Factors affecting life cycle of motor control centers

This is part one of a two-part series. The second part will appear in the March issue. Maintaining electrical infrastructure in the manufacturing plant is a challenge. In this environment, electrical gear is subjected to every extreme of heat, dirt, chemicals, misuse, abuse, and good old-fashioned hard use.

02/10/2005


This is part one of a two-part series. The second part will appear in the March issue.

Maintaining electrical infrastructure in the manufacturing plant is a challenge. In this environment, electrical gear is subjected to every extreme of heat, dirt, chemicals, misuse, abuse, and good old-fashioned hard use. The old axiom, "If it ain't broke, don't fix it" clearly no longer has a place in this world.

The advent of new streamlined manufacturing systems, highly sophisticated electronic controls, and "just-in-time" inventory supply has added complexity and criticality to once fairly simple systems. Those responsible for production have a keen interest in reliability.

Life expectancy

What is the life expectancy of electrical equipment? Obviously, it is a period of time beginning at installation and ending when it is no longer practical to maintain. It could be dependent on number of operations, technology, model, environment, etc. Figure 1 illustrates a number of interrelated factors that I believe directly influence how long equipment will remain viable.

The simple motor control center of years past may no longer be suitable for today's purpose. The Motor Control Center remained fairly basic through the 1980s — nothing much more than a metal-clad switchboard consisting of magnetic starters and fused safety disconnects. Today its function remains fundamentally unchanged.


Current technology

Advances in technology have changed the manner in which we control, protect, and interface with the production process and other plant systems.

Let's look at some basic types of motor control technology available today:

  • The old, basic configurations remain. Magnetic and manual contactors with overload sensing are still very much in demand for many applications. They still look and operate pretty much the way they did 30 years ago

  • Combination starters include the starter and overcurrent protection in the same enclosure or module. Also included is a safety interlocked disconnecting operator

  • Solid state is a new twist on the old magnetic starter using silicon-controlled rectifiers (SCRs) to connect and carry the load. While these units serve the same function as the basic mechanical contactor, the controls associated with this type of product are inherently more complex. A working knowledge of electronic control systems is essential, and the oscilloscope joins the multimeter as an essential tool in the plant electrician's kit

  • Probably the biggest advance in the past 30 years is the variable frequency drive (VFD). This development allowed the use of a normal induction type motor in applications requiring speed and torque control. Prior to this innovation, elaborate DC systems were required to control motor speed

  • Manual-static speed control was accomplished with brush shifting motors

  • Soft starters use electronics to softly accelerate a motor, thereby not subjecting the electrical feeder or mechanical apparatus to the stress associated with a sudden full-load start

  • New programmable protective features have replaced the old "heater elements" in many controllers of all types. These devices allow for multiple layers of protection and ease of adjustment

  • Add to all these types of product the E factor (meaning "enable"). The convergence of advanced technology and the business reality of doing more with less have resulted in a necessity to be connected. The requirement for instant status, information gathering, and forensic analysis has driven the inclusion of e-capability in the modern manufacturing facility. Communications and control are the name of the game.

    • Proper design and application

      The road to long life of any piece of gear begins long before installation.

      All too often some aspect of system design or application is lacking. It may be physical restrictions that prevent proper maintenance. It may also be improper application of a product type. Most manufacturers provide a range of equipment models to satisfy specific requirements of the manufacturing facility system.

      As a plant engineer, you are part of the team designing new systems. You are the one person who can bridge the gap between design, application, and maintenance and what effect these considerations will have on production. You're also the one who will receive the 3 a.m. call.

      The first consideration is selecting the appropriate motor control technology for a manufacturing line or process:

      • In the new line or system, you have the opportunity to select technology that is efficient, sustainable, and cost effective

      • Replacing a Motor Control Center (MCC) in an existing line presents a different set of challenges. Compatibility with other equipment and controls may force you to install an older level of technology

      • Spare parts are a major consideration. Most manufacturing plants stock ample spare parts. It is important to consider how your current spare parts can support a new piece of gear and what you will have to purchase. The cost of additional spare parts stock must be included in the business case, especially when calculating the payback period.

        • MCC interface

          Having decided the technology best suited for the system demands, you must consider how the MCC will interface with other system components:

          • How will loads be managed? Overcurrent coordination is extremely important. Overcurrent protective devices must be coordinated beginning at the end of the branch circuit and working back toward the power source feeding the MCC. You don't want a small downstream fault to cause a big upstream problem due to lack of proper coordination between the overcurrent protective devices in a system circuit

          • Are time delays and control setpoints properly coordinated between all critical members of the system? This is just as important as coordination of the overcurrent protection. The input, output, and throughput of a manufacturing line depend on proper coordination of controls. A smooth flow at optimum speed is certainly desirable to stop or start operation

          • What about communications and monitoring? It is essential in today's critical environment that operators know instantly the status of or changes in the system they depend upon.

            • Careful handling and installation

              Here again events occur that can severely affect the useful life of a piece of equipment even before it's installed:

              • Did it ship well? Only incoming inspection can tell. Inspection should take place obviously before you accept the gear. The presence of the manufacturer's representative is recommended. Waiting until the gear is installed to discover a problem creates many problems and may impact commissioning schedules

              • Store the gear in a dry, safe environment. Moisture or condensation can and will degrade insulators, breakers, and electronic controls. Moisture also attracts dust and dirt, which may become conductive. Some of these problems may not occur immediately, but will become a latent failure down the road

              • Those installing the gear should be thoroughly familiar with the manufacturer's recommendations. Extreme caution must be taken to avoid letting metal shavings or other debris contaminate the unit or controls. A misplaced wrench or piece of metal shavings can spell disaster. Protecting the interior of the MCC during installation will go a long way toward avoiding dangerous conditions. Accounting for tools used during installation is another prudent measure. A wrench left lying on the bus or a wrench left in a lug may cause a catastrophic fault and injury. Make sure all tools are accounted for when the job is complete

              • Those responsible for operation and maintenance of gear in the plant may not be doing the actual installation. However, it is just as important that your people are as familiar with proper installation as are the contractors or manufacturer's personnel actually performing this work. Personnel should read the installation and operations manual. Do not force controls or the mechanisms. If you're not sure what to do, call the manufacturer for guidance

              • Depending on the complexity of the equipment, it may be a good idea to include training for plant electricians and others charged with maintenance and repair

              • Remove all shipping blocks, bolts etc.

                • Benchmark testing

                  This aspect of commissioning cannot be emphasized enough. The manufacturer should be contacted to participate in the planning of the benchmark or initial commissioning test. This may be part of the purchase package. If it is not, a few dollars invested at this point may save a few thousand dollars later.

                  • A team should include the design engineer, plant engineer, and members of the plant team responsible for installation and/or maintenance as well as manufacturers' representatives, if required. Usually, these benchmark/acceptance tests include functional testing of each component, functional testing of the system, and specific performance criteria

                  • You must ensure that all programmable set points and time delays are understood, properly set, and indeed function

                  • Unrealistic tests invalidate results and diminish the value of this important requirement

                  • Verify overcurrent protection devices and settings

                  • Document and retain all setpoints and adjustments on each piece of gear. Test records should become a clearly defined package in the system master file.


                    • <table ID = 'id4704807-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id4699041-0-tbody'><tr ID = 'id4699043-0-tr'><td ID = 'id4699045-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id4699055-3-tr'><td ID = 'id4699832-3-td' CLASS = 'table'> Doug Sandberg serves as Director of Operations for ASCO Services, the OEM service provider for ASCO Technologies. His prior positions include plant electrician, field service technician, senior field service technician, and national service manager. He can be reached at 973-966-2079 or Dsandberg@ASCO.com . </td></tr></tbody></table>


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