Abstract: AC servo drives and frequency converters are new types of AC speed control devices used for position, speed, and torque control. They are widely used in machine tools, automated production lines, textiles, packaging, and many other industries. With their increasing application, the usage of frequency converters is also growing. Due to their relatively high price, especially for imported brands of servo drives and frequency converters, the repair of damaged servo drives and frequency converters is particularly important. Servo drives and frequency converters combine high-voltage and low-voltage electrical technologies, making them relatively technically complex and requiring highly skilled repair technicians. They must not only understand high-voltage and low-voltage electrical systems but also have a certain understanding of software applications. For some foreign brands of servo drives and frequency converters, since the application menus are in English, repair technicians also need a certain level of English proficiency.
Having worked in frequency converter companies both domestically and internationally for over ten years, I have been engaged in the development, repair, and maintenance of frequency converter-related hardware and production equipment. I have accumulated some experience and insights into the repair and maintenance of servo drives and frequency converters, and I hope that these insights and experiences can provide some assistance to repair work.
Keywords: observation, auscultation, inquiry, palpation.
To be a frequency converter repair technician, one must have a certain theoretical foundation in electrical engineering and electronics, be able to read drawings, and be proficient in operating common repair tools such as multimeters, oscilloscopes, and soldering irons.
Before attempting to repair a frequency converter, it's essential to understand its working principle and structure. A frequency converter is an electrical device used to change the frequency of alternating current (AC). It also has the auxiliary function of changing AC voltage. In the past, frequency converters were generally included within electrical equipment such as electric generators and rotary converters. With the advent of semiconductor electronic devices, it is now possible to produce completely independent frequency converters. A frequency converter mainly consists of a main circuit and a control circuit.
The main circuit consists of a rectifier circuit, a filter circuit, and an inverter circuit. The rectifier circuit mainly consists of rectifier diodes or thyristors, and its function is to convert the input AC power into DC power. The filter circuit mainly consists of electrolytic capacitors, which filter out the AC component from the DC power. The inverter circuit mainly consists of IGBTs, and its function is to convert DC power into AC power. See Figure 1.
The control circuit is the loop that provides control signals to the main circuit. It consists of a frequency and voltage "operational circuit," a main circuit "voltage and current detection circuit," a motor "position and speed detection circuit," a "drive circuit" that amplifies the control signals from the operational circuit, and "protection circuits" for the inverter and motor. See Figure 2.
When diagnosing a frequency converter that has been sent for repair, the process is similar to that of a doctor examining a patient. By observing, listening, questioning, and palpating, we can understand the damaged parts or specific circuits of the servo and frequency converter. Then, we can repair the specific parts and circuits. As mentioned above, servo and frequency converters are composed of many circuits. Without identifying the specific fault, it is impossible to know where to begin the repair.
When you receive a faulty servo motor or frequency converter, observe it by sight and smell. Check for obvious damage to the inverter's casing, impact damage to the internal circuit board, missing components, obvious burnt parts, and any burnt smell. In actual repair cases, a significant number of servo motors and frequency converters suffer damage to their casings and detached circuit board components due to improper handling during installation and transportation. In such cases, targeted repairs can be made by replacing the casing and resoldering the components. However, some servo motors and frequency converters experience damage due to unstable mains voltage and high harmonic content, causing the varistor and ceramic filter capacitor in the inverter's input circuit to explode. In cases where the circuit board damage is not severe, replacing the varistor and capacitor can solve the problem. If the varistor and capacitor explode and damage other circuits, repairs must be based on the specific fault symptoms. A detailed analysis of specific fault repairs follows. Another type of circuit board burnout is caused by customer carelessness or improper wiring harness labeling leading to incorrect wiring. This is most commonly seen in the main control board. Since the main control board is mostly low-voltage, the common input voltage of the interface terminals is DC24V. If the customer connects to high voltage or connects voltage to a port that is not connected to voltage, the circuit board is very likely to be burned out. By looking at and smelling these situations, the faulty part of the machine can be quickly identified.
For a frequency converter with a good appearance and circuit board surface, showing no obvious damage, you can first inquire with the customer to understand the fault symptoms. Due to technical limitations, end customers can usually only describe the symptoms or the fault display. In this case, you need to use these symptoms and display information to roughly determine the location of the fault. For example, if an elevator frequency converter can only move upwards and not downwards, you need to consider whether there is a problem with the braking system. When the elevator is moving downwards, it is in a generator state. If the feedback energy cannot be fed back to the grid, it will be consumed in the braking resistor. If the braking circuit is damaged, the feedback energy cannot be consumed, which will cause the frequency converter to alarm and forcibly shut down. During maintenance, you can specifically test the braking circuit. If the customer reports that the motor stops after running for a period of time each time, you need to consider whether the frequency converter is overheating and triggering overheat protection. In this case, you need to check if the radiator is dusty, if the fan is running properly, and if there are any problems with the circuit controlling the fan.
If none of the above information is available, then the "cutting" method should be used to understand the cause. Here, "cutting" refers to using auxiliary tools such as multimeters and oscilloscopes to manually diagnose the cause of the fault and repair it. If you have a damaged frequency converter in your hands, and you cannot understand the damage information through external observation or from the customer, then you must understand it by powering it on. Before powering on the frequency converter, you must first measure whether the main circuit of the frequency converter is good. The power modules inside the frequency converter are easily damaged because they have to withstand high voltage and high current at all times when they are working. This damage also includes short circuits between phases or between phase and ground in the main circuit. If you power on the main circuit without measuring, it may cause an explosion and expand the internal fault of the frequency converter. After the measurement is normal, you can power on the main circuit. In actual operation, if you are worried about unexpected situations when directly powering on AV220 or AC380, you can add a current limiter or voltage regulator on the power supply side. These measures can prevent the main circuit fault from expanding due to powering on. If a frequency converter fails to power on, check the main circuit. If there's no display after powering on, check the switching power supply circuit and the panel display circuit. If the display is normal after powering on, use the servo and frequency converter fault log menu and load testing to identify the damaged part. Manufacturer-produced frequency converters usually have a fault log menu, some with several historical fault code records. You can usually access the fault log to understand the cause of the current frequency converter failure. For example, if the last fault log shows an encoder failure, check the speed feedback circuit; if the fault log shows overcurrent, check the current detection circuit, and so on. If there is no fault information, perform a load test with the motor to understand the fault. First, determine the specific damaged part of the servo and frequency converter, then perform specific circuit testing and analysis.
Having identified the fault location using the "observation, listening, questioning, and palpation" method, targeted repairs can then be carried out on the specific circuit. Multimeters and oscilloscopes are used to measure the voltage, current, and waveforms of the damaged circuit nodes to pinpoint the specific faulty electronic components. During repair, comparison, reference, and replacement techniques can be used to quickly identify the damaged electronic components. It is also necessary to prepare some tooling and testing fixtures tailored to the characteristics of the circuit board, especially important for drive circuit repairs. Power modules are expensive, and powering on the entire unit before ensuring the drive circuit is properly repaired can easily damage the inverter's power module. For repairs of the main control board's interface circuits, the inverter's internal menus can be used. For example, some inverter input/output interface circuits can usually be monitored through the I/O menu, making it easy to determine which interface circuit is faulty. After repair, the menu can also display whether the repair was successful.
After repair, the inverter needs to be tested as a whole machine. An inverter sent for repair often has multiple faults. By powering on and testing the whole machine, it can be confirmed whether the inverter has been repaired properly. Some faults only appear under certain conditions, such as full load testing or high ambient temperature. In this case, it is necessary to simulate these external conditions for testing. The load needs to reach the rated value. Motor-to-motor testing can be considered. If the ambient temperature is too high, the motor can be placed in an aging chamber for heating testing.
Inverter maintenance is a job that combines theoretical knowledge, practical experience, and operational skills; the level of technical expertise reflects the quality of inverter maintenance. Therefore, we should regularly read relevant books and magazines to continuously understand the functions and characteristics of these electronic components, broaden our thinking, and inspire our maintenance work. We should then apply this knowledge to practical work to solve problems that cannot be solved during maintenance, thereby continuously improving our technical skills.
Proper routine maintenance can effectively reduce the frequency of inverter failures. In practical applications, inverter performance is affected by environmental conditions such as temperature, humidity, vibration, dust, and corrosive gases. Reasonable use and proper maintenance can extend the lifespan of the inverter and reduce production losses caused by sudden malfunctions. However, improper use and inadequate maintenance can lead to operational failures, causing the inverter to malfunction or even prematurely fail, thus affecting the normal operation of production equipment. Therefore, routine maintenance and periodic inspections are essential.
For continuously operating frequency converters, their operating status can be visually inspected from the outside. Regular inspections should be conducted to check for any abnormalities during operation. The following checks should typically be performed:
1) Is the ambient temperature normal? It should be within the range of -10℃ to +40℃, with around 25℃ being ideal. If conditions permit, it is necessary to install air conditioning.
2) Check if the output current, voltage, frequency, and other data displayed on the inverter's display panel are normal;
3) Are the characters displayed on the display panel clear, and are there any missing characters?
4) Use a temperature measuring instrument to check if the frequency converter is overheating or has any unusual odor;
5) Check if the inverter fan is running normally and if there are any abnormalities, and whether the heat dissipation airflow is unobstructed.
6) Are there any fault alarms displayed during the operation of the frequency converter?
7) Check if the AC input voltage of the frequency converter exceeds the maximum value. Refer to the frequency converter's specification sheet to understand the maximum allowable input voltage. If the external input voltage to the main circuit exceeds the limit, it will damage the frequency converter's circuit board even if the frequency converter is not running.
In addition, regular inspections should be conducted, taking advantage of the annual overhaul period to focus on areas of the frequency converter that cannot be inspected during daily operation.
1) When performing periodic inspections, the power must be disconnected before operation. After the inverter is powered off, wait for 4 minutes (the larger the capacity of the inverter, the longer the waiting time, up to a maximum of 15 minutes) after the power indicator light on the operation panel goes out to allow the DC filter capacitor of the main circuit to fully discharge. Use a multimeter to confirm that the capacitor has discharged completely before proceeding with the operation.
2) Remove the inverter control board and main board, and use a brush and vacuum cleaner to clean the inverter circuit board and internal IGBT modules, input and output reactors, etc. For dirty areas on the circuit board, wipe them with a cotton cloth dampened with alcohol or a neutral chemical agent.
3) Check the insulation of the internal wires of the frequency converter for signs of corrosion, overheating, discoloration, or damage. If any are found, they should be dealt with or replaced in time.
4) Due to vibration, temperature changes, etc., screws and other fasteners of the frequency converter often become loose. All screws should be tightened.
5) Check whether the input and output reactors, transformers, etc. are overheating, discolored, burnt, or have an unusual odor.
6) Check if the small shoulder (safety valve) of the intermediate DC circuit filter electrolytic capacitor is bulging, and whether there are cracks, leaks, or expansion on the outer surface. Generally, the service life of a filter capacitor is about 5 years, with a maximum inspection cycle of one year. When it is nearing the end of its lifespan, the inspection cycle should ideally be every six months. The capacitance of the capacitor can be measured with a digital capacitance meter. When the capacitance drops below 80% of the rated capacity, it should be replaced.
7) Check if the cooling fan is working properly. If there is a problem, it should be replaced. The lifespan of the cooling fan is limited by the bearings. Depending on the inverter's operating conditions, the fan or bearings need to be replaced every 2-3 years. If abnormal noises or vibrations are found during inspection, the fan also needs to be replaced.
8) Check whether the insulation resistance of the frequency converter is within the normal range (all terminals and grounding terminal). Note that a megohmmeter should not be used to measure the circuit board, otherwise the electronic components of the circuit board will be damaged.
9) Disconnect the R, S, T terminals and power supply cables of the frequency converter, and disconnect the U, V, W terminals and motor cables. Use a megohmmeter to measure whether the insulation resistance between each phase conductor of the cable and between each phase conductor and the protective ground meets the requirements. Normally, it should be greater than 1MΩ.
10) Before the frequency converter is put into operation after maintenance, it should be run unloaded with the motor for a few minutes and the rotation direction of the motor should be checked.
With good maintenance and operation habits, coupled with correct maintenance methods, the service life of servo frequency converters can be effectively extended, ensuring maximum equipment operating efficiency and reducing the company's operating costs.
