Anything and Everything you know about brushless excitation in Alternators

@sau-A0ib6r • Oct 9, 2024

Oct 9, 2024

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Guys, the previous discussion about brushless excitation in alternators should continue here...for refrence, visit -
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freak16

@freak16-XgiLj6 • Mar 19, 2011

Brushless Excitation is very popular recently.
It has been used in all alternators you see in railways.
THey use diodes that is power electronics for converting ac into dc.
This is the key that makes machines to run without brushes because otherwise we need commutator that need brushes.
any doubt will be welcomed
sau

@sau-A0ib6r • Mar 19, 2011

Thanks a lot for your information freak. but what i actually wish to know is this-
I have read in 'Electrical machines(book)' by Ashfaq Khan that in the power generation stations, they need to excite the field circuit (i. e. the rotor circuit) of the alternator so as to produce electricity. To excite the rotor circuit, they use dc generator which is mounted on the main generator shaft of the main alternator itself (at least this is what i understood from that book). Now, what I am confused about is that how can we mount a whole DC generator on a shaft? And if it is mounted outside the alternator body, then hoe do we feed the field circuit with DC without using brushes and slip rings?
please help me out!
thanx
narayana murthy

@narayana-z4W16q • Mar 19, 2011

hmmmm by my knowledge i know 1 thing here we can mount generator shaft to it so they can rotate on same speed
and we use brashes and slip rings to take current from D.C generator
if i am wrong correct me
freak16

@freak16-XgiLj6 • Mar 20, 2011

@sau
Yes dc generator is mounted on same shaft through gears. and feeding back to the rotor circuits of alternator.
But this is not used nowadays it is outdated.
We use self excitation that is the electricity produced by alternator is taped which is feed to 3phase bridge rectifiers and converted into dc. this dc is filtered and used to excite the field winding of rotor of alternator.
Automatic voltage regulator is also provided by this method as for regulating voltage we nee to change the field winding excitation It is very smoothly attained in this method.
this is called brushless excitation in alternators
freak16

@freak16-XgiLj6 • Mar 20, 2011

@ NM yes if you mounted on shaft directly the dc generator. It will rotate at the same speed.
but you can use gears to change the speed.
sau

@sau-A0ib6r • Mar 21, 2011

thanks again freak. One last Question. If we use the electricity produced by the alternator itself to excite it's field circuit, what is done before the alternator actually starts generating? I mean, at the very start, if we do not have excitation, how do we have output? and, if we do not have the output, what will we rectify n feed?
freak16

@freak16-XgiLj6 • Mar 21, 2011

residual magnetism.
sau

@sau-A0ib6r • Mar 21, 2011

YES! I missed that out..thanx a lot..
narayana murthy

@narayana-z4W16q • Mar 21, 2011

freak16
@ NM yes if you mounted on shaft directly the dc generator. It will rotate at the same speed.
but you can use gears to change the speed.
ofcourse we can i am not mentioning because i am too much exhausted with that mess
anyway i don't like gears
but i like add one more point arrangement of gears can add more speed depending on gear diameter
and its aligning
i think ishu can add some points regarding gears here
Arp

@arp-8yytsY • Mar 22, 2011

to start the generator, residual magnetism is one option. but in the beginning when machine have to start at the first time then there is no residual magnetism so we have to go for field flashing which means exciting it with dc battery for this purpose only.
sau

@sau-A0ib6r • Mar 22, 2011

ok.thanks Arp.
Arp

@arp-8yytsY • Mar 22, 2011

guys, i've just read the whole posts in this. what u all say to keep a dc generator is not a method for brushless excitation and even to use output for field excitation is also not a method for brushless excitation as u've to start it through battery bank as done in ntpc badarpur so not again a perfect brushless as u've to use brushes for starting
use a permanent magnet rotor poless and armature wdg on stator produce 3phase power, rectify it to dc then send to another alternator field wdg on stator produce 3 phase power in rotor send to rotating rectifier through the same shaft to main alternator field winding which is also on rotor so need no brushes,etc as here we get rotating DC.this is what is called brushless excitation.
see its complex but due to rotational voltage, spark problem in brushes especially in case of high rating alternator we've to use it.