Maintaining The Bottom Line

Sep 1, 1998 12:00 PM, By Robert J. Lawrie, Senior Editorial Consultant

How to boost productivity, reduce downtime, and save money: What you should know about motor maintenance programs.

A well-planned motor preventive maintenance (PM) program is the key to dependable, long-life operation of motors and generators. In modern plant operations, unscheduled downtime or long repair shutdowns are intolerable. These result in downtime costs that eat deeply into profits.

Most companies realize the need for good maintenance, but they sometimes resist investing in the proper tools, practices, or technical assistance that guarantee effective motor performance. So, it is important to justify a properly planned motor/generator preventive maintenance program.

Select the best approach. Organizing and setting a budget for a motor PM program is usually a difficult chore-you have much to consider. Take care not to practice the false economy of cutting costs in the wrong places. The program won't work if you don't have the proper test equipment, tools, and trained men. Consider what equipment you'll need and the time required to perform inspections, and keep accurate records. Determine which procedures are essential and who is best for the job.

Select the best motor-maintenance techniques for your type of equipment. For each type of motor or controller, a variety of maintenance methods are available. Choose the best methods and determine how to apply them. Consider whether you should check for possible bearing trouble on a motor simply by feeling the components for over-temperature and listening for unusual sounds, or by installing temperature-monitoring devices and making inspections using a stethoscope or an infrared scanner.

Motor maintenance. Lubricate regularly according to the manufacturer's instructions. On sleeve bearing and other oil-lubricated machines, check oil reservoirs regularly. In poor environments, change oil at least once a month. Never over-lubricate-excess grease or oil can get into windings and deteriorate insulation. Use only the lubricant specified for that machine. But with one exception. Advances in lubricants can make a switch to a newer lubricant worth while. Check with the manufacturer, first.

On essential motors, check bearings daily using a stethoscope, infrared scanner, or vibration analyzer. Check bearing surface temperatures with a thermometer, electronic temperature sensing devices, or stick-on temperature indicating labels. Compare temperature of hot bearings with the temperatures of normal operating bearings. Check oil rings, and watch for excessive endplay.

You should check the air gap between the rotor and stator with feeler gages at least annually. Measure at the top, bottom, and on both sides of the stator. Differences in readings obtained year-to-year indicate bearing wear.

Examine the belt tension. Belts should have about 1 in. of play. Seat the sheaves firmly with little to no play. Tighten couplings within tolerances to operate without excess noise. Check the alignment on all motor-generator sets and on motor-load couplings when trouble is suspected.

Inspect brushes and commutators of DC motors for excessive wear. Appropriate brushes are essential and should properly fit into the holders. Check spring pressure of the holders with a small scale. Adequate spring pressure ranges from 2 lbs. to 2 1/2 lbs. per sq in. of the brush cross-sectional area. Call the manufacturer or a service company to solve recurring problems of brush chatter, excessive brush wear, or sparking, streaking, or threading of the commutator.

It is also important to check the mounting of motors regularly. Inspect the mounting bolts, the steel base plates for warpage, and the concrete base for cracking or spalling. Because excessive vibration shortens motor life significantly, perform annual vibration analysis tests. You can perform field vibration analysis with portable instruments that display the vibration amplitudes and frequencies.

Keep the motor clean and cool. In high-temperature locations, consider using oversized motors. Excessive ambient temperatures will shorten motor life. Remove and disassemble important motors during summer or holiday shutdowns, allowing for a complete and thorough inspection, testing, and cleaning. Check the bearings, couplings and accessories. All of these steps will help you decide whether or not to recondition the motor.

Keep accurate records. Perform annual insulation-resistance tests and other appropriate tests. You should visually inspect motors thoroughly, then make voltage and current checks. Record and compare all values each year. The trends of the readings will indicate the condition of the motor and offer a guide to its reliability.

Motor controls. Keep control equipment clean. Blow out dirt weekly. Keep dust or contamination off of high-voltage equipment. This is important because dust may contain conductive materials, which could form unwanted circuit paths resulting in current leakage or short circuits.

Moving parts should operate without excessive friction. Check operations of contactors and relays by hand. Feel for any binding or sticking. Look for loose pins, bolts, or bearings.

Check contacts for pitting and signs of overheating, such as discoloration of metal and charred insulation. Contact pressure should be per the manufacturer's specifications. Watch for frayed flexible leads.

On essential controls, perform contact-resistance tests with a low-resistance ohmmeter regularly. A proper value of contact resistance is about 50 micro-ohms (50 millionths of 1 ohm). Don't forget to record the reading for future comparisons.

You should inspect overload relays thoroughly. Maintenance requirements for these relays include checking the rating or trip setting, taking into account ambient temperature as well as the higher inrush currents of modern, efficient motors. You should clean contacts, making them free from oxidation. To ensure reliable operation, test and calibrate relays every one to three years, using special equipment such as OL relay testers.

Maintaining equipment only during unscheduled downtimes is costly and often done too hurriedly. This information should help convince management to schedule preventive maintenance downtimes.

Show management how preventive maintenance pays dividends. Collect case histories of motor breakdowns and the cost of resulting lost production. Compare this to the lesser costof budgeted preventive maintenance.