VFDs reduce costs through process optimization

Cover story: Variable frequency drives (VFDs), modern motor controllers, that communicate readily with process automation controllers can achieve much higher efficiency and process improvements by providing power consumption details. See VFD sizes, benefits, how to match a VFD to a process, and four questions to ask when specifying a drive.
By Tom Lowery October 15, 2015

Figure 1: Modern VFDs allow for control and monitoring online or wirelessly with a tablet or mobile device. Courtesy: Schneider ElectricBy incorporating communication between modern motor controllers, such as variable frequency drives (VFDs) and process automation controllers, it is now possible to achieve much higher efficiency along with process improvement by understanding in greater detail where and why power consumption varies. Companies are taking a greater look at process efficiency to protect the environment and enhance their bottom line. Resources are being stretched more than ever, and the demand for efficient use of those resources is being seen across industries. Evolving technologies that increase process efficiency and provide detailed information on energy use have helped, offering greater insight into the level of efficiency for their respective processes.

Equipment efficiency also has been a focus of federal regulators and industry organizations. The U.S. Department of Energy, for example, has convened a panel of experts to analyze the energy use, cost, and benefits associated with commercial and industrial pumps with the goal of developing energy conservation standards. They are also in the preliminary stages of examining commercial fans and blowers with a similar goal. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI), an industry organization, recently developed the latest certification for VFDs—Standard 1210/1211—that is used in many of the applications with fans and pumps. 

VFDs drive efficiency

Many companies have looked to install modern VFDs—or update older drives—in their processes to improve efficiency. VFDs are designed to vary a motor’s speed electronically to match demand within a given process.

These devices become critical to process improvement and often contribute to overall efficiency gains, without sacrificing system performance.

VFDs are available in different sizes and are paired based on the motors that they will support. 

VFD sizes, ratings, uses

Available sizes, ratings, and usage of VFDs follow:

  • Low voltage (600 V and less), fractional horsepower up to 2,000 hp
  • Medium voltage (2,400 V and higher) 500 hp up to 30,000 hp
  • General duty usually defined as 110% current overload capacity for up to 1 minute, support pumps, fans, and other variable torque loads
  • Heavy duty requiring 150% drive output current for the same 1-minute time period 
  • Support conveyors, compressors, and other constant torque loads.

Newer VFDs offer key enhancements to improve efficiency, including power conversion and power measurement. Power conversion circuits transfer the power from the distribution source to the motor with newer models accomplishing this more efficiently than ever. Insulated gate bipolar transistors, or IGBTs, and power capacitor technology have improved to reduce the overall heat generated in the drive (their largest contributor to energy loss), allowing for a drastic size reduction—often up to half that of older units—while still providing greater efficiency. Using the new power conversion semiconductors along with higher speed processors, coupled with low impedance conductors and improved capacitors, allows VFD design engineers to greatly improve their efficiency without compromising performance.

Power measurement has been greatly improved as well through enhanced sensors. This, when combined with network connectivity (Ethernet, embedded web server, and Wi-Fi), allows users to determine the exact power consumed by an individual motor running on their VFDs. By closely monitoring individual motor energy consumption, inefficiencies can be identified and action taken to improve it. Often in a process with multiple VFD/motor combinations, the one consuming the most energy can be addressed without impacting the others or the overall process throughput. 

Modern VFD benefits

Figure 2: Energy costs represent ~40% of the pump’s total cost of ownership. Courtesy: Schneider ElectricIn the past, when a drive indicated a trip condition, the user was left to largely guess at what had caused the condition. Did an interlock open? Was there a power spike? Did the motor get overloaded? Or was it something else? Modern VFDs are now capable of alerting users to not only the exact cause of a trip condition but can record conditions leading up to the trip that may indicate entirely different causes for that eventual trip.

For example, in the event of a motor overcurrent, the VFD would alert users to the increasing amp draw by the motor and provide data showing this long before an actual trip would occur, shutting down the process or showing a sharp instantaneous motor demand for current explaining the cause.

Another innovation for modern VFDs is the ability to produce dynamic QR codes to communicate critical information to users directly. Users with a cell phone or other mobile device can scan the code, and critical information on their drive is delivered literally to the palm of their hand. This can include diagnostics and suggested corrective actions, and they can even send a report directly to the manufacturer’s customer service teams who can then analyze and suggest corrective customer actions.

Learn more about four tips on how to specify a VFD.

Four ways to specify a VFD

Figure 3: VFDs vary by size to match the process motor that they support. Courtesy: Schneider ElectricWhen specifying a drive, it is most important to know the details of a process to identify the size and model that best matches the need. Here are four key questions to ask:

1. What are the specifications for the process?

This is a great place to start because it addresses the basics. Knowing if the process is located inside, outside, or in a dusty or corrosive environment is important. Next would be the motor characteristics; speed range, voltage, and horsepower are the minimum requirements.

2. What controls/functions would benefit the process?

For many processes, the functions available on one drive versus the other can make a big difference in optimizing their process. As an example, for pump systems, new VFDs offer enhanced features, such as sensorless flow and sensorless pressure, to better support those processes. Using these built-in algorithms, users are no longer required to mount and wire a flow or pressure sensor downstream of the pump to provide feedback to the VFD.

3. How will we communicate with the drive?

Integrated hardware allows the VFD to communicate within the user’s existing systems or communicate wirelessly with them through their mobile devices. Many modern VFDs link to communication networks through RS485, Ethernet, LonWorks, Modbus, FLN, N2, BacNet, and/or MSTP.

4. What additional options would benefit the process?

Many modern VFDs offer additional integrated features, such as bypass, harmonic filters, dV/dt filters, and multi-motor output that were previously unavailable or required additional components to be purchased and integrated into the drive system. Additional control functions may also be available, including safety interlocks, safe torque off, start/stop command, and more to provide additional process benefit. 

Build a smarter process

While it may seem like an undertaking to incorporate a modern VFD into an older/legacy process, it is often easier than one may think. These systems can sometimes gain the greatest benefit from the new technology. Modern VFDs can usually be placed into the system as a "drop in" requiring little to no retrofitting. By doing so, the older system can benefit from enhanced monitoring capabilities, including operator alerts programmed to signal system changes or required equipment maintenance.

To begin optimizing a process, there’s no need to replace every existing drive with a modern VFD. A general guideline would be to select the most critical loads in the process and establish a plan of action to replace those first. If higher efficiency is desired, usually large HP loads provide faster return on investment (ROI) because they consume the most power and are often the most critical drivers of processes.

Funding can be planned longer term in an evolving system. Once some of the loads are upgraded and benefits realized, it is often much easier to obtain Opex or Capex (operations or capital expenditure) funding to continue the overall process upgrade. Gains in productivity or reduction in operating expense due to efficiency gains can be used to fund evolving projects. Utilities may also participate via rebates to provide incentives to customers to reduce overall or peak energy consumption, reducing the ROI time period.

Care should be taken to make sure that the control system is examined for compatibility with newer drives. While most existing process controllers or programmable logic controllers (PLCs) will be able to integrate the new drive seamlessly into the system, additional monitoring and operating data must be sent by the drive to some intelligent controller that can act on the data. With built-in web servers, connections to the Internet should be carefully planned so that operators can act on the accumulated data. Security is often a concern as well, so careful planning on access to this data should be considered. 

Increase pump efficiency

VFD efficiency has been studied greatly in pumping applications, where energy costs represent approximately 40% of the total cost of ownership (TCO) for the life of a pump. With the addition of a VFD, it is possible to reduce the electrical consumption by at least 30%. Maintenance is also a key cost for pump systems, representing 25% of the TCO.

While these costs are unavoidable due to the wear of components during system operation, they can be reduced by installing a drive that provides greater control of the pump to reduce the impact of the operation. This benefits the efficiency of the system by reducing the need for maintenance and the resulting lost production during down-time.

Knowledge to power

In residential applications, the evolution of thermostats—from simple dials to smart digital technologies—has provided more information on energy usage, allowing better cost control. The same can be said for modern VFD technologies for industrial processes. By providing more information on power use, process efficiency, and maintenance needs, these devices allow businesses to improve efficiency and optimize processes to reduce the resources required and support the bottom line.

– Tom Lowery is marketing operations manager industry drives business, Schneider Electric. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske@cfemedia.com.

Key concepts

  • Variable frequency drives optimize processes and save energy.
  • Information about processes can augment energy savings.
  • Properly matching a drive to an application is important.

Consider this

Cost considerations for modernizing motor use with VFDs should include process improvements as well as energy savings.

ONLINE extra

About the author: Tom Lowery is the marketing operations manager for the Industry Drives Business at Schneider Electric. With more than 25 years of experience in VFD applications, Lowery has worked within industry organizations to promote energy efficiency in many different types of equipment. He currently serves as the AHRI – VFD engineering committee chairman, who authored and handles Standards 1210 and 12111. In 2015, he was elected this year as chairman of ASHRAE TC 1.11, Motors, and Motor Control. Lowery holds a BEE degree from Villanova University and has been an active author.

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