Maintenance can be classified into two groups:
- Restorative (Repairs) Maintenance: It is the most primary type of repairs. It arises after a fault being carried out. Restorative Maintenance has many disadvantages like lesser time for the useful life of the machine, waste of valuable energy, etc. It is also known as Corrective maintenance.
- Protective Maintenance: This type of maintenance form can be referred to the schedule of planned maintenance actions (i.e. scheduled maintenance) aimed at the prevention of breakdowns and failures. Examples of Protective maintenance are changing of oil, greasing, tightening of the belt, changing of filters, etc.
It can also be defined as “anything that increases the life of equipment, and helps it runs more efficiently.”
Further, it can be divided into two subgroups of activities
- Continues monitoring;
- Periodic measurements or predictive techniques.
Protective maintenance will generally involve lubricating, cleaning and check for sparking brushes, vibration, loose belts, high temperature and unusual noises.
So a planned inspection and maintenance is needed for vast of electrical equipment to keep in proper working condition.
But the question arises do we need a proper maintenance schedule?
And the answer will be yes because without a proper maintenance schedule when something goes wrong with a motor it can lead to expensive repairs, can take some leads to replace it with a new one. This is why a proper schedule maintenance schedule is necessary; however, it will not guarantee that motor will not get any trouble, but it will limit the problems.
So the main idea behind motor maintenance is to prevent breakdown rather than repairs them.
Since here we are talking about maintenance of induction motor, and it is of two types
- Squirrel Cage Type Induction Motor:- Squirrel cage induction motor requires significantly less maintenance as it does not contain brushes, commutators, slip rings.
- Wound Rotor Type Induction Motor:- Since it contains slip ring, brushes so it does require maintenance time to time.
Considering the above discussion most common faults that can be avoided through the adoption of condition monitoring methods:
Stator Winding Faults: Normally a consequence of overheating, contaminations, etc, possibly causing shorted turns, shorted coils (same phase), phase or coil to ground and single phasing. Such failures create stator electrical imbalance as well as vibrations in the current harmonic content.
Bearing Faults: It can be caused by incorrect lubrication, mechanical stresses, wrong assembling, etc. They can affect all the bearing parts such as inner and outer races, cage and balls or rolls.
Rotor Faults: These are usually caused by broken bars or broken end rings, rotor misalignment and imbalance.
The primary focus of problems in a three-phase induction motor is in their stators and their supports. The leading causes of failures are superheating, imperfections in the isolation, mechanical bearings, and electrical failures.
So the following inspection schedule (may vary depending on the type of machines and importance of that machine) must be carried out of Induction Motors.
The maintenance program for every week :
- Examine commutator and brushes.
- Examine the starter switch, fuses, and other controls; tighten loose connections.
- See that machine brought up to rated speed in normal time or not.
- Check the level of oil in bearings.
The maintenance program for every five/six months:
- Clean motor thoroughly, blowing out dirt from windings, and wipe commutator and brushes.
- Check brushes and replace any that are more than half worn
- Examine brush holders, and clean them if dirty. Make sure that brushes ride free in the holders.
- Drain, wash out and replace oil in sleeve bearings.
- Check grease in a ball or roller bearings.
- See that all covers, and belt and gear guards are in place, in good order, and securely fastened.
- Inspect and tighten connections on motor and control.
The maintenance program for every year :
- Clean out and renew grease in ball or roller bearing housings.
- Clean out magnetic dirt that may be clinging to poles.
- Check clearance between shaft and journal boxes of the sleeve bearing motors to prevent operation with worn bearings.
- Clean out undercut slots in the commutator. Check the commutator for smoothness.
- Examine connections between commutator and armature coils.
- Test insulation by megohmmeter.
- Check air gap.