Most variable speed drive (VSD) failures are attributed to poor installation practices. Yet some failures can be avoided by using drives that have built-in features thereby minimising mistakes and errors. With today’s modern keypads users are guided through commissioning, making sure that nothing is forgotten and that installation goes smoothly.
Among the most common reasons VSDs fail include:
1. Location/ environment
Many VSDs fail because they are used in environments that are too dusty, too hot, too damp or suffer from too much vibration. Most of those failures can be attributed to heat, either because of poor siting or ventilation, or because they are positioned in direct sunlight. Always check that the drive is designed for the environment in which it is to be used.
2. Cable type
Selecting the correct motor cable is more critical than the supply cable. Fully shielded (screened), three-core symmetrical cable with a separate equipotential earth bond is recommended. The cable should not have a break in the shielding, even to go via an isolator box. Isolators need to be in metal enclosures to keep a continuous screen. Connection at the motor and drive end needs to be via metal glanding. Always check the manufacturer’s documentation.
3. Grounding (earthing)
When using VSDs, close attention to the grounding schemes are needed to avoid unwanted effects such as high frequency bearing currents (caused by induced motor shaft and frame voltages) or to reduce the effects of EMC.
4. Cable routing
Because of the power electronics used and the nature of operation of VSDs, it is essential that the routing of all cabling be very carefully considered, otherwise other electronic equipment in their vicinity could suffer from interference. The emissions which the motor cable is producing can also affect cables that are close by, even with a screened cable, so product manuals recommend minimum distances from motor cables to other sensitive signals, to ensure “pick-up” is prevented. If cables need to cross, then crossing at 90 degrees is recommended.
5. Incorrect motor data and set-up
Once the power has been applied, it is very important that the VSD is told the correct information about the motor to which it is connected. Without this, control anomalies will occur, the application will not perform correctly, and energy will be wasted.