Both frequency converters (VFDs) and programmable logic controllers (PLCs) have benefited from advances in processor technology and memory capacity. In the case of PLCs, this results in easier programming, smaller component sizes, and the ability to handle more complex mathematical and logic applications.
These same enhancements also benefit VFDs, offering better motor control, a simplified user interface and the ability to run more complex applications. The functionality of VFDs with built-in pump applications is currently replacing PLCs in pumps, bringing numerous benefits to the end user.
The main problem with the PLC option is the closed nature of PLC programming. If the end user does not have a ladder programmer, the source code to run the PLC is generated by a third party company or independent contractor. This need links the end user to the programmer for initial programming. Any subsequent changes, additions or troubleshooting will incur an additional charge. Programmers may not want to publish source code because it is a trade secret used and owned by the programmer.
So while PLCs have some advantages, such as functionality and input/output (I/O) scalability, and the ability to perform complex logic and math functions, there are also some disadvantages. These include:
While a VFD cannot be a replacement for a PLC for some complex pumping applications, a VFD can be used in place of a PLC in most simple pumping applications and even in some more complex applications such as pump multiplexing. Even when using a PLC, adding a properly programmed VFD pump drive can reduce the I/O required by the PLC, reducing hardware and programming costs.
One of the most challenging applications over the past few years has been the exclusive implementation of a PLC that integrates multiple pumps and turns them on and off as demand changes.
This is preferable to pump sizing based on expected maximum demand, which can result in the pump operating on the left side of the performance curve. This condition accelerates pump wear and makes the wires less efficient for water. In some cases, end users are using pumps that are oversized for future demand, causing the pump to operate inefficiently for extended periods of time. If this increase is not realized, the result is wasted capital investment.
Using a variable frequency drive that supports multiplexing allows the end user to purchase a pump that meets current needs and add more pumps later when additional flow is required. All that is required is a universal manifold to connect additional pumps. This technology has been successfully used in agricultural and construction booster stations for more than a decade.
These systems vary in size from a few 10 HP pumps. to pressurize buildings to several 500 hp lifting stations feeding large circular trusses. There are currently hundreds of such systems in operation in the United States.
Even if the maximum demand is known and the actual demand at any point is variable, the maximum demand can be assigned to multiple pumps to better match their performance characteristics.
Any form of multiplexing gives the user multiple options, and the failure of one pump does not result in the failure of the entire system. If any pump fails, the other pumps continue to run, and if the VFD fails, the remaining pumps can run in bypass mode. The prospect of quick VFD replacement is better than programming a replacement PLC in a short period of time.
VFDs may not be able to replace PLCs in all applications. However, VFDs can now replace many products for the benefit of the end user through reduced acquisition costs, improved setup management, technical support, staff training and overall standardization. VFD manufacturers are constantly working on expanding functionality, so the future is for a large number of VFDs with built-in firmware for specific pumps.
Dan Peters is an application engineer at Yaskawa America Inc. He can be contacted at danny_peters@yaskawa.com. For more information, please visit www.yaskawa.com.
Post time: Apr-28-2023