At present and in the future, the maintenance of the electrical control equipment, VFD (Variable Frequency Drive) will become the professional knowledge that all electricians need to master. As a reliable supplier of leading brands of VFDs for many years, OKmarts now shares ten simple and easy maintenance methods to the majority of VFD maintenance technicians. Hopefully these experience can help you in the maintenance of VFDs.

1. Look

After taking over a faulty VFD, first remove the shell and clean it. Next, you should observe the appearance, shape and integrity of relevant components on the circuit board with your eyes according to the type of VFD faults.

This method is very important for the preliminary judgment of charging resistance, filter electrolytic capacitor, IGBT / rectifier bridge module and even PCB.

75kW VFD with errors

Figure 1: A 75kW VFD with display error

Figure 1 shows the problem that when using a 75kW VFD, the display screen sometimes does not show its value, and the DC10v power output from the main board is intermittent.

After dismantling the device, we found out the cause of the fault at a glance - the rectifier diode used in the branch from the switching power supply to the main board + 15V power supply was ignited due to open welding, resulting in the serious PCB ablation.

2. Listen

This method is aimed at three aspects of diagnosing VFD failures: the first is to judge whether the DC bus charging relay / contactor is normal. This is very important to eliminate the under voltage fault in the process of inverter operation.

Then, listen carefully to the VFD used in switching power supply for abnormal noise. It is also very effective to detect whether there is overload in the switching power supply.

Finally, monitor the operation of the cooling fan of the VFD.      

3. Touch

After a VFD fails and the power is turned off, you can quickly dismantle the VFD while remember to take good protection measures.

Touch the related electronic components and integrated IC blocks on the internal circuit board of the VFD with your fingers. Once you find the temperature rise of some components is obvious, the faults must be on them or in the circuits around them.

overheated resistance arrays in 35kw VFD

Figure 2: Resistance array with intense heat.

Figure 2 shows a 35kw VFD, whose DC bus overvoltage protection was triggered by the deterioration of DC bus voltage monitoring resistance. From the picture, we can imagine the fierce heat emitted by the resistance when the incident happened!

4. Press


When a VFD works, but just intermittently, it may be caused by the problem of loose welding of some components in the internal circuit.
It is not easy to find out whether there is a problem with your naked eye for some IC chips with fine pin arrangements and many more IC chips. At this moment, we might as well use an insulated plastic stick or wooden stick (it is strictly forbidden to use conductive metal objects) to press the suspected components with appropriate force with the power on.
This method is very effective for diagnosing loose welding problem of small chip components, especially chip mounted IC. But you must protect yourself against electric shock and short circuit during the operation.

5. Knock

This method is a supplement to the fourth troubleshooting method. After all, the fourth method is only effective for small chip components, but it is not appropriate for some high-power electronic components or high-voltage lines.
For this reason, we can change the pressing to knocking, and use an insulating tool to knock the suspected fault point with appropriate force. In most cases, we can quickly find the fault object.

loose welded joint causes intermittent over pressure fault for small capacity VFD

Figure 3: Loose welded joint found by knocking.

Figure 3 shows the loose welded joint of components detected by this method, which caused the intermittent over pressure fault of a small capacity VFD.

6. Measure

This method depends on the detection of multimeter, for the moment, electricians usually use digital multimeter.

Among all the errors of VFD, about 65% are detected and solved by multimeter. For how to use the multimeter measurement, we believe that most electricians can use it skillfully.
Here we only emphasize one thing: because there are many high-voltage energy storage elements in the VFD, remember to discharge it and make sure the power is off, otherwise the multimeter is easily to get ruined!

deteriorated resistance of 15KΩ, 1/4W in a 22KW VFD

Figure 4: Resistance that seems to be in good condition but actually deteriorated.

Figure 4 shows a 1/4W colored ring resistance with marked value of 15KΩ, whose appearance and color looks fine. But after it’s tested with a multimeter, we find its value has become infinite.
Due to the deterioration of the resistance, the 22KW VFD returns the error "Unbalanced Output Current" .

7. Test

After using a multimeter for measurement, let's talk about another measurement method - test. It refers to the use of an oscilloscope that can visually show the waveform for testing.

As for the maintenance of VFD, oscilloscopes are usually used for six channel inverter pulse signals of VFDs. The driving signal of the brake power tube / module is the switch value, which does not need to be tested by oscilloscope.
When you use an oscilloscope for test, you should pay attention to whether the waveform of the signal is normal, if it can meet the working requirements and whether the amplitude and frequency range of the driving signal meet the driving requirements, etc.
This method is indispensable for the repair of inverter power tubes and modules after burning.

8. Short

This method means making short circuit by purpose.
When diagnosing the problems of VFD, especially when IGBT / IPM is removed due to damage and the pulse drive circuit is separately powered on for maintenance, if the driving optocoupler model is a316j, which has the function of IGBT / IPM fault detection, the optocoupler can not be normally functioning due to module damage or removal.
At this moment, it is necessary to use wires to short circuit the IGBT / IPM fault detection components (most of them are high back voltage diode anodes) to the negative end of the DC bus of the inverter (some labeled N or GND), so as to deceive the main controller of the VFD, making it identify that the power module is in good condition, and then to make the drive pulse signal to be transmitted normally.

high back voltage diode for detecting IGBT module

Figure 5: High back voltage diode for detecting IGBT module.

Figure 5 shows a 55kW VFD, there’s a short circuit of pulse signal driving circuit for detection of the IGBT module.

9. Disconnect


Disconnection - refers to disconnecting the circuit. As we all know, the internal circuit of VFD contains many protection functions for itself or overload. When there are problems with these protection functions, we can use the method of disconnection to detect and repair.

For example: all VFDs have the function of output over-current monitor and protection, but this function design of some products may be quite unreasonable - when there is a fault, it is impossible to clearly find out which phase has the problem, so it is a headache for many maintenance personnel.

To solve this problem, we can take the method of disconnecting the current transformer / sub unit used in each phase detection one by one from the subsequent circuit for comparison. When the input signal is disconnected and the error disappears, the error part should be clear at a glance (some models need manual reset to clear the error message). Of course, this method is also applicable to the maintenance of temperature protection circuit.

10. Discharge

Discharge - refers to discharge of electricity. There are electrolytic capacitors of various specifications and capacities in VFDs. The probability of VFD failures caused by the capacity reduction of these capacitors is quite high. For the error diagnosis of these capacitance, the maintenance personnel usually use the methods of observing the shape and measuring it with capacitance meter, but there are limitations for these two methods.

For this reason, we can use incandescent light bulbs / small electric beads to test the discharge of the tested object after charging. By this way, you can directly compare whether the capacity of the measured object meets the requirement. According to our observation, the efficiency and accuracy of this method is higher than 80%.

a capacitor labeled as 50V and 220uF

Figure 6: A capacitor that seems no problem with appearance, but its capacitance is completely lost.

The capacitor in Figure 6 is labeled with a capacity of 50V and 220uF, but its actual capacity is very small, which is found by the discharge detection method.

We hope that our 10 methods for VFD troubleshooting and error diagnosis can help you in your work.

Besides carrying out good maintenance of VFDs and repairing damaged VFDs, you can also buy new ones if necessary.

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