It is important to learn the helpful methods for VFD troubleshooting. In order to let everyone be capable of solving VFD problems, we have summarized 10 methods here.

two technicians inspecting the circuit board of VFD

Figure 1: Maintenance staff are inspecting the VFD.

1. Alarm Parameter Inspection

Case Study 1

The VFD had a fault and did not work, at the time the LED showed "UV" (abbreviation of Under Voltage).
In the manual, it means DC bus is under voltage. Because the control circuit power supply is not taken from the DC bus, but from the AC input through the transformer, the alarm should be true.

So check from the power supply, we found input power supply voltage was correct, and filter capacitor voltage was 0 V. The short circuit contactor of charging resistor did not work, so it had nothing to do with rectifier bridge.
The fault was considered in the charging resistance. After the power was cut off, it was detected by the multimeter that the charging resistance was broken. After replacing the resistor, the VFD was normal.

Case Study 2

A SANKEN IF 11Kw VFD, which has been used for more than three years, occasionally shows "AL5" (abbreviation of “alarm 5”) when it is powered on. The manual says this means CPU is disturbed.

We have found that it occurs if the contactor works when short circuit happens through charging resistor. It is suspected that the interference is caused by the contactor. After the control pin was added with the resistance capacitance filter, the fault disappeared.

Case Study 3

A Fuji E9 series 3.7kW VFD suddenly stopped working with OC3(over-current during constant-speed operation) alarm. After the power was cut off, when the machine was turned on again, OC1(over-current during constant-speed operation) alarm was given to stop the VFD.

After removing the wires from U, V and W terminal to the motor, the resistance between them measured with a multimeter was infinite.
When VFD was run without load, there was no alarm and the output voltage was normal. It could be preliminarily concluded that there was no problem with the VFD, but there was a joint in the middle of the motor cable, which was covered by the board in the trunk of the pit. Due to the aging of the insulating tape, there’s water getting in the joint when staff cleaned the factory, which caused the output short circuit.

Case Study 4

When a SANKEN SVF303 VFD operated, "5" was displayed on the panel, which means DC overvoltage in the manual. The voltage value is formed by the sampling of DC bus (about DC 530V) and the isolation of optocoupler after voltage division.
hen the voltage exceeds a certain threshold, the optocoupler works to give the processor a high level. In case of over-voltage alarm, we can check whether the resistance changes, or the optocoupler has short circuit, etc.

From the above cases, it is not difficult to find how important the alarms are to deal with problems. These alarm signals can help you find the right direction to solve these problems.

Sanken Frequency Converter VFD ES-2.2K

Figure 2: Sanken Frequency Converter VFD ES-2.2K

2. Analogy Inspection

This method can be the analogy of the same circuit itself, or the analogy between the fault board and the good board. This method could help the maintainer quickly narrow down the inspection area.

Case Study 1

A SANKEN MF15Kw VFD is damaged and sent back for repair. But the user cannot give the specific information.

First, use a multimeter to measure the terminal R, S, T. It could be found that there was a certain resistance value between R and T, and infinite resistance between other terminals. 
The diode characteristics were between the positive or negative terminals of the rectifier bridge and the input terminals R, S and T respectively. 
Why was the resistance between R and T different from that between R and S or S and T? It is because there is a control power transformer inside terminal R and T, leading to a certain resistance. It was concluded that the input part is fine.

After using the multimeter to check the resistance value between terminal U, V and W, we found that it's balanced between three phases. Then, when checking the diode characteristics of each terminal's output to DC positive and negative electrodes, it was found that the terminal U was not conducted to both electrodes.

It's suspected that there was a problem with the U-phase IGBT. When it was removed and checked, we found that the IGBT was broken. In the drive circuit, the three groups of the upper arm control circuit had the same characteristics, while the three groups of the lower arm control circuit had the same characteristics. 
Comparison shows that Q1 is damaged. After replacement, the resistance value of the terminals was consistent in each group. After powering on, confirm whether the PWM waveform is correct. Reassemble and test it.

Case Study 2

There was a VFD whose panel display was normal, as well as the digital setting frequency and operation, but the terminal control was out of order.

After checking by using a multimeter, it's found that the terminals were not at 10V voltage. Starting from the switching power supply, each group of power supply was normal, it seemed that the problem lied in the connecting wire. But without drawings, it would be difficult to find out 10V in 32 flat cables.
There happened to be a 22KW VFD in good condition, so we first recorded the voltage of each pin of the 22KW VFD flat cable to ground, and then compared with the 37KW VFD and found the difference. We found that the pin of the original slot is not soldered, and the converter is completely disconnected due to oxidation after a period of time. Then it is soldered again and repaired.

Case Study 3

There was a carding machine in a woollen mill, which used Siemens 440 VFD, including two sets of 5.5kW and one set of 7.5kW. After two years of operation, one set of 5.5kW often shut down by alarming F0011 or A0511. These two alarms indicated that the motor was overloaded. After detaching the motor belt, manually turn the motor and equipment, and there was no abnormal heavy phenomenon. If the motors controlled by the two VFDs were interchanged and the originally alarming VFD was still alarming. It could be determined that the VFD had a problem.

The analogy method can be used not only to check the internal circuit of the machine, but also to judge the problems on the spot.

Siemens Frequency Converter VFD 6ES6420-2UD17-5AA1

Figure 3: Siemens Frequency Converter VFD 6ES6420-2UD17-5AA1

3. Spare Circuit Board Inspection

It is very effective to confirm the fault and narrow the inspection scope by using the spare circuit board or the circuit board of the same model. If there is a problem with the control board, there is often no alternative but to replace it. Because most users can hardly get the schematic diagram and layout drawings, it is difficult to achieve chip level maintenance. Circuit boards other than control boards, such as power supply board and drive board, can be repaired.

4. Isolation Inspection

It is often difficult to judge which area some faults occur in. By adopting isolation inspection method, we can simplify the complex problems and find out the causes of faults quickly.

Case Study

The phenomenon of a VFD was that there was no display after power on, accompanied by the beep, beep, beep sound.
It could be concluded from experience that the switching power supply was overloaded and the feedback protection worked. After the switch was turned off, there was power supply output. After it vibrated, and the sound was turned off again.

Danfoss Frequency Converter VFD VLT2855PT4B20S

Figure 4: Danfoss Frequency Converter VFD

5. Sensory Inspection

Use our sense organs hand, eye, ear and nose to find out the cause of failure. This inspection method is adopted first. The maintenance principle of "first outside then inside" requires that personnel should first use the methods of seeing, smelling, asking and touching when encountering faults, and check the faults from outside to inside. Some faults can be found quickly through sensory inspection so as to avoid waste of time. By seeing, check whether the connection of circuit elements is loose; whether the disconnected contactor is ablated by electric shock; whether the pressure is abnormal; whether the heating element is overheated and discolored; whether the electrolytic capacitor is expanded and deformed; whether there is obvious breakdown point on the voltage withstand element. After power on, smell to know whether it is burning, and touch the heating element to know if it is hot. It is very important to ask about the situation of the failure, which helps to analyze the cause of the problem, so as to grasp the key point. Sometimes it is also a shortcut to ask peers.

Fuji Frequency Converter VFD FRN0.1C1S-7J

Figure 5: Fuji Frequency Converter VFD FRN0.1C1S-7J

Case Study

One SANKEN IP 55kW VFD was damaged during the warranty period, and there was no display after power on. After opening the cover of the machine, and carefully observing the various parts, we found that the charging resistance, the contactor coil and the shell were burnt.

By asking the user, we learned that because of low power supply voltage, the VFD often stopped working due to undervoltage, and it was specially equipped with a booster. But the user did not notice that the voltage would return to normal at night, which caused the contactor and charging resistance to be burnt. The voltage resistance of rectifier bridge and electrolytic capacitor was relatively high so they were not burnt.

6. Heating and Cooling Inspection

This method is used to check some special faults. By artificial operation, some elements with poor temperature characteristics are heated or cooled to produce or eliminate "disease" so as to find out the cause of the failure.

Case Study

A Delixi VFD was out of order. The user reported that the VFD was often changed to parameter initialization and shut down. Generally, the fault would appear again 20 to 30 minutes after resetting the parameters.

First of all, we figured that the fault should be related to the temperature, because the VFD temperature will rise after 20 minutes of operation. Heat the thermistor by the hot air welding. When the fan was started, I observed that the LED of the control panel suddenly turned off and then lighted up again, and then flickered. After 30 seconds without hot air welding, the LED of the control panel changed back to the normal display.

By adopting the isolation inspection method, pull out all the fan plugs, and conduct heat experiment again, eliminating the fault. At this time, all fans were short circuited. It could be seen that after reaching a certain temperature, the control board gave the fan running signal. As a result, the short-circuit fan caused the overload of the switching power supply, so the output was stopped, and the control board lost power quickly, resulting in parameter storage error and parameter initialization. After replacing the fan, the problem was solved.

Delixi Frequency Converter VFD CDI-E100G011/P015T4BL

Figure 6: Delixi Frequency Converter VFD CDI-E100G011/P015T4BL

7. Destruction Inspection

The method of destruction inspection refers to the elimination of internal protection measures by some means, which can simulate the faults by destroying the faulty devices, so that the faulty devices or areas could be seen obviously. It’s very important to be confident to control the development of the situation, that is, the maintenance personnel should know what the most serious damage is, and whether they can accept the most serious situation. At the same time, they should have control means to avoid more serious damage.

Case Study

A VFD with switching power supply fault is encountered. From its protection circuit action, it could be concluded that there was a short circuit branch at the output end of the transformer, but the fault point cannot be measured statically. So we use the destruction method to find fault devices when static.

Firstly, the feedback signal of the protection circuit is disconnected to make it lose the protection function, and then the DC power supply is connected. The voltage regulator is required to slowly increase the DC voltage from 0V, and the relevant devices are observed at the same time. When smoke was found, the power was turned off immediately. At the same time, the resistance was used to short circuit the DC filter capacitor to discharge rapidly. The smoke is the rectifier diode of the fan power supply. The original fan has been damaged by short circuit, and the control switch signal of the fan is always on (device short circuit caused high level on). As long as the switching power supply outputs normal voltage, the fan will be short circuited, resulting in switching power supply protection. In the static measurement, the short-circuit state of the fan can not be detected.

Delta Frequency Converter VFD VFD004CB21A-20

Figure 7: Delta Frequency Converter VFD VFD004CB21A-20

8. Knocking Inspection

VFD is composed of various circuit boards and module connectors, and there are many solder joints on each circuit board. So any false soldering and poor contact will cause failure. Knock the suspected fault part with an insulated rubber rod. If the fault of the VFD disappears or reappears, it is likely that the problem part.

Case Study

The VFD of a factory has been running normally for more than 3 years. It stopped suddenly without any signs, and there was no fault information. It would start and stop frequently when it was powered on again. There was no abnormality, and no problem found in static measurement.

After power on, knock on the shell of the VFD. It was found that the running signal would change with the knock. After inspection, it was found that the screw of external FR terminal was loose, and the running signal terminal was directly connected to the terminal instead of terminal U. The wire skin of the wire was pressed at the wiring terminal, resulting in the false connection between the control wire and the terminal after the screw was loosened due to vibration. After crimping the terminal U and re-tightening the screw, the fault was eliminated.

Lenze Frequency Converter VFD E82CV371K2C

Figure 8: Lenze Frequency Converter VFD E82CV371K2C

9. Cleaning Inspection

There are a lot of special faults which are sometimes absent, so it is difficult to judge and deal with these faults. At this time, you can first use water or alcohol to clean the circuit board, and use a soft brush to remove the dust and rust, especially where the solder joints are dense. In addition, it’s pf vital importance to clean the vias and circuits close to the 0 V copper layer, and then dry it with hot air. In this way, unexpected results will be achieved, which is at least helpful to observation.

Case Study 1

There was no display of a frequency converter. After preliminary detection, the rectifier part and inverter part were both in good condition, so inspected it with the power on. After checking, it was found that the voltage of DC bus was normal, but the starting voltage of switching power supply control chip 3844 was only 2V. The on-line detection of the divider resistance was much smaller, but the off-line detection is normal. So brush it. It turned out that the positive pin pad of a capacitor was too close to the 0V layer, and the residual flux made it semi conducting.

brush the circuit board with a soft brush

Figure 9: Brushing the circuit board.

Case Study 2

When the VFD was sent for maintenance, there were several different alarm records. After power on, in the test process, there were also various false alarms.
After carefully cleaning the solder joint of flat cable between the control board and the drive board, the problem was solved.

10. Principle Analysis Inspection

detailed diagram of principle of VFD

Figure 10: Principle diagram of VFD

Principle analysis is the most fundamental method of troubleshooting. When other inspection methods are difficult to work, we can start from the basic principle of the circuit, and check step by step to finally find out the cause of the fault. So we must understand the principle of the circuit well, and know the logic level and characteristic parameters (such as voltage value and waveform) of each point at different times. Measure it with multimeter and oscilloscope, and compare the results with that under normal condition. By analyzing and judging the cause of the fault, narrow the scope of the fault, find the cause and solve it.

Case Study

A VFD lost the signals of charging resistor, fan running and status relay. After comparative test, it was confirmed that the problem was in the control board. After analysis, the problem may be in the latch which controlled all these signals. After replacement, it was repaired as expected.

latch charging resistor 45A1JEM E4, SN74HC595N

Figure 11: Latch charging resistor 45A1JEM E4.


In a word, VFD fault inspection should be from outside to inside, from static to dynamic, from main circuit to control circuit. The following three checking steps are generally required.

* Use a multimeter to detect the diode characteristics and three-phase balance characteristics of the output terminal to DC positive and negative respectively. This can determine the quality of the module and decide whether it can output without load. If the above problems occur, no-load output can not be carried out.

* Open the cover and observe.

* If no problem is found in the above two steps, then open the case, remove the dust, and carefully observe whether the internal is damaged, whether there are blackened parts, whether the capacitor leaks, etc.

The above are ten methods about VFD maintenance. By learning these troubleshooting methods, you will know how to repair the VFD. As you further master more knowledge in practice, you can also become an user who can skillfully repair the equipment.