediate DC circuit, which causes numerous hazards. These include motor magnetic circuit saturation, damage to motor insulation, and direct impact on the lifespan of the intermediate DC circuit filter capacitors. Therefore, inverter manufacturers typically limit the overvoltage value of the intermediate DC circuit; once the voltage exceeds the limit, the inverter generates an overvoltage alarm. However, for a continuous corrosion-resistant production line, the loss from a single overvoltage alarm is extremely serious. An inverter alarm can cause the entire production line to stop, resulting in scrap. Restarting wastes a significant amount of raw materials (due to process requirements), and the scrapped pipes need to have their original anti-corrosion layer removed before re-entering the production process; the cost of processing such a single anti-corrosion pipe can be several thousand yuan.
2. Causes of overvoltage in frequency converters
Generally, the main causes of inverter overvoltage come from the following two aspects.
1) Overvoltage from the power input side
Under normal circumstances, the power supply voltage is 380V, with an allowable error of -5% to +10%. After three-phase bridge full-wave rectification, the peak DC voltage is 590V. In some cases, the power supply voltage may reach 450V, but the peak voltage is only 630V, which is not very high. Generally, the power supply voltage will not cause the inverter to trip due to overvoltage. Overvoltage on the power input side mainly refers to excessively high surges on the power supply side, such as overvoltage caused by lightning or overvoltage formed when the compensation capacitor is closed or opened. The main characteristics are a large voltage change rate (dv/dt) and a large amplitude.
2) Overvoltage from the load side
This mainly refers to a situation where, due to some reason, the motor is in a regenerative braking state, meaning the motor's actual speed is higher than the synchronous speed set by the frequency converter. The mechanical energy stored in the load in the drive system is converted into electrical energy by the motor and fed back to the inverter's intermediate DC circuit through the inverter's six freewheeling diodes. At this time, the inverter is in rectification mode. If the inverter does not take measures to dissipate this energy, it will cause the voltage of the capacitor in the intermediate DC circuit to rise. When the voltage reaches a certain limit, the circuit breaker will trip.
4. Fault Phenomena
One of the steel pipe transmission lines in the workshop is controlled by three AC motors driven by three frequency converters. The frequency of the three frequency converters can be controlled by a single potentiometer or individually (as shown in Figure 2). Normally, a single potentiometer is used for coordinated control during production for easy frequency adjustment. One day, after a repaired frequency converter was put into use, the operator reported frequent fault alarms on the transmission line. Maintenance personnel inspected the site and found that one frequency converter was displaying a main circuit overvoltage alarm. Based on past experience, this usually occurs when the steel pipe is being accelerated due to human error—failure to release the acceleration button in time—causing the accelerating steel pipe to push against the steel pipe running at normal speed in front, resulting in the preceding steel pipe and transmission wheel being accelerated (a phenomenon known as "pipe jacking"). The motor generates feedback voltage, causing the frequency converter to trigger an overvoltage alarm, i.e., an overvoltage alarm originating from the load side, as mentioned above. However, on-site verification by the maintenance personnel confirmed that this was not caused by human error.
Figure 2
5. Fault finding, analysis and handling
The operators were instructed to operate the inverter while maintenance personnel monitored it. They discovered that while the other two inverters were running at 24Hz, the faulty inverter was operating at 22Hz. Analysis indicated the cause was an overvoltage alarm in the slower-frequency inverter due to a steel pipe passing through the transmission lines of two different frequency bands. Checking the inverter's parameters revealed they were identical to those of the other normally functioning inverters. Same model inverter, same parameters, same potentiometer adjustment, yet different frequencies were produced. The analysis suggested a factory setting deviation after repair or a hardware issue causing signal output inconsistencies. However, given the need to maintain production and the lack of spare parts, they had to attempt other methods to resolve the issue as quickly as possible.
Technicians attempted to adjust parameters to bring the faulty inverter back to its normal operating frequency. Initially, noting that the faulty inverter operated 2Hz slower than the other two, they calculated and increased the bias parameter by 4%, anticipating the problem would be solved. However, during trial operation, they discovered that the V/F curve of the faulty inverter intersected with the V/F curves of the other two inverters (as shown in Figure 3). When the inverters operated at 20Hz, the frequencies of all three inverters were consistent. Below 20Hz, the faulty inverter's frequency was slightly higher than the other two. Above 20Hz, the faulty inverter began to operate at a lower frequency than the other two. Therefore, it was determined that the problem was not with the faulty inverter's bias, but rather that the slope of its V/F curve showed a negative deviation from the normal V/F curve (as shown in Figure 4). Technicians adjusted the gain parameters of the faulty inverter. Through multiple adjustments, they locked the gain parameter of the faulty inverter at 110% (the gain parameters of the other two inverters were the factory setting value minus 100%). At this point, the slope of the V/F curve of the faulty inverter was exactly the same as the slope of the V/F curve of the other two inverters. Trial operation was successful, and production was restored.
Figure 3 Figure 4
6. Summary
Based on long-term practical experience, the overvoltage faults in frequency converters for steel pipe transmission lines are mainly caused by the following reasons:
1) Human error: When accelerating the steel pipe, the acceleration button is not released in time due to operational error, causing the accelerating steel pipe to hit the steel pipe running normally in front, resulting in the steel pipe in front and the transmission wheel being accelerated. The motor generates feedback voltage, causing the frequency converter to alarm for overvoltage.
2) Significant differences in drive line pitch: This means that the rotation angles of the solid tires at different positions differ too much, resulting in inconsistent steel pipe travel speeds. In high-speed production, when changing the motor frequency to track the pipe is no longer sufficient for continuous production, workers may manually increase the angle of the solid tires at the pipe-tracking point to increase the forward pitch of the steel pipe (the distance the pipe travels in one rotation). However, if the adjustment is too large or inappropriate, it can also cause pipe jacking.
3) Inverter-related issues: as described in the text.
4) In theory, there is also an overvoltage alarm from the input side, but this is not common in actual problem handling.
