Inverters are not reliable; sometimes they need to be repaired or replaced. The frequency converter is usually an indicator of process changes or application problems. Many inverters use LCD or LED displays for communication, or communicate through open interlocks or fault indications. In most applications, the frequency converter interacts with operator control, process control signals, and PLC. When the actual problem is related to the process, the interaction problem between the inverter and these external controls may seem to be a driving problem. Discussing the process and drive symptoms with the machine operator can often help identify the problem area (see the sidebar "Talking to the Machine Operator").

If the external control works normally, use the inverter to systematically identify the problem. If the status indicator is not working, verify the input AC power. If the status indicator is not displayed after verifying or restoring AC power, verify the control power and restore it if necessary.

If the inverter has run successfully but suddenly fails to start, or the drive starts but runs abnormally, please check whether the diagnostic status display indicates a fault. The instruction manual of the inverter should have fault description and troubleshooting steps. Use diagnostics or keyboard control to monitor variables such as input voltage, DC bus, carrier frequency, output frequency, voltage, current and I/O, and control status. These parameters are displayed on common VFDs. The I/O status uses bits to monitor the required start conditions to ensure they are enabled and to determine what might prevent the start. The control state represents the speed reference source, which can be used to verify the incoming speed or direction signal.

High bus fault

High bus is a common fault caused by external factors. Instantaneous voltage spikes in the AC line or "overhaul loads" generated by machine inertia may cause high bus failures. The load continues to rotate at a speed faster than the motor commanded speed. When this happens, the VFD protects itself by tripping the high bus fault and turning off the insulated gate double j transistor (IGBT). If it indicates a high bus fault, make sure that the AC power supply is consistent and adjust the deceleration time to match the load capacity. If the process requires rapid deceleration, you can add dynamic braking or power control circuits (see the sidebar "Dynamic Braking and Power").

Overcurrent fault

Another common fault is overcurrent. When troubleshooting an overcurrent fault, first check all power connections to make sure they are properly connected. When overcurrent and control problems occur, loose connections or broken wires are often the culprits. Loose power connections can cause overvoltage and overcurrent, blown fuses and damage to the inverter. Loose control wiring leads to unstable drive performance, resulting in unpredictable speed fluctuations or inability to control the inverter. If it is provided on the inverter, use the automatic adjustment function. The auto-tuning function on many drives enables the drive to recognize the connected motor, allowing rotor information to be used in the processor algorithm to achieve more accurate current control. The frequency converter can also compensate the magnetic flux current, which can better control the current that produces the torque. It will be troublesome for the motor to pass up and down.

The second step is to check the mechanical load or excessive friction of worn or damaged parts. Repair or replace components as needed. Check the input voltage and acceleration. If the input voltage is too low or the acceleration rate is set too fast, an overcurrent fault may occur. Reduce the acceleration or stabilize the input voltage to correct this fault.

High starting load current

High current/load readings may indicate unexplained changes in the mechanical bond or process speed or load. The power requirements of many pumps and fans are proportional to the cube of the speed (S3). It only takes a few minutes to run the load every minute to overload the inverter. The components should be checked before starting to avoid overload conditions. Conveyors loaded during non-working hours should be unloaded before starting. When the pump is not in use, the settled solids should be cleaned to avoid clogging the pump. Avoid ice or moisture that may form on the load. Wet material is heavier than dry material and may put more load on the conveyor, causing the motor and inverter to overload.

One way to reduce the high starting load is to use a frequency converter with an extended acceleration rate. This feature can start loading slowly and smoothly instead of pulling it to the start state. This type of start-up is easier on mechanical parts and has lower production line requirements because the inverter only absorbs 150% of the load of 100 MDASS ML.

Unstable operation

If the inverter is operating abnormally but does not indicate a fault, it may be an external factor, or the drive itself may have failed. Knowing the cause of a drive failure can help determine the root cause of the problem. The root cause that is often overlooked is usually the instability in the process, forcing the inverter to function under harsh conditions. By looking for signs of discoloration or cracking, visually inspect the inverter for injuries or overheated components. Burnt out or cracked parts will hinder normal inverter operation. Replace the defective components and test the inverter before putting it into operation. Power quality is another electrical problem that affects frequency converters. Due to electrical storms or system overloads, changes in utility equipment or unexpected power surges may affect VFD performance.

Pollution failure

Pollution of the operating environment is a preventable problem of inverter failure. Check whether the inverter is polluted by dust, moisture or other airborne particles that may conduct electricity. Traces or arc marks on components or circuit board traces indicate evidence of contamination failure. If there is too much pollution, the VFD must be isolated from the pollution source by changing the environment or providing an appropriate NEM Class A enclosure. If there is obvious air pollution in dust, humidity or corrosive vapor, the inverter must be at least in a NEMA-12 enclosure.

Also check whether the internal cooling fan and component radiator of the inverter are contaminated. A blocked fan forces the frequency converter to operate outside its temperature specification, which may lead to premature failure due to insufficient cooling. Check the fan for grease and other contaminants that may cause the bearing and other parts of the fan to fail. The inside and outside of the VFD, including fans, blowers, filters and heat sinks, should be cleaned once a month to reduce the risk of pollutant failure.

Temperature failure

The environment in which the drive must operate must be within the specified temperature range. Measure the temperature inside and outside the enclosure to ensure that it is within the environmental specifications determined by the manufacturer. Failure to meet the required temperature specifications can lead to premature failure of the frequency converter because many power components rely on sufficient cooling to operate normally. If the ambient temperature is too high, add additional cooling to the enclosure, or relocate the inverter to an area where the ambient temperature is within the specification range. Low ambient temperature can also cause problems. Condensation may form and cause component or inverter failure.

Other failures

Many faults are caused by misuse of the inverter. Process changes, such as changes in load or speed; power issues, such as power company's capacity conversion; or changes in environmental operating conditions are not obvious, but may be the main reason for inverter failure. When trying to determine the cause of the failure, evaluate the consistency and condition of the process.

If the inverter still does not work after performing the above inspections, please contact the manufacturer. Most inverter suppliers have well-trained technical support personnel who can provide the help needed to diagnose the problem. If replacement is required, technical support staff can help you choose replacement parts or new drives. As a smart device embedded in the manufacturing process, inverters can provide in-depth understanding of applications and device performance. By providing maintenance personnel with the information needed to understand and explain the problem, it is possible to quickly identify inverter problems, and sometimes process or operational problems, so that plant operations can be restored and productivity can be increased.