generators

vptran84

hi,

I'm currently learning about self-excited and separate shunt generators, and i'm having a hard time comprehending the residual magnetism.

i was just wondering if a generator have lost all of its residual magnetism, can it still build up an output voltage? can someone explain this to me if they have time?

Also how would you get a generator to work after it has lost all of its residual magnetism?

any help would be greatly appreciated.

Averagesupernova

A self excited generator cannot work if it has lost all of its residual magnetism. Older vehicles had generators instead of alternators. They had a commutator and brushes instead of diodes. The main charging current in modern alternators does not go through the slip-rings and brushes, it comes from the stator which is wound and arranged to provide 3 phases to the diodes. In generators however the main charging current passed through the commutator and brushes. Typically if the generator was serviced in some way it was 'polarized'. This simply meant passing current through some part of the generator to create residual magnetism. I imagine the field coil. I own an older military suplus 120 Vac generator that is self-exciting. It always come up to voltage but sometimes it takes a couple of seconds. It has a set of slip rings that the main current passes through as well as a commutator that rectifies the voltage coming off of the armarature to power up the field coils.

MarkT

On self excited DC motors/generators that lose their residual magnitism. You can use a technique called 'Flashing the field' aka, applying a DC voltage to the field and it should come up.

If it builds up in the wrong direction then it can be reversed. This should be done with all other circuits off and the Armature disconnected.

On Separately excited DC motors it should not be a problem because there is a seperate exciter generator or known as an MG set.