Introduction to Preventive Maintenance of Electrical Equipments
Maintenance generally contains regularly scheduled inspection, greasing, oiling and probably minor repairs. Several causes of failure of alternator and electrical equipment are poor maintenance process that includes flushing out oil wells, greases cups, and testing of rotor and slip rings for concentricity. Shop overhaul is necessary for all electrical equipment at least one time in five years.
To avoid main repairs:
1. Test all connections and wiring.
2. Ensure that moisture does not penetrate the winding insulation. Existence of moisture lowers insulation resistance. Test insulation using a megger.
3. Remove the moisture through heating the windings using hot bulbs or applying low voltage to winding to develop heat and dry. Do not permit the temperature to rise above 900C that may damage the insulation.
4. Dust in the machine should be removed through using a blower with low pressure of air.
5. Take out the grease and oil using carbon tetrachloride (CTC). While using CTC the area should be well ventilated to avoid fumes and toxics.
6. Test voltage of alternator at terminals and panel boards. If the generator is operating satisfactory, load the alternator slowly for 2 to 4 hours to evaporate remaining moisture. When testing it should be within 5% of rated voltage and 3 phases load should be well balanced.
7. All the time open bus bar switches and after that stop the alternator. Be sure that all the switches are in 'off' condition before working on the equipment.
8. Reversed coil connections of pole windings can be detected through passing Direct current by winding and testing poles through soft iron strip or bar. If polarity is correct soft iron piece will be held tightly. If not the bar will not be held in its place. Very much care is to be taken if D.C. (direct current) exciter is to be removed for test up of main alternator itself.
9. Slip rings on rotor are made up of Bronze or non ferrous metal that are polished through fine sand paper or polishing stone. If the rings are worn-out extremely, the rotor should be separate and the rings be decreased down in diameter on the lathe machine. Insulation resistance is after that computed through Megger, ring to ring and rings to shaft. Accumulation of carbon / metal dust in the vicinity of rings should be cleaned carefully.
10. Earlier than starting the motor or alternator - clean the motor or alternator surrounding area to ensure that there is enough open space for air movement, also be sure of dry windings.
11. Ensure from name plate data that type, design of the motor for that work and load.
12. Test that operating speed reaches in minimum time, if not there might be overload or centrifugal switch or starting coil is defective. If motor is running in not proper direction test for the proper connections according to the manufacturing data.
13. Test for unusual noises. Poor alignment of end plates, that causes the rotor core to strike against the stator core. Bearing might be defective. If motor turns overloaded and begins smoking, there might be over loading or defective starting winding or switch. Clean the commutator or reset brushes or adjust spring tension of brushes, if there is sparking at brushes.
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