When should we replace our working VSDs?

When to replace existing VSDs is an important decision for many organisations. Modern variable speed inverter drives (VSD) are very reliable. With earlier drives, the two biggest weaknesses were the cooling fans and the power capacitors, but in newer products, these components are designed to have an operating life of ten years, which means they will probably last a lot longer. When to replace them depends on how many you have and how important they are for your process or facility.

Carrying spares is one option, but modern drives are generally reliable and keeping a range spares drives can be expensive, even assuming you can get the costs signed-off. Often, sites have VSDs from a number of vendors, and whilst they may be the same in principle, they are rarely interchangeable. The leading drives manufacturers regularly replace or upgrade their models as they integrate new electronics and control functions or develop smarter energy saving algorithms. So what do other users do?

What do others do?

In a recent example at Bristol Airport, it was decided to replace a number of twelve-year-old drives controlling the AHUs that provide heating and cooling to the terminal building. According to Steve Cox, Engineering and Maintenance Manager for Bristol Airport, explained “Although we had no problems with breakdowns, to improve reliability we decided to be proactive and replace them with more modern and efficient units.”

Replacing the existing drives would also allow the airport to take advantage of the latest fault-finding features of modern HVAC drives. In particular, they were interested in the newer functions. The load monitoring function is very useful, as if a belt fails, the load will change, causing a control alarm rather than needing a visual inspection. Newer drives were also easier to integrate into the site’s new building management system (BMS).
Says Cox. “Our engineers thought the ABB drive was easier to interact with and to program, as it had very logical, user-friendly menus that made for easy navigation and fault finding. It was easy to find the functions we needed without getting lost in sub-menus.

Added benefits

Although energy saving was not a priority for this application, the drives have the ability to operate the AHU motors at varying speeds at different times of the day to match actual demand. The real-time clock allows them to be operated at full speed at 06:00 when the airport is starting to get busier and ramp the motors down to 30 Hz at midnight when there is less activity.

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