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Finding a Better Protocol Conversion Solution between PLCs and Devices

In the November 2014 issue of Moxa Connection, the article How to Optimize HVAC to Save Energy emphasized the importance of managing the energy consumption of HVAC (Heating, Ventilation, and Air Conditioning) systems in facilities, and walked you through how a protocol conversion solution can be used to bridge PLCs and VFDs (Variable Frequency Drives; also called Variable Speed Drives) to save energy by automatically controlling motor speed. In this article, we present an actual case study of how a chemical fiber plant in China is using this strategy to save energy in their production line.

Background: Customer’s Requirements

VFDs have been widely used for many years in China’s chemical fiber industry to reduce energy consumption. However, in the past, the VFDs were controlled manually. That is, an engineer would need to adjust the speed of a VFD manually to ensure the best manufacturing quality, and they would need to check the result and adjust the VFD manually many times over. This type of manual operation is not only time consuming, it is also non-optimal, since the engineer cannot maintain precise enough control over the VFD. For this reason, a chemical fiber engineering company in Dairen decided to implement automatic VFD control using PLCs. However, since the VFDs used the Modbus RTU protocol, and the PLCs used the PROFIBUS protocol, they could only implement a PLC-to-VFD solution if they were able to find an appropriate protocol converter with the following features:

  1. A cost-effective solution that can simultaneously transmit large amounts of data from VFDs scattered around the plant, and convert between Modbus RTU VFD and PROFIBUS PLC.
  2. Suitable for industrial environments that are subject to high levels of electromagnetic noise.
  3. Must be easy to configure and deploy.

Industrial Ethernet Gateway Solution

One option would be to install a PROFIBUS communication module in each VFD, but the cost of purchasing and setting up so many modules is prohibitive. Moxa’s MGate 4101-MB-PBS Modbus RTU to PROFIBUS protocol conversion gateway provides a better solution by allowing engineers to connect Modbus RTU devices directly to the PROFIBUS network. Since the MGate can fetch and store data from all of the Modbus RTU devices, the MGate can be used as a PROFIBUS slave to exchange and process data with the PLC. In addition, the MGate 4101-MB-PBS is designed with a number of important industrial features, including a wide -40 to 75°C operating temperature range, and IEC 61000-4 ESD/Surge/EFT Level 3 compliance for high electromagnetic compatibility. The high EMS design is an important feature for VFD communication because VFDs often generate enough electrical noise to completely degrade the gateway’s serial interface signal. In addition, since the MGate can connect up to 31 Modbus RTU RS485 devices to a PROFIBUS PLC, it’s easy to understand how Moxa’s MGate can save up to 50% of the cost of using VFD PROFIBUS communication modules, particularly since with MGate, the cabling and labor cost is reduced significantly. The bottom line is that the MGate plus VFD solution helped the plant reduce their energy consumption by up to 25%.

Moxa’s MGate also supports QuickLink, an innovative Moxa technology that simplifies the configuration of gateways to enable seamless communication between Modbus and PROFIBUS protocols. QuickLink supports intelligent AutoLearning and AutoMapping, and replaces traditional methods of gateway configuration that are tedious and time-consuming. QuickLink is a standard feature of Moxa’s MGate Manager utility, and allows system integrators to finish their gateway configuration in just minutes, and most importantly, ensures that configurations are done right the first time, virtually eliminating the possibility of human error.

Check out the YouTube video for a demo of QuickLink or click here to download our free white paper. For additional case study details on how to extend energy savings even further by using a more efficient data acquisition system, download our application note.

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