Frequency inverters are critical components of many drive systems. A frequency inverter changes the output voltage frequency and magnitude to control the speed, power, and torque of an induction motor. This is often carried out under closed-loop control to respond to changing load conditions. They are commonly used in applications including industrial drives, pumps and air conditioners.
Without any maintenance, inverters will inevitably fail over time due to the limited lifetime of electrolytic capacitors, which must be periodically replaced. If the inverter is allowed to overheat, electrolyte evaporation will accelerate the wear of these components and inverters may fail prematurely. Other common causes of failure include inappropriate or poorly-installed cabling, blown fuses, voltage fluctuations, and vibration. As with any industrial equipment, a preventative maintenance schedule is advisable. This not only prolongs the life of inverters, but more importantly it avoids unplanned downtime which results in additional costs such as lost production.
Electrolytic capacitors work by using an anode which forms an oxide layer; this oxide then acts as the dielectric to store the charge. The oxide is covered by a film of electrolyte, acting as the cathode. Electrolytic capacitors are sensitive to overvoltage conditions and can be destroyed by an overvoltage of just one Volt. Failure can result in fire or explosion.
‘Dry’ electrolytic capacitors, such as tantalum capacitors, use a solid electrolyte and do not wear out, although random failure or overvoltage damage are still possible. However, many inverters use capacitors with a non-solid electrolyte. Evaporation of the electrolyte in these ‘wet’ capacitors means that, even under ideal operating conditions, they have limited lifetime, and this can be greatly reduced when operated at elevated temperatures.
Many inverter breakdowns are caused by failing capacitors. With proper maintenance, that can usually be avoided. Failures may be caused by overheating due to poor installation, failure of cooling fans or heat sinks becoming clogged with dust. If wet capacitors are not changed within their recommended service period, they may also simply wear out. Insufficient power quality can also cause sudden failure of capacitors, so it is important that power supplies and fuses are able to prevent overvoltage conditions from reaching the inverter.
Incorrectly-specified cables, or poor installation into terminals, is another common cause of inverter failures. Wires can become loose, resulting in circuit breaks; undersized cables can burn out. Inverters induce electromechanical vibrations, and these can lead to friction, heat, wear and loosening.
As with any electrical device, ingress of contaminants such as water or dust can also be a cause of failure.
Installation and Inspection
Like most electronic parts, frequency inverters are affected by heat and should, therefore, be installed in a cool and well-ventilated location. Dust and other environmental contaminants should also be avoided when siting the inverter. It is also vital that the cables are correctly specified for the required currents and installed into terminals correctly. Regular inspections should then be carried out to ensure that the operating environment is being correctly maintained and to check the performance of the inverter.
Environmental checks should look for any build-up of dust, signs of humidity or dripping water, and temperature. Checks for water should include prior traces. The temperature should be checked both in the room and inside the inverter cabinet.
The inverter should also be checked for vibration and noise during operation. A simple touch-check on the cabinet can be sufficient. If there is any noticeable vibration, the individual components should be checked, such as transformers and fans.
Ideally, operational data should also be checked. This might include the load current, voltage and rectifier temperature.
At regular intervals, a more in-depth inspection should be carried out involving physical checks on components and cleaning. These will require the power to be disconnected before they are carried out. Where applicable, drive parameter settings should first be backed up to the control panel and the service engineer’s laptop.
Wires and terminals should be visually inspected for damage and terminal screws should be checked and tightened if they are loose, using a torque wrench where applicable. Insulation tests may also need to be carried out.
Dust and any other debris should be cleaned and vacuumed from within the cabinet and its surroundings. This may also involve removing cooling fans to vacuum inside the heat sink. Any air filters should be cleaned or changed as required.
Components such as cooling fans, smoothing capacitors and fuses should be replaced if they show signs of damage or if they have reached a standard replacement period, typically of several years.
Box :Voice of the service provider
Inverter Drive Systems (IDS) is an approved value provider for ABB inverters and has been an ABB partner for over 20 years. Blaise Ford, managing director, says: “ABB inverters are fundamentally reliable electronic devices, especially if installed correctly and situated in the correct environment. Inverters need to be kept cool, dry and away from dust and vibration. If this is done, then an inverter can run for over 20 years.
“This reliability, however, hides a problem. Because ABB inverters are so reliable, maintenance is often neglected, which means that some inverters do fail prematurely. ABB offers a routine maintenance schedule (available via
www.is.gd/sepexe) for each model. In general, the cooling fans (the only moving part on an inverter) need replacing every three to five years and the capacitors need to be replaced at around nine years.
“If this is done to schedule, then the inverter will have a significantly longer life. If the inverter is controlling a critical application, it should be inspected, checked, tested and cleaned more frequently, usually on an annual basis. If an inverter fails, there will clearly be the cost of repair or replacement, but very often it is the downtime that costs significantly more.”
Blaise confides that IDS tends to advise the following when it comes to repairing inverters: “If it is a small inverter, replace it. If the inverter is large, we would attend the site to fix it; for inverters on a critical application, we replace it with a hire inverter while the failed unit is being repaired. But by far and away the most cost-effective method is to ensure the inverters are serviced regularly and maintained.”
IDS specifies, supplies, installs, commissions, repairs, hires and services inverters with a 24/7 emergency service.