An effective motor maintenance program boosts productivity, reduces unnecessary downtime, maximizes motor efficiency, and saves dollars.
Unscheduled downtime in modern plant operations results in high costs and possible loss of business. That's why a well-planned preventive maintenance program is key to dependable, long-life motor operation. However, a preventative maintenance program for motors won't work if you don't have the proper test equipment, tools, and training for the process. When developing a plan, consider the equipment you'll need, and allow adequate time for performing inspections and maintaining accurate records.
A variety of maintenance methods may apply for each type of motor, controller, or related equipment in your facility. You should choose the best methods for your facility and then determine how best to apply those methods. For example, consider whether you should check for possible bearing trouble on a motor by feeling components for high temperature and listening for unusual sounds, or installing temperature-monitoring devices and making inspections using a stethoscope or an infrared scanner. The following presents basic guidelines to follow whenimplementing an effective motor maintenance program.
Motor maintenance. Lubricate regularly according to manufacturer's instructions. On sleeve bearings and other oil-lubricated machines, check oil reservoirs on a regular basis. For example, in poor environments, change oil at least once a month. Never over-lubricate; excess grease or oil gets into windings and deteriorates insulation. Be sure to use only the lubricant specified for the machine in question. However, check into the possibility of using modern lubricants, which have excellent life and lubricating qualities.
On essential motors, or those you frequently duty cycle, check bearings daily (or at least weekly) using a stethoscope, infrared scanner, or vibration analyzer. Check bearing surface temperature with a thermometer, electronic temperature sensing device, or stick-on temperature indicating labels. Compare the temperature of hot bearings with the temperature of normally operating bearings. Check oil rings and keep an eye on excessive endplay.
Check air gap between the rotor and stator with feeler gages at least annually. Make the measurements at the top, bottom, and both sides of the stator. Differences in readings obtained from year-to-year indicate bearing wear.
Check belt tension. Belts should have about 1 in. of play. Be sure to firmly seat sheaves with little or no play. Couplings should be tight within tolerances, and should operate without excessive noise. You should make an alignment check on all motor gensets and motor-load couplings when you suspect trouble.
Inspect brushes and commutators of DC motors for excessive wear. Be sure all brushes are of the proper type, hardness, and conductivity, and that they fit properly in brush holders. Check spring pressure of brush holders with a small scale. In most instances, pressure should be 2 to 2.5 lb/sq in. of brush cross-sectional area. Call manufacturers or a service company to solve recurring problems of brush chatter, excessive brush wear, sparking, streaking, or threading of commutator.
Check mounting of motors regularly. Inspect mounting bolts and steel base plates for possible warping and concrete base plates for cracking or spalling. Annually, perform vibration analysis tests. Excessive vibration may be hard to detect by hand; but it could be enough to shorten motor life significantly. You can accomplish field vibration analysis with portable instruments that identify vibrations and display their amplitudes and frequencies. Then, you can identify and correct the source of the problem.
Keep the motor clean and cool. In dirty operating environments, blow out dirt with dry compressed air (no more than 50 lb) as often as needed. In high-temperature locations where high reliability is a priority, consider the use of oversized motors. Excessive ambient temperatures will shorten motor life. Pull and disassemble important motors during summer/holiday shutdowns for a thorough inspection, testing, cleaning, check of bearings, couplings or accessories, or complete reconditioning.
Keep accurate records. Perform annual insulation-resistance tests and other appropriate test. You should also visually inspect important motors, as well as perform voltage and current checks. Record and compare all values each year. The trend of the readings indicates the condition of the motor and offers a guide to its reliability. Motor control maintenance. Keep control equipment clean. In dirty operating environments, blow out dirt weekly, otherwise a quarterly or semiannual cleaning should be adequate.
Check operation of contactors and relays by hand, feeling for any binding or sticking. Moving parts should operate without excessive friction. Look for loose pins, bolts, or bearings. If the control is dirty, wipe or blow it clean.
Check contacts for pitting, and signs of overheating such as discoloration of metal or charred insulation. Make sure contact pressure is adequate (see the manufacturer's specifications) and is the same on all poles. Watch for frayed flexible leads.
On essential controls, perform contact-resistance tests with a low-resistance ohmmeter on a regular basis. Proper contact resistance should be about 50 micro-ohms. Record the readings for future comparison to identify trends.
Inspect, clean, and check overload relays for proper setting. In general, maintenance requirements for these relays include checking that: the rating or trip setting takes into account ambient temperature as well as the higher inrush currents of modern, energy-efficient motors; contacts are clean and free from oxidation; the relay will operate dependably when needed.
To ensure reliable operation, test and calibrate relays every one to three years. You can use special equipment to perform these tests, such as OL-relay testers.
