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How to Optimize HVAC to Save Energy

According to the U.S. Department of Energy, HVAC (Heating, Ventilation, and Air Conditioning) systems consume 40% to 60% of a facility’s power. It stands to reason, then, that optimizing the HVAC system is one of the first steps a plant operator should take to reduce operational expenses.

Approach 1: Manual Motor Control with a Variable-Frequency Drive

The typical HVAC distribution system uses several fans and pumps to move air and water around a plant, with the “drive” one of the most important components required to achieve this. As part of daily operation, the HVAC system needs to be adjusted to maintain a fixed temperature and stable environment. For example, office buildings must ensure that the working environment is comfortable enough for their employees, and in a plant, some processes or refrigeration units require keeping the temperature at an extremely low value. Before, end-users could only turn the power on and off to control the drives manually. But now, end-users can take advantage of variable-frequency drive (VFD) technology to control drive speed. The VFD is a very important aspect of modern industrial technology, particularly since the drive speed of a VFD can be used to control a measurable environmental factor (temperature, for example) to keep it at a fixed target value. Another important benefit of VFDs is that drives can be started more smoothly, which helps protect the drives and reduce damage. In fact, many applications don’t require the drives to work at full speed, which means that most of the time, the drives do not need to operate at full steam. Being able to adjust the speed of a drive can save a lot of energy, and this is one of the main reasons why more and more end-users would like to incorporate VFDs into their systems. Moreover, end-users can now get VFD status information, allowing them to see if the VFD is operating abnormally so they can repair it as soon as problems arise. Fixing problems quickly can help reduce maintenance costs and downtime.

Approach 2: Automatic Motor Control with a PLC-VFD Combination

Research has shown that incorporating VFD technology into an HVAC system can result in an energy savings of up to 60%. That’s all well and good, but perhaps still not good enough. Many end-users using VFDs with their HVAC systems don’t use automatic control, even though most modern VFDs support basic communication options that can be used to set up automatic VFD control. More specifically, most VFDs come with an RS-485 interface that supports the Modbus RTU protocol. The challenge for end-users is that they need to use PLCs to implement the “automation” part of the control, and different PLCs use different protocols.

Another challenge that end-users face is how to set up sensors that can feed back the necessary environmental readings to control the VFD drive speed.

There are several ways to deal with protocol incompatibilities, but the most cost-effective way is to use a protocol gateway, which is a standalone device that converts data from one protocol to another. For example, one option would be to use a PROFIBUS PLC and Modbus RTU VFD. In this case, the gateway actively retrieves the VFD state variables, and then stores them in internal memory. As for the PLC, it uses the PROFIBUS protocol to process data exchanged with the gateway. If the PLC needs to send a command to the VFD, it uses the PROFIBUS protocol to transfer the new value to the gateway’s internal memory, after which the gateway sends the new value through a Modbus RTU to the VFD.

Protocol Conversion, Check! But, What’s Next?

Moxa’s MGate gateways provide a complete protocol solution, including support for Modbus RTU/TCP, PROFIBUS, PROFINET, and EtherNet/IP, and MGate gateways meet most VFD integration requirements. However, for many users, only meeting some requirements isn’t enough. Two major must-have requirements are easy installation and easy maintenance. Since one gateway could be connected to several VFDs, it would be extremely valuable for end-users if the gateway provided a user-friendly interface and configuration tools. MGate does just that by providing a user-friendly web interface for easy installation, with all protocol settings configurable through the web browser. In addition, to address the “easy maintenance” requirement, features such as a system log, alarm, and protocol analysis tool provide end-users with valuable information. MGate provides diagnostic information that allows end-users to troubleshoot and solve problems easily. The diagnostic information includes possible internal memory mapping errors and protocol parameter errors, and the built-in protocol analyzer supports a traffic capture feature for conducting detailed analyses. Because protocol settings are the most complicated aspect of gateway configuration, the availability of key diagnostic information can reduce the time end-users spend on troubleshooting by a significant amount.

Learn more about the innovations that make it easier to install and maintain energy-efficient HVAC systems here.

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