Motor starters - Part 8: Causes of variable frequency drive failure
Time:2024-04-22
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Variable frequency drive (VFD) is an electronic motor controller used to regulate the speed of synchronous or induction motors and drive loads in the power industry. A VFD is also known as a frequency converter, inverter driver, AC driver, adjustable frequency driver, adjustable speed driver, micro driver, or variable speed driver, whose function is to select the voltage frequency and change it before it is used to drive the motor.
VFDS are prone to failure, so effective troubleshooting skills are needed to avoid downtime. Troubleshooting any VFD is no easy task, especially because of the presence of both electronic and electrical components in its structure.
Causes of inverter failure
Before starting the VFD troubleshooting process, it is recommended that you familiarize yourself with some common causes of failure, especially if you do not have any background knowledge of power electronics. Understanding what causes VFD failures will help you determine the actual source of the problem. That being said, let‘s take a look at the internal and external factors that lead to inconsistent performance or complete failure of drives.
Cause one: Poor environmental conditions
The specifications of a VFD are similar to those of a normal computer and are therefore susceptible to any form of moisture, dust, debris, overheating, and corrosives. Several environmental conditions that can shorten the service life of a VFD are discussed below.
pollutant
Converters stored in areas with a lot of debris, moisture, dust, etc., can experience clogged cooling vents, especially when contaminants interact with oils and lubricants.
Keep in mind that the purpose of the vent is to cool the VFD, and if it is blocked, the VFD will overheat. Operating outside the temperature range will result in a shortened service life. Contaminants such as conductive metals may cause a short circuit in a VFD‘s PCB. In order to eliminate such contaminants, the inverter should be cleaned at least monthly. For excessive pollution, completely eliminate VFD from the environment.
High humidity
High humidity environments such as water and sewage treatment plants can be dangerous to inverters because they can corrode the internal printed circuit boards. Moisture enters the circuit board through the VFD‘s cooling vents. In order to reduce this problem, the inverter should be operated in a moisture-free environment and kept in a clean, dry place.
High operating temperature
Each VFD has a specific operating temperature. Failure to meet the specified operating temperature will shorten the service life of the VFD, because most of the components that make up the VFD circuit require sufficient cooling to operate. Therefore, internal and external temperatures must be measured to ensure that the VFD operates within the specified range of 10 o C (50 o F) to 40 o C (104 o F).
VFDS are prone to failure, so effective troubleshooting skills are needed to avoid downtime. Troubleshooting any VFD is no easy task, especially because of the presence of both electronic and electrical components in its structure.
Causes of inverter failure
Before starting the VFD troubleshooting process, it is recommended that you familiarize yourself with some common causes of failure, especially if you do not have any background knowledge of power electronics. Understanding what causes VFD failures will help you determine the actual source of the problem. That being said, let‘s take a look at the internal and external factors that lead to inconsistent performance or complete failure of drives.
Cause one: Poor environmental conditions
The specifications of a VFD are similar to those of a normal computer and are therefore susceptible to any form of moisture, dust, debris, overheating, and corrosives. Several environmental conditions that can shorten the service life of a VFD are discussed below.
pollutant
Converters stored in areas with a lot of debris, moisture, dust, etc., can experience clogged cooling vents, especially when contaminants interact with oils and lubricants.
Keep in mind that the purpose of the vent is to cool the VFD, and if it is blocked, the VFD will overheat. Operating outside the temperature range will result in a shortened service life. Contaminants such as conductive metals may cause a short circuit in a VFD‘s PCB. In order to eliminate such contaminants, the inverter should be cleaned at least monthly. For excessive pollution, completely eliminate VFD from the environment.
High humidity
High humidity environments such as water and sewage treatment plants can be dangerous to inverters because they can corrode the internal printed circuit boards. Moisture enters the circuit board through the VFD‘s cooling vents. In order to reduce this problem, the inverter should be operated in a moisture-free environment and kept in a clean, dry place.
High operating temperature
Each VFD has a specific operating temperature. Failure to meet the specified operating temperature will shorten the service life of the VFD, because most of the components that make up the VFD circuit require sufficient cooling to operate. Therefore, internal and external temperatures must be measured to ensure that the VFD operates within the specified range of 10 o C (50 o F) to 40 o C (104 o F).
Cause 2: The power cable connection is loose
Aging electrical components and loose power connections can cause the VFD to stop operating as intended. Both problems are related to too much mechanical vibration and extreme overheating, which leads to arcing, a condition that can damage the VFD‘s circuitry.
Visual or manual inspection of the VFD power connection is not sufficient to determine whether the connection is good, so it is recommended to use specialized equipment such as temperature probes and digital pyrometers to confirm that the power connection is up to standard.
Figure 1.VFD cable. Photo courtesy of Simon Mugo
Cause 3: The bus is seriously faulty
This is a common fault in frequency drives and the cause has been determined to be external. Some of the external factors that create this challenge include instantaneous spikes in voltage in AC power lines and overhaul loads stemming from the inertia of connected machines. This condition causes the connected load to continue to rotate at a faster speed than the speed specified by the drive motor. When this happens, the VFD will protect its internal components by triggering a high DC fault trip as well as shutting down the IGBT connected to the inverter side circuit.
If there is an indication of extreme bus failure in the AC driver of the diagnostic display, ensure that the machine has a stable AC power supply and is operating on a motor controlled by the VFD with a deceleration time equal to the connected load. If the VFD is used in an application that requires rapid deceleration, dynamic braking can be used to protect the VFD from extreme bus failures.
Cause 4: The startup current is high
A high starting current on the VFD display reading may indicate an unexpected change in processing speed or connection load.
Visual or manual inspection of the VFD power connection is not sufficient to determine whether the connection is good, so it is recommended to use specialized equipment such as temperature probes and digital pyrometers to confirm that the power connection is up to standard.
Figure 1.VFD cable. Photo courtesy of Simon Mugo
Cause 3: The bus is seriously faulty
This is a common fault in frequency drives and the cause has been determined to be external. Some of the external factors that create this challenge include instantaneous spikes in voltage in AC power lines and overhaul loads stemming from the inertia of connected machines. This condition causes the connected load to continue to rotate at a faster speed than the speed specified by the drive motor. When this happens, the VFD will protect its internal components by triggering a high DC fault trip as well as shutting down the IGBT connected to the inverter side circuit.
If there is an indication of extreme bus failure in the AC driver of the diagnostic display, ensure that the machine has a stable AC power supply and is operating on a motor controlled by the VFD with a deceleration time equal to the connected load. If the VFD is used in an application that requires rapid deceleration, dynamic braking can be used to protect the VFD from extreme bus failures.
Cause 4: The startup current is high
A high starting current on the VFD display reading may indicate an unexpected change in processing speed or connection load.
For example, the power requirements of VFD-controlled pumps and fans increase in proportion to the cube of their speed, resulting in VFD overload if the VFD load is operating at a slightly higher speed than expected.
To protect the VFD from overload, test and check the components of the VFD drive before starting it. For example, the conveyor belt is unloaded before the start button is pressed to avoid moisture and debris on any part of the VFD load.
Cause 5: The capacitor is faulty
It is worth noting that the DC bus section of the VFD contains large capacitors that play an important role in filtering AC ripple.
After the use of electrolytic capacitors, they are expected to produce electromechanical tearing and wear, which is a factor leading to their shortened life. These capacitors are expected to age faster than other VFD components. Capacitor failure can also lead to VFD failure. Therefore, you must consider the factors that can cause the life of the capacitor to shorten, which includes high temperatures.
Cause 5: The capacitor is faulty
It is worth noting that the DC bus section of the VFD contains large capacitors that play an important role in filtering AC ripple.
After the use of electrolytic capacitors, they are expected to produce electromechanical tearing and wear, which is a factor leading to their shortened life. These capacitors are expected to age faster than other VFD components. Capacitor failure can also lead to VFD failure. Therefore, you must consider the factors that can cause the life of the capacitor to shorten, which includes high temperatures.
The electrolytes of capacitors are sensitive to high temperatures, so it is unwise to run the VFD above the specified operating temperature, which will affect the service life of the capacitor. Capacitors are recommended to operate at temperatures lower than specified.
Cause 6: The overcurrent is faulty
Overcurrent fault is another cause of inverter failure, which is rooted in the frequency converter acceleration too fast when starting. To resolve this issue, check the power connections to determine if they are properly connected. Keep in mind that a poorly connected power cord can cause overvoltage or overcurrent and may also cause a blown fuse, ultimately damaging the VFD. Also, check the load on the connection to make sure it has no damaged parts or excessive friction.
Key points of VFD failure
Before starting any troubleshooting operation, it is important to fully understand the various causes of VFD failure.
Check the working condition of the inverter to ensure its safe operation.
Harsh environmental conditions such as pollutants, high humidity and extremely high operating temperatures can shorten the service life of a VFD.
The operation of the VFD is affected by loose power line connections due to extreme overheating and excessive mechanical vibration.
Extreme bus failure is a condition caused by transient peak voltages and is a common cause of VFD failures.
High starting currents on the VFD can cause the VFD to overload, so it is recommended to match the speed on the load to the speed on the drive.
Capacitor failures can affect the DC bus of the inverter, preventing it from filtering AC as intended.
Cause 6: The overcurrent is faulty
Overcurrent fault is another cause of inverter failure, which is rooted in the frequency converter acceleration too fast when starting. To resolve this issue, check the power connections to determine if they are properly connected. Keep in mind that a poorly connected power cord can cause overvoltage or overcurrent and may also cause a blown fuse, ultimately damaging the VFD. Also, check the load on the connection to make sure it has no damaged parts or excessive friction.
Key points of VFD failure
Before starting any troubleshooting operation, it is important to fully understand the various causes of VFD failure.
Check the working condition of the inverter to ensure its safe operation.
Harsh environmental conditions such as pollutants, high humidity and extremely high operating temperatures can shorten the service life of a VFD.
The operation of the VFD is affected by loose power line connections due to extreme overheating and excessive mechanical vibration.
Extreme bus failure is a condition caused by transient peak voltages and is a common cause of VFD failures.
High starting currents on the VFD can cause the VFD to overload, so it is recommended to match the speed on the load to the speed on the drive.
Capacitor failures can affect the DC bus of the inverter, preventing it from filtering AC as intended.
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