How can VSDs make HVAC applications more efficient?

Published:  21 December, 2021

One way to make heating, ventilation and air-conditioning (HVAC) systems more efficient is through the use of variable speed drives (VSDs),as Marek Lukaszczyk* explains

VSDs are connected to more motors than ever in HVAC systems because they can provide significant energy savings. This is due to better speed control. In applications where traditional belts or gearboxes are used to lower speed, the motor still runs at full speed. Conversely, using a VSD in the same way serves to actually reduce the motor speed. The upshot of this effect is that fewer amps are drawn by the motor. In turn, there is less energy consumption, resulting in lower energy bills.

But, this isn’t the only reason VSDs are the best choice for HVAC systems.

Optimal capacity modulation

HVAC systems are designed to function at peak load, even though this requirement only occurs for a relatively short period of time in any given year. As a result, one of the most effective ways to improve the energy efficiency of a HVAC system is to make use of a VSD. The VSD can be deployed to vary the speed of one or more motors based on load requirements. Dramatic cuts in energy consumption can therefore be achieved.

Precise levels of control

The use of VSDs for HVAC applications provides more precise levels of control. A system utilising a VSD can exert more precise control over a wider range of flow rates and output, while simultaneously reducing energy requirements and pump wear. For example, a pressure sensor placed inside of an air duct can provide feedback to a drive (Proportional- integral- derivative) PID regulator, comparing desired pressure with actual pressure and delivering optimisation by adjusting the fan-motor speed. Besides a heating, cooling, and ventilation air- handling unit (AHU), proportional control can also be applied to cooling tower fans and chilled-water pumps.

One VSD to control multiple motors A VSD can be used to control multiple motors in some HVAC configurations. The only caveat is that the correct design considerations are applied, particularly with regard to implementing sufficient overload protection for each individual motor. This stipulation is necessary because a VSD can only detect the total connected load and not which individual motor is drawing high current. It should also be noted that not every variant of overload protection device can be applied at the VSD output. Sizing the VSD appropriately is a further requirement. If these prerequisites can be met, using a single VSD to control multiple motors brings several benefits beyond simple cost savings. To provide an indication, control complexity can be reduced, as can panel space.

Network monitoring

If networking capabilities are required for the HVAC system, VSDs are a good choice. The only requirement is the inclusion of a suitable communications card for BACnet, Modbus or any other proprietary network. Having this capability means VSDs can communicate a diverse range of data on factors like system energy consumption, equipment health and diagnostics. Power and energy consumption data points are available through most modern VSDs. A major requirement for buildings featuring HVAC systems is having knowledge of where power is being used, so monitoring this type of parameter will allow plantroom staff to collect this data, and trend it accordingly.

For example, monitoring the energy consumption of an air-handling unit (AHU) within an office area or conference room might show differences in usage on a month-to-month basis. There could be several underlying reasons behind this fluctuating data, including outdoor temperature changes, increased occupancy or the AHU filters beginning to load, which in turn heightens pressure on the fan.

With monitoring, staff will be able to map out certain trends and impart better system maintenance to optimise energy efficiency and reduce bills. This type of activity will also help identify root causes in the event of HVAC system failure. With a networked VSD, scores of discrete alarms can be monitored, logged and trended, so that staff gain a comprehensive history of events. Having this invaluable information resource also helps the maintenance team return the HVAC system to its full operational state as quickly as possible. There is much, much more data available over the network interface to the latest HVAC VSDs. These extend from straightforward monitoring points, perhaps of voltage or current, to high- level analysis points that could include power factor or harmonics. Using VSDs in HVAC systems is better for both profitability and the planet. In HVAC applications, VSDs offer dramatic energy savings, more precise and simplified system control and better network monitoring. With such a host of benefits, VSDs are the obvious choice for HVAC systems.

*Marek Lukaszczyk is European and Middle East marketing manager at the global manufacturer and supplier of motors and drive technology, WEG

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