Calling Electrical Boffins!
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Join Date: Jun 1999
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Calling Electrical Boffins!
Guys and Gals (ATPL students or current engineer/pilots). .. .I need some help please. I am just doing some PPL revision in preparation for practical flying, and have a question. I am not sure whether I am going too deep here for PPL studies, but I have been reading and reading and reading and getting interested. I must point out that electrics is not my strong point!. .. .With reference to AC generators I would like to know if I have understood the following about external excitation.. .. .Am I correct in assuming that a Generator winding and core that is externally excited via the battery source to create the electro magnet is constantly fed with electrical current from the battery to excite it, or does it become self exciting once the engine has been started? My train of thought, is that, if the battery failed the external excitation source would fail, therefore the alternator would not provide the output voltage to the bus bar and electrical power would be lost all together, is this correct?. .. .I hope the above makes sense! Any responses would be greatly received.. .. .Thank you.. .. .CAT 3C AUTOLAND.. .. .PS. I also posted this in the questions forum, but thought it may be worth posting here aswell <img border="0" title="" alt="[Smile]" src="smile.gif" />
Join Date: Jan 2001
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Hi,. .A simple answer to your question is to look at the system on a large aircraft..... .. .The jet engine normally has a generator attached to it's own ancillary gearbox, as the engine is started, it rotates the gearbox and the gene' rotates also. The gene' gets "excited" and produces current, fed to the Voltage Regulator etc etc. If the battery fails the engines still rotate and the gene's are still producing power...all very well until the battery dies completely and the DC control circuits drop out and thats when you normally lose the AC and control!. .. .Hope this quick answer helps but you might be going a little too deep for PPL!. .Regards Rodders <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />
Join Date: May 2001
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Not going against what rodders says about big jets. . .. .But you can also get self exciting alts which have an exciter on the shaft.. .This takes the form of a DC generator using perm magnets, so as long as the shaft is rotating there is a DC supply.. .. .But as rodders says its way to deep for PPL and you will get a 2" thick manual on this stuff if you do your ATPL exams. Both oxford and bristol ground school notes are pretty good on it.. .. .If you have a look at this web page. .. .<a href="http://www.perr.com/gen.html" target="_blank">http://www.perr.com/gen.html</a>. .. .under generator basics there is a good description how the thing gets its self going.
Join Date: Jan 2002
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From how I understand Trevor Thom (see vol4, p225):. .. .1. if your plane has a generator, it is fine producing electricity without any excitation current. .. .2. an alternator (which most planes have these days), it requires excitation, and "even if you hand-swing the propeller, the alternator won't come on line unless the battery has at least some residual voltage"
Join Date: May 2001
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The alternator core has some residual field left in it due to the fact that its wound round soft iron to increase the magnetic flux. This will be enough to start the process of field building.. .. .The main aircraft DC bus is running at above bat voltage (otherwise the bat would never recharge).. .So after the alternator comes on line the bat recharges and does nothing until the alt goes off line.. .. .I wouldn't suggest you try swinging a prop with the battery disconnected. But you could try bump starting your car with the battery disconnected.. .. .MJ
