Motor Starters Part 8: Causes of Variable Frequency Drive Failures
This article addresses the causes of failures in variable frequency drives to save engineers unnecessary downtime.
A Variable Frequency Drive (VFD) is an electronic motor controller used to regulate the rotating speed of synchronous or induction electric motors and to drive load in the power industry. Also referred to as frequency converters, inverter drivers, AC drivers, adjustable frequency drives, adjustable speed drives, microdrives, or variable speed drives, VFDs function such that they pick a voltage frequency and vary it before it is used to drive the motor.
VFDs are prone to failure, so effective troubleshooting skills are necessary to avoid downtime. Troubleshooting any VFD is not an easy task, especially due to the presence of both electronic and electrical components within its structure.
Causes of Variable Frequency Drive Failure
Before starting the process of VFD troubleshooting, it is advisable that you make yourself conversant with some of the most common causes of its failure, especially when you don’t have any background knowledge of power electronics. Understanding what causes the VFD failure will be a recipe for you to determine what is the actual source of the challenge. Having said that, let us have a look at the internal and external factors that play a role in inconsistent performance or complete breakdown of the VFD unit.
Cause 1: Poor Environmental Conditions
VFDs are built with specifications similar to those of normal computers and are, therefore, susceptible to any form of moisture, dust, debris overheating, and corrosive agents. Below is a discussion of several environmental conditions that could shorten VFDs’ lifetime.
VFDs that are kept in areas of high debris, moisture, dust, etc., will experience clogged cooling vents, especially if the contaminants interact with oils and lubricants.
Remember, the purpose of the vents is to cool the VFD and, when blocked, VFDs overheat. Operating outside an optimal temperature range will lead to reduced life. Contaminants like electrically conductive metals may create short circuits in the VFD’s PCB. To eliminate such contaminants, the VFD should be cleaned at least once a month. For excessive contamination, eliminate the VFD from the environment altogether.
High-humidity environments like water and sewage treatment plants can prove dangerous to the VFDs because they corrode the inner printed circuit board. Moisture gets into the circuit board through constant sacking by the VFDs’ cooling vents. To reduce this problem, VFDs should operate in a moisture-free environment and be stored in a clean, dry place.
High Operating Temperatures
Every VFD has a particular operating temperature. Failing to meet this specified operational temperature will shorten VFD life since most of the components that build up the VFD circuitry require enough cooling for maximum operation. Therefore, must measure both the internal and external temperatures to ensure the VFD operates within the specified limits that range between 10oC (50oF) to 40oC (104oF).
Cause 2: Loose Power Cable Connections
Aging of electrical components and loose power connections can make the VFD stop performing as expected. The two problems are associated with too many mechanical vibrations and extreme overheating, which leads to electrical arcing, a condition that destroys the VFD’s electrical circuitry.
Visual or manual checking of the VFD power connection is never enough to ascertain a good connection and, therefore, specialized equipment like a temperature probe and digital pyrometer is advised to confirm the power connection is up to standard.
Figure 1. VFD cables. Image used courtesy of Simon Mugo
Cause 3: Extreme Bus Fault
This is a common fault in VFDs whose cause has been determined to be external. Some of the external factors contributing to this challenge are the instantaneous spiking of voltage in the AC supply power line and the overhauling load that originates from the connected machine inertia. Such conditions cause the connected load to keep rotating faster than the speed specified by the driving electric motor. When this occurs, the VFD will shield its internal components by triggering a trip on a high direct current fault plus switching off IGBTs connected to its electrical circuit on the inverter side.
If there is an indication of the extreme bus fault within the AC drives of the diagnostic display, ensure there is a steady AC supply to the machine and work on the deceleration time motor being controlled by the VFD equal to the connected load. If the VFD is being used in an application requiring rapid deceleration, then dynamic braking can be used to protect the VFD against the extreme bus fault.
Cause 4: High Starting Current
A high starting current on the VFD display reading could indicate unexpected changes occurring in the processing speed or connected load.
For illustration, the VFD-controlled pumps and fans have a power requirement that increases proportionally to their rotating speed cube, and if the VFD load runs at a slightly higher speed than expected, will cause VFD overload.
To protect the VFD against this overload, test and inspect the components of the VFD drives before starting it. For instance, unload the conveyer belt before pressing start to avoid moisture and debris on any part of the VFD load.
Cause 5: Capacitor Fault
It is significant to note that the VFD’s DC bus section comprises large capacitors that play an important role in filtering the AC ripples.
When electrolytic capacitors have been used, then expect them to incur electro-mechanical tears and wear, which is a factor that makes them have a reduced lifespan. These capacitors are expected to age faster than other VFD components. The fault in the capacitor also contributes to the failure of the VFDs. Therefore, you have to consider the factors that can cause a reduced life span in a capacitor which includes high temperatures.
Capacitors have an electrolyte sensitive to high temperatures so it is unwise to operate the VFDs above the specified operating temperatures, which can interfere with the capacitor’s lifespan. It is advisable to operate capacitors at temperatures lower than specified.
Figure 2. VFD capacitors. Image used courtesy of Simon Mugo
Cause 6: Overcurrent Fault
Overcurrent fault is another cause of VFD failure, which originates from fast acceleration during VFD start-up. To troubleshoot this challenge, check on the power connections to ascertain if they are well-connected. Remember that poorly connected power cables create overvoltage or overcurrent and can also lead to fuses blowing, eventually damaging the VFD. Also, check the connected load to ensure it has no broken parts or excessive friction.
Key Takeaways of VFD Failure
- The various causes of VFD failure must be well-understood before starting any troubleshooting operation.
- Check the working conditions of VFDs to ensure they are safe for operation.
- Poor environmental conditions such as contaminants, high humidity, and extremely high operating temperatures play a role in reduced VFD lifespan.
- Loose power cable connections, which originate from extreme overheating and too many mechanical vibrations, affect the VFD's operation.
- Extreme bus fault, a condition caused by instantaneous spiking voltage, is a common cause of VFD failure.
- High starting current on the VFD causes the VFD overload, therefore, it is advisable to match the speed on the load to that on the drive.
- Capacitor fault may affect the DC bus of the VFD, making it fail to filter the AC as expected.
The next article in this series will address how to troubleshoot and eradicate VFD failures.