VFD Troubleshooting Flowchart

A Variable Frequency Drive (VFD), also known as a Variable-Speed Drive (VSD), Adjustable-Frequency Drive (AFD), Frequency Converter (FC), Adjustable Speed Drive (ASD), Microdrive, AC drive, or an Inverter driver, is an electronic controller used to regulate the rotational speed of induction or synchronous electric motors.

The VFD controls motor speed by varying the frequency of the supply voltage being used to power that motor. To practically execute this, a typical VFD system consists of four main components; namely, the Rectifier Circuit unit, DC Bus and Filter unit, Inverter Circuit unit, and the control unit. These four units function together to adjust the AC frequency of the voltage supplied to a connected motor, thereby controlling the motor’s RPM (motor speed in revolutions per minute).

First, the VFD takes in fixed frequency AC voltage into the rectifier section, which consists of diodes (4 or 6) or thyristors that convert the input AC voltage into unfiltered DC voltage. This DC output passes through the filtering section that consists of large capacitors functioning as ripple rejection filters. They eliminate frequency-dependent AC ripples in the pulsating DC voltage from the rectifier section, thereby smoothening it.

Next, in the inverter section, the steady DC voltage is converted back to AC voltage using transistors that act as high-power electronic switches. The resulting AC voltage has an adjustable frequency and is sent out to the connected AC motor at the desired frequency. The control unit which is mainly a Programmable Logic Controller (PLC), is responsible for coordinating the high-power transistor switching operation, as well as frequency output and power output from the inverter to the AC motor being controlled by the VFD.

VFD systems enable improved start-up and tighter operational speed control of induction and synchronous AC motors, with reduced energy consumption and cost savings. They are thus widely used in many industrial and commercial plants, that rely heavily on AC motors and adjustable speed drives to automate various processes. For example, you’ll often come across a VFD-controlled motor in pumps, chillers, air handlers, conveyor belt systems (i.e. bottling lines), compressors, fans, and highly advanced multi-drive machines.

The increased usage of VFDs presents maintenance and troubleshooting challenges because as with any electronic system, these important industrial control equipment can also fail. This is where quick and effective VFD troubleshooting becomes very necessary to efficiently minimize plant downtime. But troubleshooting a VFD is always a challenge, given its complicated combination of sensitive electronic circuits and electrical components. In this article, we’ll explain some VFD troubleshooting techniques in form of a detailed flowchart, which plant operators and engineers can use to check and get a failed VFD running again.

Common Causes of VFD Failure 

Before you begin troubleshooting a VFD, it’s important that you’re familiar with some of the common reasons why a VFD can fail, especially if you don’t have a background in power electronics. Understanding the various causes of VFD faults will assist you to determine the root source of the problem. With this in mind, let’s examine some external and internal factors that can contribute to incorrect performance or complete failure of a VFD unit.

A) Poor Working Environmental Conditions

Variable Frequency Drives are built like computers, they are thus highly susceptible to moisture, debris, dust, corrosive agents, and overheating. Here are some of the improper environmental conditions that would shorten the service life of a VFD.

Contaminants: Using a VFD in an environment that is consistently taking in moisture, debris, dust, or airborne particles may clog the drive’s cooling vents, especially when these contaminants are combined with oil. Blocked fans result in inadequate cooling forcing the VFD to operate outside the specified temperature range, which can cause premature drive failure.

On the other hand, electrically conductive contaminants like metallic particles can short circuit the VFD’s circuit boards. Also, tracing or arcing marks across VFD components or circuit board traces are an indication of contamination.

To combat possible catastrophic VFD failure due to the buildup of contaminants, both the exterior and interior of the VFD including fans, heatsink fins, filters, and blowers, should be cleaned at least monthly. But if the contamination is excessive, the VFD should be isolated from the source of contamination by changing the working environment or providing appropriate NEMA-rated enclosures. NEMA-12 enclosures can be used for VFDs exposed to dust, corrosive vapors, or moisture.

High Humidity: When a VFD is used in high humidity environments, such as in wastewater treatment plants, it’s at risk of circuit board corrosion if moisture is constantly sucked into its cabinet through the cooling vents. To avoid problems caused by high humidity, ensure that the working environment and storage space for your Variable Frequency Drive (VFD) is clean and dry.

You can also consider using a dehumidifier to dry out the air, but ensure that the dehumidifier doesn’t leak near any electronic component. For wash-down or extreme humidity environments NEMA 5, 4, or 4X enclosures would be necessary.

Extreme Operating Temperatures: The environment within which the Variable Frequency Drive operates should be within specified temperature limits. As failure to meet the manufacturer’s temperature specifications can shorten the operating life of your VFD because most of the VFD’s power components require adequate cooling to operate properly.

Measure the external and internal temperature of the VFD enclosure to ensure that it’s within the ambient temperature range specified by the VFD manufacturer. If the ambient temperature is too high provide additional cooling, or relocate the drive to an environment where the ambient temperature is as specified. Low ambient temperatures do affect the operation of the VFD as well, especially when condensation forms. Thus, if the ambient temperature is too low, consider using a heater.

B) Loose Power Connections 

A VFD may fail to perform as it used to, due to loose power connections or aging electrical components. These two problems are mainly caused by excessive heat and high levels of mechanical vibrations. This can lead to electrical arcing within the VFD circuitry that can cause issues to other parts of the VFD system. Electrical arcing also creates a dangerous working environment for human operators.

Checking the power connections visually may not be enough to diagnose a loose connection within the VFD circuitry; you may need to use a handheld digital pyrometer or a temperature probe. Because the connections are hotter than the connecting wires, it’s an indication of a loose connection. After isolating the loose power wiring connection, ensure that you tighten it appropriately.

C) High Bus Fault 

This is a common fault in VFDs which is caused by external factors such as an instantaneous voltage spike in the AC power line or an “overhauling load” generated by the inertia of the connected machine. In such a case, the load will continue rotating faster than the specified motor speed. When this happens, the VFD usually protects its elements by tripping on a high DC bus fault and switching off the Insulated-Gate Bipolar Transistors (IGBTs) in the inverter circuit.

If a high bus fault is indicated on your VFD’s diagnostics display, ensure that the AC power being supplied is consistent and adjust the deceleration time of the VFD-controlled motor to match the load. If the application in question requires rapid deceleration, you may need to add dynamic braking or a regenerative power control circuit to protect the VFD and prevent a high bus fault.

D) Overcurrent Fault

Overcurrent is also a frequent fault in VFD systems normally caused by too-fast acceleration during start-up. When troubleshooting overcurrent faults, the first thing to check is all the power connections and ensure that they are attached properly. This is because loose power connections cause overcurrent or overvoltage, blown fuses, and consequent VFD damage.

Second, you can use the Auto-Tuning Feature available in some VFDs to help prevent overcurrent. This function enables the VFD to identify the connected motor, and thus access the rotor information which can be used in the control unit algorithms for more accurate current control.

In addition, to prevent overcurrent fault in VFDs, check the attached mechanical load for broken or worn out parts, or excessive friction. Replace or repair any broken or worn components as needed, and minimize the friction accordingly. Most importantly, ensure that you check the incoming supply voltage and acceleration rate. As an overcurrent fault is likely to occur when the acceleration rate is set too fast or if the incoming supply voltage is too low. In such a case, decrease the acceleration rate or stabilize the incoming voltage to correct the overcurrent fault.

E) High Starting-Load/Current

A high starting load or high starting-current reading on the VFD display could probably indicate mechanical binding or some unexplained changes in the connected load or process speed. For example, the power requirements for many VFD-controlled fans and pumps increase proportionally to the cube of their rotational speed (S3). Therefore, running a VFD load just a few RPMs (revolutions per minute) faster than the commanded speed can overload your VFD.

To avoid an overload situation, make a point of inspecting all the components being driven by the VFD before switching them on. For instance, unload conveyors before startup, clear all debris on pumps, and avoid moisture or ice on any VFD load. This is because wet materials tend to be heavier than dry materials, and can cause VFD overload by adding unexpected load on the system.

Also, you can use a VFD with an extended acceleration rate to reduce a high starting load. Instead of jerking a load to a start, the feature starts a VFD load slowly and smoothly. This type of load start is easier on the VFD’s mechanical components and has minimal power line requirements since the VFD draws only 100% to 150% of its load current.

F) Capacitor Fault

As previously stated, the VFD circuit includes large capacitors in the DC Bus unit which filter out AC ripples. If electrolytic capacitors are used, they’re normally at risk of electro-mechanical wear, which limits their lifespan. They also age faster than other VFD components.

Capacitor fault in itself can cause the VFD to fail. For this reason, it’s important you check the factors that can limit the capacitor’s service life such as high temperatures. Capacitors are extremely sensitive to temperature, and their electrolytes evaporate faster at higher operating temperatures. Therefore, operating the VFD in temperatures over the range recommended by the manufacturer (mainly due to high current) will reduce the lifespan of the capacitor components. Essentially, capacitor service life increases if they’re consistently used at lower than normal operating temperatures.

VFD Troubleshooting Flowchart 

Remember, a Variable Frequency Drive is a sensitive electronic device, which response to fluctuations in system or process conditions and ultimately shuts down on fault indication, depending on which component of the system is malfunctioning. So, how do you go about troubleshooting VFD problems?

The best way to start troubleshooting a VFD system is by first checking its status indicator display or the controller display, to determine whether the problem is internal or external to the drive. Then check the basics like power connections, humidity, operating temperature, etc. The flowchart illustrated below outlines a general procedure for troubleshooting both internal and external VFD problems.

Flowchart Explanation 

If your Variable Frequency Drive has been operating properly, but suddenly it fails to start, or it starts but does not run as required, checks its diagnostics status display to see if it indicates a fault. Most VFDs communicate via Liquid Crystal Displays (LCD) or LED (Light Emitting Diode) displays, or through an open interlock to indicate an internal fault. After identifying the type of fault, refer to the instruction manual provided by your VFD manufacturer for fault descriptions and corresponding troubleshooting steps.

Some troubleshooting steps will require you to use a keypad or diagnostics control to monitor variables such as incoming AC voltage, DC bus, I/O, control status, carrier frequency, current, output frequency, and output voltage. But in most cases, these parameters are displayed on the VFD. The control status display indicates the source of the VFD’s speed reference, and can thus be used to verify the direction of signals and incoming speed.

Also, in most industrial control applications, the VFD system interacts with Programmable Logic Controllers (PLCs), process control signals, and operator controls. Thus, an issue with the communication between these external controls and the VFD may appear to be a drive problem, when the problem could actually be with the process. But if the external controls are working correctly, use the diagnostics status display of the VFD to systematically identify the problem, as shown in the flowchart above.

In some situations, the status indicator display can be nonfunctional, and when that happens verify the incoming AC power. If the diagnostics indicator still does not display even after verifying or restoring the AC power supply, then verify the control power and restore it if need be. With the VFD diagnostics status display working properly, you can determine whether the VFD failure is due to internal or external factors. If it’s due to internal problems, you can still use the display status indicator to troubleshoot the issue as discussed in Section B of this explanation.

A) Troubleshooting a VFD for External Problems

When diagnosing VFD failure, it’s advisable that you begin with a basic preventative maintenance checklist. For a good preventative maintenance schedule, here are the steps to follow:  

• Carry out a thorough Visual Inspection of the entire VFD System: Check for dripping or running water, extreme temperatures in the operating environment, high humidity, debris/excessive dirt, or corrosive agents near or under the drive.  
• Check all Power Wiring Connections for Tightness: As discussed earlier, loose connections in the power circuit of the VFD between the incoming AC power and the motor are a major cause of most VFD faults like IGBTs damage, input rectifier failure, overcurrent trips, and burnt terminals on switches and contactors.  
• Check the Incoming Line Voltages and Current: Incoming three-phase AC power should not exceed a 5% deviation. An unbalanced line voltage can cause significant problems in the Variable Frequency Drive such as under-voltage or overvoltage trips due to line sags or surges. Note, that most VFDs today can still run a motor even when one phase/line of the incoming power is completely dead. Also, check that the input current to the drive is as recommended to avoid overcurrent and high starting-current faults.  
• Check the Drive’s Output Current and Voltage: On most VFDs, the voltage from the inverter circuit should be balanced within a couple of volts before going to the motor, and the output current should also be balanced. Large variations in the drive’s output voltage and current can cause the motor to shake violently as well as other motor problems. And as mentioned earlier, motor-related problems are a major cause of VFD malfunctioning or complete failure. 

B) Troubleshooting a VFD for Internal Problems  

If you have ruled out external problems, what else can cause your Variable Frequency Drive to function improperly or not run at all? Well, the problem could be internal to the drive and you need to check the drive’s diagnostics status display for fault indication. The most common internal problems in a VFD drive include Overcurrent Fault, High Starting-Current/Load Fault, and Capacitor Fault; the various ways of troubleshooting these problems were discussed in the previous section-Common Causes of VFD failure. On identifying the indicated fault, correct it appropriately or eliminate its cause/source as described in the flowchart above.

Note: When troubleshooting a capacitor fault, you need to visually inspect the DC Bus Capacitors for signs of damage like a deformed or cracked casing, or a popped-out pressure plug which could indicate an issue with the capacitor’s electrolyte. Then use an oscilloscope to measure the resistance across the capacitors and see if it’s as recommended. In addition to troubleshooting and correcting the aforementioned faults, here are a few other internal checks you need to carry out when troubleshooting your VFD.

Check the VFD’s Input Rectifier: Before testing the rectifier circuit ensure that you remove the supply power and wait until no voltage is present on the DC bus circuit (you can use a multimeter to check this). Then, carry out a diode check and see if you read a forward bias diode voltage drop, ranging between 0.3 V…0.6V on each diode’s input terminal.  

If you read both forward bias and reverse bias, the input rectifier bridge is most likely shorted. But if you read open circuit for both reverse bias and forward bias, then it’s likely the charge resistor could be open. 

Check the Output of the Variable Frequency Drive: This is also a diode check in which you look for a forward bias diode drop. If you read both forward bias and reverse bias, the output device is controlled by the VFD and could be shorted. However, if you encounter an open circuit reading, then either the DC Bus fuse is open or the output device is damaged.  

Therefore, you need to physically inspect all the VFD’s output devices for any visible or internal failures. As sometimes an output device can physically explode but you still get a forward bias diode voltage drop. 

Check the VFD’s Internal Power Supplies: In most VFDs, the internal power supply is obtained from the DC Bus circuit through one or more switching power supplies. A quick way to check this is to ensure that the VFD keypad lights up. If it doesn’t light up, that means your switching power supply is dead and needs to be replaced. 

Note: If your Variable Frequency Drive does not start or function properly even after performing the aforementioned checks, contact the manufacturer for further assistance.