Preventive Maintenance of 3 phase induction Motor

PREVENTIVE MAINTENANCE

Most electrical equipment requires planned inspection and maintenance to keep it in proper working condition. Periodic inspections prevent serious damage to machinery by locating potential trouble areas. Observant personnel will make full use of their senses to diagnose and locate problems in electrical machinery: the sense of smell directs attention to burning insulation; the sense of touch detects excessive heating in windings or bearings; the sense of hearing detects excessive speed or vibration; the sense of sight detects excessive sparking and many mechanical faults.

preventive maintenance

Sensory impressions usually must be supplemented by various testing procedures to localize the trouble. A thorough understanding of electrical principles and the efficient use of test equipment is important to the electrician in this phase of troubleshooting.

PERIODIC INSPECTIONS

The ideal motor maintenance program aims at preventing breakdowns rather than repairing them. Systematic and periodic inspections of motors are necessary to ensure best operating results. In a good preventive maintenance program with detailed checks, the person in charge should have a record card on file for every motor in the plant. Entries on the card should include inspection dates, descriptions of repairs, and the costs involved. When the record indicates that a motor has undergone excessive and /or costly repairs, the causes can be determined and corrected.

Inspection records also serve as a guide to indicate when motors should be replaced because of their high cost of operation. They also reveal faulty operating conditions, such as misapplication or poor drive engineering.

Inspection and servicing should be systematic. However, the frequency of inspections and the degree of thoroughness may vary, as determined by the plant maintenance engineer. Such determinations are based on 1) the importance of the motor in the production scheme (if the motor fails, will production be slowed seriously, or stopped?), 2) the percentage of the day the motor operates, 3) the nature of the service, and 4) the motor’s environment. An inspection schedule, therefore, must be flexible, and adapted to the needs of each plant. Equipment manufacturers’ specifications and procedures should be consulted and followed.

The following schedule, which covers both ac and dc motors, is based on average conditions insofar as operational use and cleanliness are concerned. (Where dust and dirty conditions are extremely severe, open motors may require a certain amount of cleaning every day.)

EVERY WEEK

1. Examine commutator and brushes, ac and dc.

2. Check oil level in bearings.

3. See that oil rings turn with shaft.

4. See that exposed shaft is free of oil and grease from bearings.

5. Examine the starter switch, fuses, and other controls; tighten loose connections.

6. See that the motor is brought up to speed in normal time.

EVERY SIX MONTHS

1. Clean motor thoroughly, blowing out dirt from windings, and wipe commutator and brushes.

2. Inspect commutator clamping ring.

3. Check brushes and replace any that are more than half worn.

4. Examine brush holders, and clean them if dirty. Make certain that brushes ride free in the holders.

5. Check brush pressure.

6. Check brush position.

7. Drain, wash out, and replace oil in sleeve bearings.

8. Check grease in ball or roller bearings.

9. Check operating speed or speeds.

10. See that end play of shaft is normal.

11. Inspect and tighten connections on motor and control.

12. Check current input and compare it with normal.

13. Examine drive, critically, for smooth running, absence of vibration, and worn gears, chains, or belts.

14. Check motor foot bolts, end-shield bolts, pulley, coupling, gear and journal set screws, and keys.

15. See that all covers, and belt and gear guards are in place, in good order, and securely fastened.

ONCE A YEAR

1. Clean out and renew grease in ball or roller bearing housings.

2. Test insulation by megohmmeter.

3. Check air gap.

4. Clean out magnetic dirt that may be clinging to poles.

5. Check clearance between shaft and journal boxes of sleeve bearing motors to pre vent operation with worn bearings.

6. Clean out undercut slots in commutator. Check the commutator for smoothness.

7. Examine connections of commutator and armature coils.

8. Inspect armature bands.

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