Question

Give a brief outline of the care and maintenance that should be given to the stator and rotor of an AC generator.

Always ensure that generator prime mover is shut down and locked off. Circuit breaker should be put off and warning sign boards to be placed before doing any maintenance.

1. Check all connection in the stator terminal box are tight. Check for signs of oil and water

contamination of cable insulation within the terminal box.

2. Keep cooling air intake and exhaust are free of dirt and dust. Clean the filters at regular intervals.

3. Ensure that the space heaters come "on*' automatically whenever the alternator is stopped for a long time. They should go "off" when the alternator is started.

4. Inspect and clean alternator rotor and stator windings by removing dust with dry lint free cloth. An industrial type vacuum cleaner with rubber or plastic nozzle (so as not to damage the winding insulation) is very effective in removing dirt. Oil on the winding r educes insulation resistance and shortens its life. Use electro cleaner for degreasing. Minor abrasions to winding insulation can be repaired after cleaning by applying air drying varnish.

5. Slip rings on the rotor must be checked for even wear and that carbon brushes have free movement in their boxes. Correct brush pressure can be checked by pull type spring balance and compared with maker's instruction. A pull of around 0.14 to 0.21 kg/cm2 is usual. Worn out brushes (below 2 cms approximately) suffer reduced spring pressure and cause sparking at slip ring contacts.

Replace brushes with correct type and 'bed' them to the curvature of the slip rings. This can be done by placing a thin strip of glass paper (not emery paper) over the slip ring, with its cutting surface under the carbon brush. Pull glass paper around slip ring till brush surface has the same contour as the slip ring. Remove all traces of carbon dust with a vacuum cleaner.

6. In case of brushless alternator, the rotating diodes should be kept free from dirt, oil and dampness.

7. Measure the insulation resistance of stator and rotor windings. Remember to disconnect any electronic circuit components which may be damaged by a 500V insulation test. Measure the insulation of the stator winding and the rotor winding to earth, and between stator phases. A minimum value is 1 MD, but a lower value may be acceptable to a surveyor based on lkΩ/volt.

8. No load running checks should be don before synchronising and loading. On load check for excess temperature rise and load sharing stability when running in parallel.

Question

State what is likely to occur if the driving power of an ac generator suddenly fails when 2 generator are operating in parallel.

Generator No.2 will run as an induction generator drawing its excitation from No.l.

Both generator currents will rise rapidly with No.l becoming more lagging while No. 2 runs with a leading pi. (indicated on p.f. meter).

A 'loss of excitation' trip (if fitted) or an overcurrent trip should trip No.2 generator possibly causing an overload on No.l.

Alternatively, No.l trips on overcurrent (current) which deprives No.2 of excitation and its breaker trips out on undervoltage.

Result - Total power failure!