Understanding intelligent motor control centers
Lately, there has been a lot of interest in the use of "intelligent" motor control centers (MCCs). But defining intelligence is not as easy as it sounds. To answer the question of intelligence, first take a look at traditional MCC technology.
Lately, there has been a lot of interest in the use of “intelligent” motor control centers (MCCs). But defining intelligence is not as easy as it sounds. To answer the question of intelligence, first take a look at traditional MCC technology.
The role of a basic MCC is to provide a compact, modular grouping for motor control and electrical distribution components (Fig. 1). Think of an MCC as a filing cabinet, with drawers full of combination starters, lighting contactors, and other electrical distribution and control products. Historically, MCC units were electromechanical, with basic functions that included a power switching device, short-circuit and overload protection, local and remote actuation, and controller state indication.
There are a number of reasons for MCC popularity:
MCCs provide a single source for coordination of components. Electrical distribution and motor control equipment can be purchased as a pre-assembled, pre-tested system—usually at a less expensive installed cost when compared to separately mounted controls.
MCCs require significantly less line-side power wiring than separate controls, which makes them easier to install.
MCCs come in space-efficient packaging, with excellent configuration flexibility. They also centralize maintenance.
MCCs offer excellent fault containment and electrical component isolation.
MCCs are used in applications where there are multiple, remotely controlled loads linked to a central control point. In industrial plant applications, the central control point is usually an environmental control system, with HVAC, pump, and fan loads. Industrial process applications include many load types, such as conveyors, pumps, fans, and mixers. These loads often require coordinated control and can be the source of important process or machine diagnostic information. Distributed control systems (DCSs) or PLCs normally are used to provide this control and data acquisition functionality.
Intelligent motor control systems
Use of open networks, distributed I/Os, and electronic components is one way of defining an MCC as intelligent. Typically, intelligent systems have three things in common:
Control is achieved through a microprocessor-based system
Network technology is used to replace hardwiring
Some degree of enhanced diagnostic or protective functionality is included.
While many associate these technologies with intelligence, another definition has nothing to do with the technology involved. In today’s competitive business environment, perhaps the definition of an intelligent MCC should be as simple as a unit that provides equivalent or greater functionality more economically. In other words, an intelligent MCC is simply an MCC that maximizes the value of the components in a given application.
But how does one determine maximum value? Is it based on the cost of the equipment? That definition doesn’t take into account the total cost of the unit. In other words, the cost to design, specify, purchase, install, commission, operate, maintain, and upgrade an MCC are all calculated into the total cost of ownership.
Each individual in the chain is likely to define intelligence related to his or her perception of value. For example, consultants who design and specify equipment are interested in the reliability and performance of the components. A contractor may be primarily concerned with installed cost. A facility manager is likely to be worried about ease of operation and maintenance costs. And operations personnel are focused on the functionality and durability of the entire system. In other words, regardless of a person’s function, a truly intelligent MCC will optimize the value of the equipment specific to that function.
There are several ways in which MCCs can be made more cost-effective.
Bit-level networks to replace traditional hardwiring
MCCs in this category are characterized by simplicity. They will almost always deliver the lowest installed cost. They allow localization of failures to improve maintenance and usually provide some additional functionality not economically feasible with hardwired systems.
Installation and commissioning time is significantly improved over traditional MCCs. They do not require extensive system configuration or parameter management and 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 do not require “intelligent devices” to provide benefits.
Locating distributed I/O within each MCC (preferably within each shipping split) reduces installation and commissioning time for a lower installed cost. Construction is common across applications and provides clear segregation between automation and motor control. This segregation 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 is available for all popular networks.
Intelligent devices imbedded in the MCC not only provide network communications, but extensive functionality 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 additional information, diagnostics, and protective features easily justify the incremental cost of the equipment.
Specification of intelligent MCCs requires taking into consideration the variables of cost, capability, and complexity. Therefore, it is important to realistically assess your capabilities and objectives. Keep in mind that additional functionality usually comes with additional cost or complexity that you may not need.
The following list is a sampling of questions to consider when selecting and specifying intelligent MCCs.
Will I maximize my return on investment by lowering the installed cost of the MCC equipment or by providing significant additional functionality?
Will last minute changes be likely?
Is future expansion or modification likely?
How valuable is a reduction in installation and commissioning time?
Can the equipment be integrated into my overall control and information architecture?
Does my control system or network have enough capacity to handle the data provided?
Do I have a technical staff capable of maintaining the system?
What are the cabling and connection requirements of the system?
A good MCC supplier will offer a range of options; one size does not fit all. The manufacturer should be able to help you answer these questions, so you can select the best approach for your application. The supplier also should be able to integrate with multiple networks and existing control systems. They must be willing to work with third parties and value-added system integrators. And they should be able to provide total solution packages where appropriate. Finally, a complete functional test of the equipment prior to shipment is essential to realizing reductions in commissioning time. Be sure the vendor intends to conduct the test by actually energizing the I/O, not with simple point-to-point wiring checks.
Edited by Jack Smith, senior editor, 630-320-7147, firstname.lastname@example.org