Checking connections is a step many people miss or do incorrectly. Heat cycles and mechanical vibration can lead to sub-standard connections, as can standard PM practices. Re-torquing screws is not a good idea, and further tightening an already tight connection can ruin the connection. Bad connections eventually lead to arcing. Arcing at the VFD input can result in nuisance over voltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output can result in over-current faults, or even damage to the power components. Loose control wiring connections can cause erratic operation. For example, a loose START/STOP signal wire can cause uncontrollable VFD stops. A loose speed reference wire can cause the drive speed to fluctuate, resulting in scrap, machine damage, or personnel injury.

Additional steps

1. As part of a mechanical inspection procedure, don’t overlook internal VFD components. Check circulating fans for signs of bearing failure or foreign objects, usually indicated by unusual noise or shafts that appear wobbly. 
2. Inspect DC bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse. 
3. Take voltage measurements while the VFD is in operation. Fluctuations in DC bus voltage measurements can indicate degradation of DC bus capacitors. One function of the capacitor bank is to act as a filter section (smoothing out any AC ripple voltage on the Bus). Abnormal AC voltage on the DC bus indicates the capacitors are headed for trouble. Most VFD manufacturers have a special terminal block for this type of measurement and also for connection of the dynamic braking resistors. Measurements more than 4VAC may indicate a capacitor filtering problem or a possible problem with the diode bridge converter section (ahead of the bus). If you have such voltage levels, consult the VFD manufacturer before taking further action. With the VFD in START and at zero speed, you should read output voltage of 40VAC phase-to-phase or less. If you read more than this, you may have transistor leakage. At zero speed, the power components should not be operating. If your readings are 60VAC or more, you can expect power-component failure. 
4. What about spare VFDs? Store them in a clean, dry environment, with no condensation allowed. Place this unit in your PM system so you know to power it up every 6 months to keep the DC bus capacitors at their peak performance capability. Otherwise, their charging ability will significantly diminish. A capacitor is much like a battery it needs to go into service soon after purchase or will suffer a loss of usable life. 
5. Regularly monitor heat-sink temperatures. Most VFD manufacturers make this task easy by including a direct temperature readout on the keypad or display. Verify where this readout is, and make checking it part of a weekly or monthly review of VFD operation. You wouldn’t place your laptop computer outside, on the roof of a building or in direct sunlight. A VFD, which is basically a computer with a power supply, needs the same consideration. Some VFD manufacturers advertise 200,000 hours almost 23 years of Mean Time Between Failures (MTBF). Such impressive performance is easy to obtain, if you follow the procedures outlined above