Can the inverter replace the PLC in practical applications?
Published time: 2019-07-08
Both variable frequency drives (VFDs) and programmable logic controllers (PLCs) benefit from advances in processor technology and memory capacity. In the case of PLCs, this makes programming easier, components smaller, and able to handle more complex mathematical and logic applications. These same advances have also benefited VFDs, providing better motor control, simplified user interfaces and the ability to perform more complex applications. The ability to have VFDs for embedded pump-specific applications is now replacing PLCs in pumping applications and offers end users many advantages.
The main problem with PLC options is the proprietary nature of PLC programming. Unless the end user has a ladder logic programmer on the staff, the creation of the source code to run the PLC is written by an external company or an independent contractor. This necessity connects the end user to the programmer for initial programming. Additional fees are required for any subsequent revisions, additions or troubleshooting. The programmer may not want to publish the source code because it represents the trade secrets that the programmer uses and owns.
Therefore, while PLCs have advantages such as functionality and input/output (I/O) scalability and the ability to perform complex logic and math functions, there are some drawbacks. These include:
The source code is proprietary to the programmer.
The end user needs to write a detailed specification that the programmer uses to develop the requested PLC functionality. Changes that may have been omitted from the specification require an order change at the end user's cost. End users have limited ability to solve problems that may exist in the PLC code. End users do not always get functional documentation on PLC operation. The programmer can leave the end user without changing or checking the code.
While VFDs may not be able to replace PLCs for some complex pumping applications, in most simple pumping applications and even more complex applications, such as pump multiplexing, VFDs can be used instead of PLCs. Even with a PLC, adding a properly programmed pump VFD can reduce the I/O required by the PLC, reducing hardware and programming costs.
Some of the advantages of VFDs with embedded pump functionality are that the standard programming features listed by the VFD manufacturer are in the ready-to-use documentation. End users can access VFD programming. VFD manufacturers provide programming training courses for end-user employees. End users can make programming changes as needed. VFD manufacturers can provide end-user technical support in a variety of formats, such as knowledgeable sales people, authorized service providers, and telephone technical support. Costs vary by manufacturer, while others provide technical support for free, so ask wisely what types of help are available.
What types of operations can be performed by a VFD with embedded pump function?
Line fill or precharge function to decelerate fill empty lines to prevent water hammer from automatically adjusting life variables when line fill is completed, such as constant pressure, level or flow multiple sensors The input can automatically respond to system changes. For example, the constant pressure system's suction control multi-channel pump can maintain a wide range of demand changes while maintaining efficiency. The pump first detection loss has high action, fault or alarm high, low feedback detection without flow detection or Sleep, automatically restarts the lube oil pump in the lube application as needed, assists in controlling the tank level or low city pressure, or restricts and accumulates multiple set points and more functions depending on the manufacturer
One of the most complex applications of exclusive implementation of PLCs over the past few years has been to multiplex multiple pumps and turn them on and off as demand changes. This is preferred for determining the size of the pump at the expected maximum demand, which can cause the pump to run to the left of the performance curve. This condition will accelerate the wear of the pump and cause the wire to lose water efficiency. In some cases, the end user oversizes the pump in anticipation of future demand, causing the pump to operate in an inefficient mode for extended periods of time. If this increase is not achieved, the result is a waste of capital investment.
A more elegant option is to combine multiple pumps and discharge them into a common manifold. The use of multiplexable VFDs allows end users to purchase pumps that meet current needs and add additional pumps later when additional flow is required. All that is needed is a common manifold that is connected in an additional pump. For more than a decade, this technology has been successfully applied to agricultural and construction booster stations. Even if the maximum demand is known and the actual demand at any point is variable, the maximum demand can be divided by multiple pumps to better match its performance curve.
Any form of multiplexing provides the user with a variety of options, and losing one pump does not cause the entire system to shut down. If any pump fails, the other pumps are still running, and if the VFD fails, the remaining pumps can be bypassed. Quickly obtaining an alternative VFD is better than getting a replacement PLC programming in a short time.
VFD may not be able to replace PLCs in all applications. However, VFD is now able to replace the advantages of many end users with reduced procurement costs, enhanced setup controls, available technical support, employee training and overall standardization. VFD manufacturers have been working to add functionality, so in the future will be more VFDs with embedded pump-specific firmware.
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