In recent years, frequency converters have been used more and more widely in industrial production and daily life. Consequently, becoming proficient in the use of frequency converters and mastering the basic fault repair of frequency converters has become a major challenge for power workers. Although frequency converters of different brands and models are not exactly the same, as long as you grasp the commonalities of frequency converters, you can learn how to use one type of frequency converter and then apply the same principles to other frequency converters.
Among the many parameters of a frequency converter, we should learn to select and master the most basic parameters, because most of the parameters are generally not used in our actual applications and can be used at their default values. In particular, we should learn to adjust the characteristic parameters to make the frequency converter work in the best condition.
To illustrate the issue more clearly and intuitively, we will analyze it through the following specific examples:
A 15KW water pump is controlled by PID to stabilize the pump outlet pressure.
1. Design concept: Select a 15KW frequency converter with PID function. The input reactor can be ignored. The motor speed is adjusted by PID by comparing the set frequency with the pump outlet pressure. The design should consider manual/automatic switching.
II. Inverter control circuit design diagram:
Additional notes: Other parameters can use the inverter's default values to improve the inverter and motor control characteristics, such as acceleration, deceleration, and U/F curves, which should be adjusted and set according to actual operation during commissioning; PID adjustment parameters need to be adjusted and improved by the 40 parameter group during actual commissioning. It is particularly important to note that the pump outlet pressure and motor speed should be reversed, that is, the speed should be reduced when the pressure is high.
III. Explanation of Control Principles:
1. When manually controlled:
With the manual/automatic selector switch off (DI2=0) and the manual selector switch closed (DI1=1), the inverter will use AI1 as the reference to control the speed and adjust the motor speed. If a fixed frequency point is required, the combination of constant speed 1 (DI3) and constant speed 2 (DI4) switches can be used to obtain three speeds, set to 20Hz, 40Hz, and 50Hz. The set frequency can be modified in parameters 1202, 1203, and 1204.
2. When automatically performing PID control:
Close the manual/automatic switch (DI2=1) and the automatic PID switch (DI6=1). At this time, DI1, DI3, and DI4 will not function. The motor will be PID regulated with AI1 as the setpoint and AI2 as the actual value. If the pump outlet pressure is high, the motor speed will be automatically reduced to reduce the outlet pressure, and vice versa.
3. In case of an emergency:
Pressing the emergency stop button (DI5=0) will prevent the motor from starting, serving as a safety feature. The motor speed can be viewed using a frequency meter, output from AO2. Note the corresponding settings of the analog signal and the meter's range. Two indicator lights are provided to show the motor's running and stopping status, with signals obtained from the normally open and normally closed contacts of output relay 2.
4. Fault Analysis and Repair:
Inverter is a high-precision, high-tech electronic component. When troubleshooting its faults, we should analyze and repair it from simple to complex. For us, the general field maintenance personnel, we do not need to have an in-depth understanding of its internal structure, but we should grasp the general patterns of its fault occurrence and analyze and handle them.
When a frequency converter malfunctions, the first step is to read the fault code from the inverter's display panel. While the specific codes may vary slightly between different inverters, they generally include faults such as overcurrent, overvoltage, overload, undervoltage, overtemperature, analog signal loss, and communication loss. Additionally, a red indicator light on the panel can usually indicate a fault.
When analyzing faults, we can also check the actual switching signals, analog signals, and actual inverter operating data from the actual test data of the inverter to determine whether they are normal. In addition, we should pay attention to whether the characteristic parameters of the inverter are set reasonably. The settings of parameters such as U/F curve, acceleration and deceleration time, current limit, and various protections need to be checked and analyzed in particular.
Case 1: An operator set the operating frequency of a DCS-controlled frequency converter to 30Hz and found that the actual motor speed was very slow, or even stopped.
Analysis and Troubleshooting: Since the original control was normal, the original external wiring and control method should be fine. The inverter fault codes were overcurrent and overload, indicating that the load was too large. According to feedback from the site, the motor was fine. The change in process had increased the inverter load. The inverter current limit was checked and found to be normal. During the trial run, the operating parameters were checked and the input switch and analog data were normal. However, when the input analog signal was about 30Hz, the output frequency could not be increased, indicating that the inverter had poor starting load capacity. The U/F curve was adjusted to increase the voltage at low starting frequency, and the fault was eliminated.
Case 2: One frequency converter frequently tripped and stopped during operation, reporting an over-temperature alarm.
Analysis and Handling: Since this frequency converter had been operating normally initially, the over-temperature alarm tripping usually only occurs when the frequency converter has been running at full load for a long time, resulting in insufficient heat dissipation. The motor load was checked and found to be normal, but upon disassembling the frequency converter, it was discovered that its heat sink had a thick layer of dust, severely hindering the frequency converter's heat dissipation performance. After thoroughly blowing away the dust, the frequency converter returned to normal operation.
As the example shows, maintaining a frequency converter is not complicated. Some common minor problems are not related to the converter itself, but rather to our neglect of its operating environment. The real cause of converter burnout is the accelerated aging of its internal components when these conditions are not met. Therefore, during routine maintenance, we must carefully check the inverter's operating condition and ensure the surrounding environment is dust-free, water-free, corrosion-free, and at a constant temperature.
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