Intelligent MCCs provide more functions, savings
There's been a lot of interest lately in using "intelligent" motor control centers (MCCs), but what defines an intelligent MCC? Typically, intelligent systems share three characteristics: control achieved via a microprocessor-based system; network technology used to replace hardwiring; and enhanced diagnostic or protective functionality.
There's been a lot of interest lately in using "intelligent" motor control centers (MCCs), but what defines an intelligent MCC? Typically, intelligent systems share three characteristics: control achieved via a microprocessor-based system; network technology used to replace hardwiring; and enhanced diagnostic or protective functionality. Intelligent MCCs fall into three main categories:
MCCs using bit-level networks to replace traditional hardwiring. Characterized by simplicity, these MCCs almost always deliver lowest installed cost; allow localization of failures to improve maintenance; and usually provide some added functions not economically feasible with hardwired systems. Installation and commissioning time is improved over traditional MCCs. Extensive system configuration or parameter management isn't required and the system can be maintained by most electricians. Concurrent engineering also is greatly facilitated. Bridges are available to most bit-level or byte-level networks and configuration is straightforward—an entire MCC is typically represented as one node. Computational resource and network bandwidth requirements are low. These systems are easily retrofitted to existing installations and so don't require "intelligent devices" to provide benefits;
Use of distributed I/O mounted in the MCC. Locating distributed I/O points in each MCC reduces installation and commissioning time and cuts installed cost. Construction is common across applications and provides clear segregation between automation and motor control. This can be important where division of responsibilities, existing spares, and training are overriding issues. Processors can be added to most distributed I/O systems to provide local control capabilities. Integration into PLC-based control schemes is straightforward and distributed I/O devices are available for popular networks; and
MCCs that use intelligent devices. Intelligent devices embedded in the MCC provide network communications, as well as functions not available on standard devices, such as network configuration, diagnostics, extensive process information, and advanced protection for each unit. Significant technical expertise is required, but the systems have excellent life cycle benefits in process applications where the added information, diagnostics, and protective features justify the incremental cost of the equipment.
In addition, when specifying intelligent MCCs, there are several cost, capability, and complexity-related variables to consider. It's important to realistically assess capabilities and objectives, remembering that added functions usually come with unneeded, added cost or complexity.
Because one size MCC doesn't fit all applications, a good supplier will offer a range of options. The manufacturer should be able to answer questions, and assist in selecting the best approach for the application. The supplier also should be able to integrate with multiple networks and existing control systems; be willing to work with third parties and system integrators that can add value; and provide total solution packages where appropriate. Finally, a complete functional test of the equipment prior to shipment is essential to realize reductions in commissioning time. Be sure the vendor intends to conduct the test by actually energizing the I/O connections, beyond simple point-to-point wiring checks.
David Ray, product specialist, Square D Co., Seneca, S.C.
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