Networking VFDs in high-performance buildings

Historically, the use of variable frequency drives (VFDs) has been limited to large motors and projects with healthy infrastructure budgets. However, as each iteration of the energy code mandates variable flow control of mechanical systems on smaller systems, VFDs have become more commonplace. While their implementation has been typically limited to providing code-required variable flow, VFDs are capable of communicating a wealth of information on system energy consumption, equipment health, and diagnostics.


This article is peer-reviewed.Learning objectives

  • Realize that variable frequency drives (VFDs) are standard for improving energy efficiency in buildings. 
  • Understand the ability to integrate VFDs with the building’s automation or management system.
  • Learn how VFDs can enhance high-performance buildings.

The past 20 years the commercial building industry has seen tremendous change in countless aspects of building design, construction, and operation. Variable frequency drives (VFDs), once a luxury reserved only for high-budget projects, have become the accepted means for complying with energy requirements for motor-speed reduction on all but fractional-horsepower motors. Traditionally, VFDs are controlled and monitored via simple hard-wired signals, while the extensive data and control available via network integration is left unused. There are good reasons why this has historically been done, but are those reasons still valid? What control and monitoring is left on the table? What value is obtained when network integration is completed? What challenges exist when considering integration?

Historical operational issues

The dynamics of commercial building construction and operations have changed over the past several years. The International Energy Conservation Code (IECC), building management systems (BMS), and building operations have undergone tremendous changes. While the IECC and ASHRAE 90.1—Energy Standard for Buildings Except Low-Rise Residential Buildings have driven the basic energy code requirements, sustainable building design and various green building organizations have pushed the envelope toward sustainable design. 

ASHRAE produced Standard 189.1 for Design of High-Performance Green Buildings, and in 2015, the International Code Council released its first International Green Construction Code. Fundamentally, depending on the climate zone in which a building is located, these codes have driven a reduction in the energy-use index of 39.3% from 1989 to 2010, according to a 2014 review by the U.S. Department of Energy. Through the efforts of countless organizations, sustainability has become part of our way of life—and with it, social pressure on businesses to not only be sustainable, but also to make it part of the public relations message. Though many organizations are choosing not to pursue the formal green building certifications available, many see the benefits that high-performance buildings offer and design new projects with this in mind.

Figure 1: Networking variable frequency drives (VFDs) gives the operator visibility to myriad additional data points for troubleshooting and energy optimization. The additional data, when properly trended, also provide valuable information to technicians in the event of equipment failure. Courtesy: Page

Around 20 years ago, most businesses maintained a staff of building operators—HVAC mechanics, electricians, plumbers, and building engineers—whose sole responsibility was to ensure quality, reliable service to the building occupants. These individuals knew their buildings and the tricks to keep them running. They had tenure at these facilities and knew which individual was always hot or cold. However, recently, more companies have begun to view building operations less as a critical part of the business, and more as unprofitable overhead. In many cases, building operations are now even considered a cost center by some businesses, with pressure to operate as lean as possible, which includes minimizing utility costs. They are frequently viewed as a necessary evil, because without an effective operations team, the occupants cannot perform their jobs. However, the resultant attitude is that many companies have either downsized the operations staff or outsourced their building to the lowest bidder. This results in significant turnover in staff, which also can include a reduction in physical bodies. In either case, facility knowledge is lost, which also includes the ability to diagnose and correct operational issues.

Technical issues

Two decades ago, a major spec issue for BMS projects was whether the system was Windows-based or DOS-based. Capturing meaningful trended data was difficult, as many systems connected the direct digital controls (DDC) controller to the server via twisted-pair serial wiring. Data throughput was a major challenge. Systems now typically operate on a thin-client (Web browser interface), connected to servers with large hard drives and communicate with the DDC controllers via Ethernet cabling. Data throughput and storage is no longer a problem in many facilities. Though not within the scope of this article, it is important to note the extensive use of Internet protocol (IP) communications in a present-day BMS does present a security vulnerability when the BMS resides on the same network as the corporate information technology (IT).

As the commercial energy code, public relations, and a new generation of employees drive for more energy efficiency, tighter construction budgets mean fewer safety factors for infrastructure design. The smaller safety factors mean that less significant operational issues, which in previous years were transparent to the occupants, now create disruptions to the building operations. The view of building operations as a cost center now also means that losses in energy efficiency and occupant disruptions are more highly visible and critiqued.

In short, businesses expect greater energy efficiency with less disruption to the occupants, but with smaller budgets and less staff who have less experience at the facility. So, how does VFD networking help achieve this goal?

Enter VFD networking

Networking, or network integration, refers to the process of connecting to devices in the building via a communication protocol for the purpose of monitoring and/or control. The BMS of the past had one pair of wires for each signal, which typically used either contact closures, 0-10 V dc, or 4-20 mA signals to either control or monitor a single piece of data. Depending on the installation, these discrete inputs and outputs may or may not have been physically connected to the device performing the control function. Networking allows direct communication between the controlling device and the VFD. Controls contractors typically factor in the count of monitoring and control points in their price. In an office building where there could easily be a few dozen VFDs, the cost of adding a hard-wired point to each VFD can quickly result in significant costs. Due to these limitations, VFD control and monitoring traditionally were limited to start commands, status feedback, speed command, and possibly a general alarm status. By networking VFDs, far more data is available. Information, such as motor frequency feedback, specific alarms, and whether the VFD is in “hand,” “auto,” or “local off” modes can be monitored and displayed by the BMS. While this data has long been available via networking, one wonders if reasons other than cost prevented this functionality from being used.

Figure 2: Monitoring data from variable frequency drives (VFDs) is only part of the solution. Trending at appropriate sampling rates to correctly see equipment and system response in the event of an alarm is critical to performing a root-cause analysis. Courtesy: Page

Among others, the number of communication protocols on the market was dizzying. Each controls manufacturer had their own proprietary protocol, while none of the manufacturers were particularly strongly integrating with other protocols. This made coordination of communications between VFD manufacturers and controls vendors a headache with which few were willing to deal. In 2004, ASHRAE issued ASHRAE 135-2016: A Data Communication Protocol for Building Automation and Control Networks (BACnet Standard) in an attempt to create a standard communication protocol for the HVAC industry to aid in the process of sorting through the muck. While the process has taken time, and the protocol has its drawbacks, the underlying goal of creating a common language that allows multiple manufacturers’ equipment to communicate with one another has been achieved. It is important to note that past reliability issues with equipment networking have been substantially reduced through time and experience in installing communication networks as well as the BTL (BACnet Testing Laboratories) Certification process, which certifies individual devices for compliance with the BACnet communication standard. There are many additional challenges associated with the physical network infrastructure, which are beyond the scope of this article, though many are tied back to the logistics of how communications networks are physically installed.

Another major roadblock to increased integration is the manner in which BMS contractors manage their pricing structures and front end. With the exception of Niagara, most BMS server databases and graphics development platforms are proprietary. Many owners and engineers also are unaware of the difference in the efforts required to add a networked point versus a physical point. After the device has been networked, the process of adding additional points, provided the equipment is already configured to broadcast the desired point, typically takes a matter of minutes (multistate points, such as equipment status, may take longer due to the information available). Compare this to ensuring a physical input/output (I/O) point exists on a controller, having an electrician run conduit (depending on the project) and wire to the device, and making terminations, and the relative cost should be clear. However, many controls vendors still charge the same for networked points as they do for physical points. It is important to note, however, that in both cases there will be cost for adding the points to graphical interfaces, adding alarm conditions, and adding trends. Though BACnet allows controllers and equipment from different manufacturers to talk to one another, the proprietary nature of most server databases and graphics platforms puts most owners at the mercy of their installed vendors’ pricing to make those new points visible to the operators.

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