Anything and Everything you know about brushless excitation in Alternators

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

  • freak16
    freak16
    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
    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 murthy
    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
    @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    But this is not used nowadays it is outdated.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
    @ 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
    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
    residual magnetism.
  • sau
    sau
    YES! I missed that out..thanx a lot..
  • narayana murthy
    narayana murthy
    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
    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
    ok.thanks Arp.
  • Arp
    Arp
    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.

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