Views: 50 Author: Site Editor Publish Time: 2024-03-07 Origin: Site
Variable Frequency Drives (VFDs) are not always reliable; sometimes, they need repair or replacement. VFDs often serve as indicators of process changes or application issues. Many VFDs communicate using LCD or LED displays, or through open interlocks or fault indications. In most applications, VFDs interact with operator controls, process control signals, and PLCs. When actual problems are related to the process, interaction issues between the VFD and these external controls may appear as a drive problem. Discussing processes and drive symptoms with machine operators can often help identify the problem area (see sidebar "Talking to Machine Operators").
If external controls are functioning properly, systematically identify issues using the VFD system. If the status indicator lights are not working, verify the input AC power. If the status indicator lights still do not display after verifying or restoring AC power, check the control power and restore it if necessary.
If the VFD has been running successfully but suddenly fails to start, or if the drive starts but operates abnormally, check if the diagnostic status display indicates a fault. The VFD's user manual should have fault descriptions and troubleshooting steps. Use diagnostics or keypad controls to monitor variables such as input voltage, DC bus, carrier frequency, output frequency, voltage, current, and I/O, as well as control status. These parameters are displayed on common VFDs. I/O status uses bits to monitor the required start conditions to ensure they are enabled and identify potential hindrances to starting. Control status indicates the speed reference source, which can be used to verify incoming speed or direction signals.
High bus faults are common issues caused by external factors. Transient voltage spikes in the AC line or "service load" generated by machine inertia can lead to high bus faults. The load continues to rotate at a speed faster than the motor's commanded speed. In such cases, the VFD trips a high bus fault and shuts down the Insulated Gate Bipolar Transistors (IGBTs) to protect itself. If a high bus fault is indicated, ensure AC power consistency and adjust deceleration times to match the load capacity if necessary. If rapid deceleration is required, dynamic braking or power control circuits can be added.
Another common fault is overcurrent. When troubleshooting an overcurrent fault, start by checking all power connections to ensure they are properly connected. Loose connections or wire breakage are often culprits when overcurrent and control issues arise. Loose power connections can result in overvoltage and overcurrent, leading to blown fuses and VFD damage. Loose control wiring leads to unstable drive performance, resulting in unpredictable speed fluctuations or the inability to control the VFD. If available on the VFD, use the auto-tune function. Many drives' auto-tuning functions allow the drive to identify the connected motor, enabling the use of rotor information in processor algorithms for more accurate current control. VFDs can also compensate for flux current, allowing better control of the current producing torque. Both over and undercurrent conditions can be troublesome for motors.
The next step is to check for mechanical loads or excessive friction on worn or damaged components. Repair or replace components as needed. Check input voltage and acceleration. If the input voltage is too low or the acceleration rate is set too fast, an overcurrent fault may occur. Lower the acceleration or stabilize the input voltage to correct this fault.
High current/load readings may indicate unexplained changes in mechanical coupling or process speed/load. The power requirements of many pumps and fans are proportional to the cube of the speed (S3). Running loads for just a few minutes per minute can overload the VFD. Components should be checked before startup to avoid overload situations. Conveyor belts loaded during non-working hours should be unloaded before startup. Pumps should be cleared of settled solids when not in use to avoid pump blockages. Avoid ice or moisture formation on loads. Wet materials are heavier than dry ones and may generate more load on conveyors, leading to motor and VFD overload.
One way to reduce high starting loads is to use a VFD with extended acceleration rates. This feature allows for a slow and smooth startup of loads, rather than jerking them into the starting state. This type of startup is easier on mechanical components and has lower production line requirements since the VFD only absorbs 100 to 150% of the load.
If the VFD is operating abnormally but not indicating faults, it may be due to external factors or the drive itself may have failed. Understanding the cause of VFD failures helps determine the root cause of the problem. Often overlooked, the root cause is usually instability in the process, forcing the VFD to operate under adverse conditions. Visually inspect the VFD for signs of discoloration or cracking, indicating damage or overheating of components. Burned or fractured components will impede normal VFD operation. Replace defective components and test the VFD before putting it back into operation. Electrical issues such as power surges from thunderstorms or system overloads, changes in public utilities, or unexpected voltage surges may affect VFD performance.
Environmental contamination is a preventable cause of VFD failure. Check for dust, moisture, or other potentially conductive airborne particles contaminating the VFD. Traces or arc marks on component or circuit board traces indicate evidence of pollution failure. If pollution is excessive, isolate the VFD from the pollution source by changing the environment or providing an appropriate NEMA-rated enclosure. If there is obvious airborne pollution from dust, moisture, or corrosive vapors, the VFD should be at least in a NEMA-12 enclosure.
Also, check if the internal cooling fan and component heatsinks of the VFD are contaminated. Clogged fans force the VFD to operate outside its temperature specifications, potentially leading to premature failure due to insufficient cooling. Check if the fans have grease and other contaminants that may cause bearing and other fan component failures. The internal and external components of the VFD, including fans, blowers, filters, and heat sinks, should be cleaned monthly to reduce the risk of pollution-induced failures.
The drive must operate within a specified temperature range in its environment. Measure the temperature inside and outside the enclosure to ensure it falls within the manufacturer's specified environmental specifications. Failure to meet the required temperature specifications can lead to premature failure of the VFD, as many power components rely on adequate cooling for normal operation. If the ambient temperature is too high, additional cooling should be added to the enclosure, or the VFD should be relocated to an area where the ambient temperature falls within the specified range. Low ambient temperatures can also cause issues. Condensation may form and lead to component or VFD failure.
Many failures are caused by misuse of the VFD. Changes in processes, such as variations in load or speed; electrical issues, such as capacity changes from the power company; or changes in environmental operating conditions that may not be readily apparent, can be major causes of VFD failure. When attempting to determine the cause of failure, assess the consistency and conditions of the process.
If the VFD still does not function after performing the above checks, contact the manufacturer. Most VFD suppliers have trained technical support personnel who can provide assistance in diagnosing the problem. If replacement is necessary, technical support personnel can help you select replacement parts or a new drive. As an intelligent device embedded in the manufacturing process, the VFD can provide a deep understanding of application and equipment performance. By providing maintenance personnel with the information needed to understand and troubleshoot problems, VFD issues can be quickly identified, and sometimes process or operational issues can be identified, leading to the restoration of factory operations and increased productivity.
