Technical notes of interest to Marine Engineers

Flashing a generator

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How to "flash" a generator when output is lost due to loss of excitation

Residual magnetism in the generator exciter field allows the generator to build up voltage during start-up. This magnetism is sometimes lost due to shelf time or improper operation, among other reasons. Restoring this residual magnetism is possible and is sometimes referred to as "flashing the exciter field".

To restore the small amount of residual magnetism necessary to begin voltage buildup, connect a 12 volt battery to the exciter field while the generator is at rest, as follows:

  1. Remove exciter field leads F+ and F- from the voltage regulator. CAUTION: Failure to remove the field leads from the regulator during flashing procedures may destroy the regulator.
  2. Measure the exciter field resistance from the F+ to the F- lead. You should be able to read some resistance as you are measuring a continuous winding. An infinite resistance reading would indicate an open in the exciter field. Also check to be sure there is no path to ground.
  3. Connect F+ to the positive pole of the battery.
  4. Hold the F- lead by the insulated portion of the lead wire, touch F- to the negative pole of the battery for about 5 to 10 seconds, then remove.
  5. Reconnect F+ and F- to the regulator. Repeat the procedure if the generator fails to build voltage. 


Brian wrote, in August 2005...

The article is fine for small generators where 12V is available, but for completeness you should also discuss how to flash the field on a larger DC generators such as are found on older vessels.

I had to do this just the other day when the oiler switched over wrong and closed the main bus breaker on a 40 KW DC generator, that was not running, motorized it and reversed its polarity. The procedure I followed was:

  1. Shut down the prime mover on the reversed generator. Make SURE the breaker is open before opening the access ports or touching the brushes.

  2. Lift ALL brushes clear of the commutator, either by pulling them up enough that the spring arm presses against their side and holds them up, or for safety put a piece of rubber gasket material between the brush and the commutator. Be sure you lift all brushes of multiple brush sets.

  3. After double-checking that all brushes are insulated from the commutator, close the breaker with power on the bus for a few seconds with the shunt field rheostat in the minimum resistance position, then open the breaker, You can do it again for good luck if you like.

  4. Make sure the breaker is open, then remove any rubber insulating material you used and return the brushes to their proper position in the holder.

  5. Do a final visual inspection that nothing has been left inside, and lift each brush a little and feel it spring back to touch the commutator. 

  6. Start the prime mover. Bring it up to speed and inspect the commutator and brushes to ensure there is no arcing. Put generator under load and check again.

Brian Bailey