1 Overview

 

Working principle of frequency converter: Frequency converter is a power control device that applies frequency conversion technology and microelectronics technology to control AC motors by changing the frequency of the motor's operating power supply. The frequency converter is mainly composed of rectifier (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, detection unit, microprocessing unit, etc. The inverter relies on the switching of the internal IGBT to adjust the voltage and frequency of the output power supply, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation.

 

 

 

With the continuous development of the automation field, the application of frequency converters has also penetrated into all walks of life. The development of frequency converters is also constantly introducing new ones, with increasingly larger functions and correspondingly improved reliability. However, if it is used improperly, operated incorrectly, or maintained not in time, malfunctions may still occur or operating conditions may change, shortening the service life of the equipment. Therefore, daily maintenance and repair work are particularly important.

 

 

 

2. Things to note

 

The operator must be familiar with the basic working principles and functional characteristics of the inverter, and have basic knowledge of electrician operation. Before inspection and maintenance of the frequency converter, the main power supply of the equipment must be cut off and the main light of the frequency converter must be completely extinguished.

 

3. Daily inspection items

 

Before powering on the inverter, the temperature and humidity of the surrounding environment should be detected. Excessive temperature will cause the inverter to overheat and alarm. In severe cases, it will directly cause damage to the inverter's power components and short circuit the circuit; too humid air will cause a direct short circuit inside the inverter. When the inverter is running, pay attention to whether the cooling system is working properly, such as whether the air duct exhaust is smooth and whether the fan makes abnormal sounds. Generally, frequency converters with relatively high protection levels such as: frequency converters above IP20 can be installed directly in the open, and frequency converters below IP20 should generally be installed in cabinets. Therefore, the heat dissipation effect of the frequency converter cabinet will directly affect the normal operation of the frequency converter. Whether the exhaust system, such as the fan, rotates smoothly, and whether there is dust or obstructions in the air inlet are all areas that cannot be ignored in our daily inspections. Check whether the motor reactor, transformer, etc. are overheated and have a peculiar smell; whether the inverter and motor make abnormal noises; whether the current display on the inverter panel is too large or the current changes too much; whether the output UVW three-phase voltage and current are balanced, etc.

 

4. Regular maintenance

 

Clean the air filter cooling duct and internal dust. Check whether the screws, bolts, plug-ins, etc. are loose, and whether there is a short circuit in the input and output reactor's resistance to ground and between phases. Normally it should be greater than tens of megohms. Check whether the conductors and insulators are corroded. If so, wipe them clean with alcohol in time. When conditions permit, use an oscilloscope to measure the stability of the voltage of each circuit output by the switching power supply, such as 5V, 12V, 15V, 24V and other voltages. Measure the waveforms of each channel of the driver circuit to see if there is any distortion. Whether the UVW phase waveform is a sine wave. Check whether there are traces of sparking on the contacts of the contactor. In serious cases, replace it with a new one of the same model or with a larger capacity than the original one; confirm the correctness of the control voltage and conduct a sequential protection action test; confirm that there is no abnormality in the protection display circuit; confirm that the inverter is running alone the balance of the output voltage. Regular inspections are recommended and should be done once a year.

 

5. Replacement of spare parts

 

Frequency converters are composed of a variety of components, some of which will gradually degrade and age after long-term operation. This is also the main reason for the failure of the frequency converter. In order to ensure the long-term normal operation of the equipment, the following components should be replaced regularly: Electrician's Home

 

(1) Cooling fan

 

The power module of the frequency converter is the device that generates the most heat. The heat generated by its continuous operation must be discharged in time. Generally, the life of the fan is about 10kh~40kh. According to the continuous operation of the frequency converter, the fan must be replaced once every 2 to 3 years. The direct cooling fan has two wires and three wires. One wire of the second wire fan is the positive wire and the other wire is the negative wire. Do not connect the wrong wire when replacing it; the three wire fan has the positive wire except the positive wire. , there is a detection line outside the negative electrode, be sure to pay attention when replacing it, otherwise it will cause the inverter to overheat and alarm. AC fans are generally 220V or 380V, so don’t make the wrong voltage level when replacing them.

 

(2) Filter capacitor

 

Intermediate DC loop filter capacitor: also known as electrolytic capacitor, its main function is to smooth the DC voltage and absorb low-frequency harmonics in DC. The heat generated by its continuous operation plus the heat generated by the inverter itself will accelerate the drying up of its electrolyte. Directly affects its capacity. Under normal circumstances, the service life of the capacitor is 5 years. It is recommended to check the capacitor capacity regularly once a year. Generally, if the capacity decreases by more than 20%, it should be replaced.

 

 

 

6. Test

 

6.1 Static testing

 

(1) Test the rectifier circuit

 

Find the P and N terminals of the internal DC power supply of the frequency converter, adjust the multimeter to the resistance level And basically balanced. On the contrary, connect the black meter rod to the P terminal, and connect the red meter rod to R, S, and T in sequence, and there will be a resistance value close to infinity. Connect the red meter rod to the N terminal and repeat the above steps, and you will get the same result. If there are the following results, it can be determined that the circuit is abnormal. A. The three-phase resistance value is unbalanced, which can indicate that the rectifier bridge is faulty. B. When the red meter rod is connected to the P terminal, the resistance is infinite, and it can be concluded that the rectifier bridge is faulty or the starting resistor is faulty.

 

(2) Test the inverter circuit

 

Connect the red meter rod to the P terminal, and connect the black meter rod to U, V, and W respectively. There should be a resistance of dozens of ohms, and the resistance of each phase is basically the same. The reverse phase should be infinite. Connect the black meter rod to the N terminal and repeat the above steps to obtain the same result. Otherwise, the inverter module may be faulty.

 

6.2 Dynamic testing

 

After the static test results are normal, the dynamic test, that is, the power-on test machine, can be performed. The following points must be noted before and after powering on:

 

(1) Before powering on, you must confirm whether the input voltage is correct. Connecting a 380V power supply to a 220V inverter will cause explosions (explosion of capacitors, varistors, modules, etc.).

 

(2) Check whether each broadcast port of the frequency converter is connected correctly and whether the connection is loose. Abnormal connections may sometimes cause the frequency converter to malfunction, and in severe cases, it may cause a crash.

 

(3) After powering on, detect the fault display content and initially determine the fault and its cause.

 

(4) If no fault is displayed, first check whether there is any abnormality in the parameters, and after resetting the parameters, start the inverter under no load (without connecting the motor), and test the three-phase output voltage values of U, V, and W. If phase loss, three-phase imbalance, etc. occur, the module or driver board is faulty.

 

(5) When the output voltage is normal (no phase loss, three-phase balance), perform a load test. When testing, it is best to test at full load.

 

7. Fault judgment

 

(1) The rectifier module is damaged

 

Usually caused by grid voltage or internal short circuit. After eliminating the internal short circuit, replace the rectifier bridge. When handling faults on site, you should focus on checking the user's power grid conditions, such as grid voltage, whether there are welding machines and other equipment that pollute the grid.

 

(2) The inverter module is damaged

 

Generally caused by damage to the motor or cable and failure of the drive circuit. After repairing the drive circuit and measuring the drive waveform in good condition, replace the module. After replacing the driver board during on-site service, attention must also be paid to checking the motor and connecting cables. After confirming that there is no fault, run the inverter.

 

(3) No display after power on

 

Generally, it is caused by damage to the switching power supply or damage to the soft charging circuit, resulting in no DC power in the DC circuit. For example, the starting resistor is damaged, or the panel may be damaged.

 

(4) Overvoltage or undervoltage is displayed after powering on.

 

Generally caused by input phase loss, circuit aging and moisture on the circuit board. Find out its voltage detection circuit and detection points, and replace the damaged components.

 

(5) Overcurrent or grounding short circuit is displayed after powering on.

 

Usually it is due to damage to the current detection circuit. Such as Hall elements, operational amplifiers, etc.

 

(6) Start to display overcurrent

 

Generally it is caused by damage to the drive circuit or inverter module.

 

(7) The output voltage is normal without load, but overload or overcurrent is displayed after loading.

 

This situation is generally caused by improper parameter settings or aging of the drive circuit or module damage.