Getting the best out of VSDs in HVAC installations

16 October 2013

There is no doubt that, in HVAC installations, replacing ordinary motor starters with variable speed drives will almost always produce useful energy savings as well as reducing wear and tear on the plant. But, says Stephen Takhar of Vacon, those who want to get the full range of benefits that VSDs can offer should choose their drives with care.

VSDs for HVAC applications should, for example, have an integrated PID controller, as this is needed to provide automatic control of fan and pump speed using signals from pressure and flow sensors. A real-time clock is also highly desirable, so that the drives can be programmed to minimise energy usage at times when the building is not occupied.

A multimotor feature that allows a single VSD to control several motors, and built-in logic functions that eliminate the need for a separate PLC are big money savers in a lot of applications. In addition, facilities for monitoring energy consumption is becoming increasingly important, to help users meet the metering requirements of Part L2 of the Building Regulations.

Fire mode operation is another big plus in fan applications, as it allows the drive to automatically override its own protection systems in the event of a fire, which means that the fans are kept running - and extracting smoke - for as long as possible.

Then there are communication features to be considered. Many HVAC drives are connected to building management systems, so suitable interface facilities are needed, most usefully taking the form of an Ethernet port and support for the TCP/IP communications protocol.
Drives for HVAC applications must also be physically small, as space for HVAC control systems is invariably limited. A high ingress protection rating, say IP54, is also required, as it allows the drive to be mounted directly on a wall, rather than needing to be built into a control panel. And, of course, essential elements like chokes, filters, keypad and display should be built in.

Nor should the reliability and durability of the drives be overlooked. The best will use components that have a typical life of ten years or more, with particular attention being given to the capacitors and fans, which are the most frequent cause of failure in low specification drives.

Ease of use is, of course, essential as even the best specified drive is of little value if its features are so hard to access that they never get used.

Finally, the drives should be well supported by their supplier. This means that expert technical guidance and advice should be readily available covering everything from the choice of drive, through installation and commissioning, to fault finding and maintenance.
VSDs have a lot to offer in HVAC applications, but to get the best results, it is important to choose the right drives for the job. Making this choice isn’t rocket science, but a little consideration given to the points mentioned will always pay big dividends.

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