Preventive maintenance the key to motor control center reliability

The low-voltage motor control center is a key element in electrical control systems because of the vital operating role they play in controlling motors and production processes. Over the years, MCCs have evolved from cabinets that housed basic electro-mechanical devices such as circuit breakers, contactors and overload relays to centers of automation that may include variable frequency drives, soft starters and programmable controllers.

Because failure or malfunction of electrical systems — specifically MCC-housed equipment — can present a serious hazard to personnel and property, it is imperative to comprehend how MCCs are maintained. By shifting from reacting when equipment fails to proactively maintaining machinery through preventive and predictive practices, manufacturers can help mitigate such risks and prevent failures from occurring (Fig. 1).

Using infrared thermography

Regular maintenance of MCCs occurs in two ways — by inspecting energized or de-energized equipment. One of the most common methods of inspecting energized equipment is infrared thermography. While infrared thermography can be part of an overall preventive or predictive maintenance program, it is not the only method of inspection.

Infrared thermography is a non-invasive inspection technology that uses an infrared camera to monitor temperatures and thermal patterns while equipment is running at full load to detect changes in temperature. This is often a telltale sign that equipment is performing out of spec. Used on a wide variety of equipment, including MCCs, this technology helps manufacturers predict equipment failure and plan corrective action before a costly shutdown, equipment damage or personal injury can occur.

 Fig 3

This inspection method can be difficult, and equipment relatively costly. To gain the best results from an infrared program, it's important that inspection personnel are properly trained to perform the task.

 Fig 4

The second type of MCC preventive maintenance is inspecting de-energized equipment. This requires more training than visual inspections. To begin, engineeers must follow specific guidelines on de-energizing, isolating and grounding the equipment to be inspected.

When performing maintenance on de-energized equipment, refer to the following general guidelines: (For a complete list of specific maintenance steps, always check the manufacturer's user or installation manual, as well as NEMA and NFPA standards).

• Structure— Check for moisture or any signs of dampness or drippings inside the MCC. Condensation in conduit and moisture from an outside source is a common cause of MCC failure. Eliminate any source of moisture and seal off conduit, cracks and openings that have allowed or could allow moisture to enter the MCC. Dry, replace and/or clean wet insulation material. Replace damaged or malfunctioning parts. Ensure that you've identified and eliminated the source or cause of wetness or contamination.
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• Buses and splice connections— For horizontal to vertical bus connections, some manufacturers do not require servicing for the life of the MCC. Follow your equipment manufacturer's specific recommendations. For MCCs that permit servicing bus connections, check the integrity of the bus splice connections (Fig. 4). Bus splices are normally identified with labels on the MCC structure or referenced in the MCC elevation drawings or user manuals. You'll usually find recommended torque values on the structure, in wiring diagrams or in the manufacturer's user manual.
• Wiring and branch circuit protection devices— Assure conductors are not damaged, worn or obstructing moving mechanical parts. Check wires and cables for signs of overheating such as discolored insulation; inspect fuses for discoloration and check for loose power and control connections (Fig. 5). If any of these conditions are present, determine the cause and reposition/replace wiring as necessary.
• Disconnect handle mechanisms— Check for proper function and freedom of movement of the disconnect handle and door interlock mechanisms (Fig. 6). Lubricate as directed according to the manufacturer's instructions. Replace broken, deformed, malfunctioning or badly worn parts.
• Starters and contactors— Check for excessive wear and dirt accumulation on starters and contactors. Vacuum or wipe components with a soft cloth to remove dirt (Fig 7). Do not use compressed air to clean these components because they may become damaged in the process. Also, do not use contact spray cleaners, which may cause sticking on magnetic pole faces.

Generally, discoloration and/or slight pitting do not harm contacts. If contacts are overly worn, replace (not repair) them in pairs to avoid misalignment and uneven contact pressure. Be very careful about filing contacts; doing so can easily damage them and reduce their life expectancy.

If a fault occurs, the excessive current may damage the MCC structure, components, bus or conductors. After a fault occurs, de-energize, disconnect and isolate all involved equipment to prevent accidental contact with live parts. Verify with a DMM or VOM that power has been removed. Sometimes tie breakers, or other power sources that may not have been properly documented can back-feed a system.

Inspect and (if needed) repair all involved equipment before placing it back into service. Verify that all replacements have proper ratings and are suitable for each application. For a complete list of items to review after a fault, refer to NEMA Standards Publication No. ICS 2, Annex A (Maintenance of Motor Controllers After a Fault Condition.)

• Regular maintenance of MCCs includes inspecting energized and de-energized equipment.
• Infrared thermomgraphy is one common way to inspect energized equipment.
• Inspecting de-energized equipment requires specialized training, especially in safety.
• Safety is also an issue if a fault occurs. De-energize, disconnect and isolate all involved equipment to prevent accidental contact with live parts.
• If you have questions about MCC maintenance you may contact Mr. Martinez directly at (414) 382-4857 or at brmartinez@ra.rockwell.com. Article edited by Jack Smith, Senior Editor, PLANT ENGINEERING magazine, (630) 288-8783, jsmith@reedbusiness.com.