Induction Motor Maintenance

Induction motor maintenance is the work that keeps a motor in a specified operable condition for its useful life. Maintenance of these machines falls into two groups:

1. Restorative (Repairs) Maintenance: This work happens after a fault. It can shorten remaining life and waste energy. Plants also name it corrective maintenance.
2. Protective Maintenance: Scheduled actions to prevent breakdowns. Examples include changing oil, greasing, tightening belts and changing filters.

Maintenance is any action that lengthens equipment life and keeps it running well.
It then splits into two subgroups of activities

• Continuous monitoring;
• Periodic measurements or predictive techniques.

Protective maintenance generally includes lubricating, cleaning and checks for sparking brushes, vibration, loose belts, high temperature and unusual noises.
Planned inspection is needed for a large stock of electrical equipment if it is to stay in working condition.
A written maintenance schedule is still worth having.
Without one, a motor fault can mean an expensive repair or a long wait for a replacement. A schedule still leaves some failures, but it limits how often they occur.
The aim of motor maintenance is to prevent breakdown rather than wait for repairs.

The two common maintenance of induction motor constructions are:

1. Squirrel Cage Type Induction Motor:- A Squirrel cage induction motor needs less maintenance because it has no brushes, commutators or slip rings.
2. Wound Rotor Type Induction Motor:- Slip rings and brushes need periodic inspection and replacement.

Condition monitoring is aimed at the faults below:

Stator Winding Faults: Overheating and contamination can produce shorted turns, shorted coils in the same phase, a coil or phase to earth, or single-phasing. Those faults unbalance the stator electrically and add extra harmonics in the current.

Bearing Faults: Wrong lubrication, mechanical stress or poor fitting can damage inner and outer races, the cage and the balls or rollers.

Rotor Faults: Broken bars, broken end rings, rotor misalignment and unbalance are the usual cage defects.
On a three-phase induction motor the stator winding, bearings and mounting still account for most failures. Overheating, insulation damage, bearing wear and electrical faults are the usual causes.
A typical inspection plan for Induction Motors is below. Intervals vary with machine type and how critical the drive is. Commutator, brush and pole items apply to wound-rotor or DC machines, not to a squirrel-cage motor.

The maintenance program for every week :

1. Examine commutator and brushes.
2. Examine the starter switch, fuses and other controls; tighten loose connections.
3. See that machine brought up to rated speed in normal time or not.
4. Check the level of oil in bearings.

The maintenance program for every five/six months:

1. Clean motor thoroughly, blowing out dirt from windings and wipe commutator and brushes.
2. Check brushes and replace any that are more than half worn
3. Examine brush holders, and clean them if dirty. Make sure that brushes ride free in the holders.
4. Drain, wash out and replace oil in sleeve bearings.
5. Check grease in a ball or roller bearings.
6. See that all covers and belt and gear guards are in place, in good order and securely fastened.
7. Inspect and tighten connections on motor and control.

The maintenance program for every year :

1. Clean out and renew grease in ball or roller bearing housings.
2. Clean out magnetic dirt that may be clinging to poles.
3. Check clearance between shaft and journal boxes of the sleeve bearing motors to prevent operation with worn bearings.
4. Clean out undercut slots in the commutator. Check the commutator for smoothness.
5. Examine connections between commutator and armature coils.
6. Test insulation by megohmmeter.
7. Check air gap.