Variable frequency drives (VFDs) are used in a number of industrial applications to save energy by controlling the speed of motors. The devices play an important role considering electric motor-driven systems (EMDS) account for about 45 percent of all electricity consumption. This article shows how industrial Ethernet gateways can be used to create a communication gateway between VFDs and programmable logic controllers (PLCs) that is easy to install and maintain.

According to the International Energy Agency (IEA), EMDS collectively use more electricity than any other single electrically powered application. Their applications consume more than twice as much electricity than lighting, which comes in a distant second. It is estimated that globally their applications account for about 45 percent of all electricity consumption and create about 6,040 megatons of CO2 emissions per year. In addition, by some estimates the applications account for 70 percent of the electric energy used by industry, with common uses including pumps, fans, compressed air delivery, conveyors and motive power for other machinery.

The bottom line is that to save energy and promote energy efficiency in industrial applications, the amount of energy consumed by EMDS applications must be reduced. This is where VFDs come in. The drives can control motor speed to save energy, decrease inrush current and apply a specified amount of torque.

The drives can be integrated into a control system through either a digital input/output (I/O), analog I/O or communication interface connection. The communication interface is accessible via either RS-232, RS-485 or Ethernet, and provides operators a convenient way to control the VFD and obtain information about it that might otherwise be unavailable. The main problem, however, is handling conflicting protocols. Most VFDs only support the Modbus remote terminal unit (RTU) protocol because it is easy to support and used widely for industrial automation applications. PLCs, however, are more likely to use an industrial Ethernet protocol, such as Modbus TCP or EtherNet/IP.

Communication requirements for VFD monitoring & control

A supervisory control and data acquisition (SCADA) system usually includes a human machine interface and a personal computer (PC), as well as PLCs and RTUs. The PC is normally used to issue supervisory level commands, with most of the control performed by the PLCs and/or RTUs. Even though PLCs and RTUs perform similar functions, key differences exist. PLCs, for example, have more sophisticated embedded control capabilities, making them more suitable for local control operations such as production lines. On the other hand, RTUs are suitable for connecting to devices distributed over a wider area because they have telemetry hardware capable of sending digital data to the supervisory system.

Because VFDs are often used to control motors that operate pumps, fans or conveyor belts — which means they are part of a local control system — it would be expected that the VFDs are connected to a PLC. For example, a typical architecture for controlling air quality in a factory building requires the SCADA operator to set a CO2 target level in parts per million (ppm). The resulting control system reads the current CO2 level from meters designed expressly for that purpose, and then the PLC issues the appropriate control signal to tell the VFDs how to adjust the speed of fans. To ensure that the optimization control system operates as expected, provisions need to be made to guarantee that the VFDs use the correct industrial protocol to communicate with the PLC.

Existing solution

First, PLCs available from different vendors do not necessarily support the same fieldbus protocol. For example, most Siemens PLCs support PROFINET, Rockwell PLCs support EtherNet/IP, and Schneider PLCs support Modbus TCP. Individual vendors accommodate other fieldbus protocols by providing communication modules that are installed right next to the PLC. The advantages of using PLC communication modules include ease of installation and reliability. Because the modules are connected directly to the PLC, users do not need to install additional wiring. On the downside, the disadvantages of using PLC communication modules are their high cost and limited versatility.

On the VFD side of the connection, most vendors use Modbus RTU is the protocol used. When the need arises to accommodate other fieldbus protocols, most vendors provide printed circuit boards (PCB) that plug directly into the VFD to enable the necessary protocol conversion. The advantages of using VFD communication modules are essentially the same as the advantages of using PLC modules. In addition, because the PCBs plug directly into a slot inside the VFD, they do not take up any extra space. The cost is not particularly high if only one PCB module is needed, but if  two, three or more units need to be installed, two, three or more times as much money will need to be spent to get the solution up and running. The amount of time required to install all of the modules should also be factored in.

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Industrial Ethernet gateway benefits

An alternative to investing in PLC and/or VFD communication modules is the industrial Ethernet gateway. A gateway is a standalone device that converts a signal from one protocol to another. For example, a gateway could convert Modbus RTU to Modbus TCP, or convert Modbus RTU to PROFIBUS. The gateway achieves this with a built-in central processing unit (CPU) and memory storage capability. In effect, the VFDs and PLC communicate directly with the gateway instead of with each other. The process is transparent to the VFDs and PLC since the gateway handles all of the necessary protocol conversions.

Using an industrial Ethernet gateway solution requires time learning how to use the gateway and effort configuring and managing gateways as they are added to the system. However, compared to using PLC and/or VFD communication modules, the gateway provides several benefits. A gateway that comes with a user-friendly interface and good management functions minimizes the additional effort required to configure and manage the gateway.

A good way of distinguishing the gateway solution from the PLC/VFD module solution is to compare the two solutions from a hardware and software perspective.

Cost-effective hardware integration

When using a VFD communication module, its housing must be removed to install the module. This operation could take a significant amount of time, particularly if dozens of VFDs need to be upgraded, in which case the VFD module solution could be costly. In fact, it might be hard to find a system integrator who will agree to upgrade an existing system in this way. As far as PLC modules are concerned, installation is easier, but one PLC slot must be reserved that otherwise could be used for a different purpose. If PLCs are in a different location than the devices, PLCs only support a serial connection. Using a gateway to connect devices to a common Ethernet base is the preferred solution because of the flexibility it adds to the system. Another factor to consider is that with an Ethernet-based environment, fiber can be used to avoid interference from electromagnetic noise.

Versatile software integration

The most important benefit of using a gateway solution is versatility. It is easy to see this if different aspects of the VFDs need to be monitored after the system is up and running, or if there is a need for additional VFDs to be installed in the system after it has already been in operation for one or more years. In fact, this is a common situation faced by engineers responsible for managing a factory. With a gateway solution, I/O space can be reserved in the PLC and the gateway configuration for future expansion.

When it comes time to add new VFDs, only the gateway will need to be configured instead of tweaking the PLC software. If a PLC module solution is used, PLC software will need to be updated by adding commands for dealing with the module. Another advantage of using a gateway is that it has built-in CPU and memory that enables the gateway to handle protocol conversion by itself, instead of using the PLC for this purpose. With a gateway solution, systems will be simpler and easier to maintain.

Casper Yang is senior manager for Industrial Device Connectivity at Moxa Inc., and Edward Lin is product manager for Industrial Ethernet Gateway at the company.  Yang leads the teams behind a serial device server and industrial Ethernet gateway business. Lin is in charge of product development for industrial protocol gateways, with expertise in Modbus, PROFIBUS/PROFINET, EtherNet/IP and other protocols.