introduction
AC drives, with their superior characteristics compared to DC drives, are often the preferred transmission solution in many applications. Modern variable frequency speed control (VFDs) generally uses 16-bit or 32-bit microcontrollers as the control core, achieving fully digital control and speed regulation performance similar to DC speed control. However, maintaining a VFD is more complex than maintaining a DC speed control, and ordinary technicians in industrial and mining enterprises often find it difficult to handle faults. This article analyzes the causes and solutions for common VFD faults.
1. Parameter setting-related faults
Whether a frequency converter can meet the control requirements of a drive system depends heavily on its parameter settings. Incorrect parameter settings can lead to poor system control or even system malfunction.
1.1 Parameter Settings
For a newly purchased frequency converter, the manufacturer typically sets default values for each parameter at the factory. Under these parameter values, the system can operate normally via the control panel, but control panel operation cannot meet the requirements of most drive systems. When modifying frequency converter parameters, consider the following aspects:
1) Confirm the motor parameters. For example, in the 99 sets of parameters of ABB frequency converter, you can set the power, current, voltage, speed and maximum frequency of the motor. These parameters can be obtained directly from the motor nameplate.
2) Setting the starting method of the frequency converter: Generally, frequency converters are set to start from the panel at the factory. Taking ABB frequency converters as an example, they can be started from the panel, external terminals, communication methods, etc. Users can choose according to their actual situation.
3) Selection of frequency setting method Generally, frequency inverters can be set in three ways: panel setting, external voltage or current setting, and communication setting. Of course, for some frequency inverters, the frequency setting method can be one or a combination of these three methods.
After correctly setting these three parameters, the frequency converter can basically operate normally. To obtain better control, the relevant parameters must be modified according to the actual situation.
1.2 Handling Parameter Setting-Related Faults
Once a parameter setting fault occurs, the inverter will not operate normally. In this case, it is best to restore all parameters to factory settings and then reset them according to the steps described above. The parameter restoration method varies depending on the manufacturer of the inverter (refer to the instruction manual). For example, for ABB inverters, the first type of parameters can be modified one by one, while the second and third type of parameters can be restored to factory settings by changing the application macro. For Mitsubishi 540 series inverters, the parameter values can be restored using the clear function in the HELP menu.
2. Overvoltage faults
For frequency converters, there is a normal operating voltage range. When the voltage exceeds this range, the frequency converter may be damaged. There are two common types of overvoltage.
2.1 Input AC power overvoltage
This situation refers to the input AC power voltage exceeding the normal range. It usually occurs during holidays when the line load is light, the voltage rises, or a line fault occurs. In this case, it is best to disconnect the inverter's power supply, check the line, find the cause of the fault, repair it, and then restart the inverter.
2.2 Generation-related overvoltages
This situation is relatively common, mainly because the actual speed of the motor is higher than the synchronous speed, causing the motor to be in generator mode, while the frequency converter does not have a braking unit installed. There are two possible causes for this fault.
1) When a frequency converter drives a large inertia load, if its deceleration time is set too short, the frequency converter's output frequency decreases rapidly during deceleration, while the load decelerates more slowly due to its own resistance. This causes the speed of the motor driven by the load to be higher than the synchronous speed corresponding to the frequency converter's output frequency. The motor is in a generator state, and since the frequency converter lacks energy feedback functionality, the DC circuit voltage of the frequency converter rises, exceeding its protection value, resulting in a fault. This often occurs in the drying section of paper machines. To handle this fault, a braking unit can be added (for applications requiring rapid braking), or the frequency converter parameters can be modified to set a longer deceleration time.
2) This fault can also occur when multiple motors drive the same load. This is mainly due to a lack of load distribution, meaning the motors are not synchronized. Taking two motors driving the same load as an example, if the actual speed of one motor is greater than the synchronous speed of the other, the motor with the higher speed acts as the prime mover, while the motor with the lower speed is in generator mode, causing the fault. In paper machines, this fault often occurs in the press and wire sections. It can be addressed by using a load distribution controller; alternatively, the frequency converter parameters can be modified to reduce the occurrence of the fault. The characteristics of the frequency converters located in the paper machine's drive speed chain can be softened. For example, for ABB frequency converters, this is set to 9907 (motor rated frequency), and for Mitsubishi frequency converters, parameter 03 (base frequency) can be modified.
3 Other faults
1) Overload: The load is too heavy, and the selected motor and frequency converter cannot drive the load. It may also be caused by poor mechanical lubrication. If it is the former, a higher power motor and frequency converter must be replaced; if it is the latter, the production machinery must be repaired.
2) Overcurrent may be caused by a short circuit in the inverter's output. In this case, the circuit should be checked. If the inverter still has an overcurrent fault after disconnecting the load, it means that the inverter circuit of the inverter is seriously damaged and the inverter needs to be replaced.
3) Undervoltage indicates a problem with the power input circuit, which may be caused by severe overload or poor contact in the circuit.
4) Overheating: If a high temperature alarm occurs but the actual motor does not have a temperature sensor, it may be caused by interference, and the fault can be masked. If the motor has a temperature sensor, the motor's heat dissipation should be checked. Another possibility is that the inverter temperature is too high, in which case the inverter's fan and ventilation status should be checked.
For other types of malfunctions, it is best to contact the manufacturer for solutions.
Finally, it should be noted that once a hardware failure occurs in the frequency converter, such as in the rectifier circuit or inverter circuit, the frequency converter is severely damaged (this can be checked by measuring whether there is a short circuit at the input and output terminals). In this case, a specialized technician must be hired to repair it. Technicians should not disassemble the frequency converter themselves or power it on again, so as not to expand the scope of the fault.
