PPRuNe Forums AC and DC in a charging system

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 11th Jul 2012, 16:45 #1 (permalink) Join Date: Jul 2006 Location: UK Posts: 12 AC and DC in a charging system Sorry if this is in the wrong forum, I was hoping that somebody could offer some guidance on a task I've been set. I've been asked to "discuss the principals of alternating current and direct current in a charging system". To me the question is very vague, so I'm assuming they mean a circuit with a capacitor to store charge. In DC, I understand the current flows in one direction, and as the capacitor charges to the DC value of the source, the current flow and charge decrease and increase respectively exponentially until the capacitor is fully charged and the current flow is zero. Hence why capacitors appear to block DC. The capacitor then holds the charge until the source is removed. However I'm struggling with AC. Am I right in saying that as the current flows in one direction then another, the capacitor charges and discharges with alternating polarity? Could somebody please elaborate on how a capacitor behaves in AC please? Thank you in advance, I appreciate any help on this. Best Wishes.
 11th Jul 2012, 18:06 #2 (permalink) Join Date: Sep 2009 Location: bahrain Age: 24 Posts: 341 Capacitors in ac circuits will attenuate the current because they charge in one direction then discharge and charge in the other direction, hence the capacitive reactance comes, In a capacitive AC circuit it is noticed that the voltage lags the current by 90 degrees phase angle due to this effect, the more the capacitance the less the capacitive reactance hence more Current flows in the circuit because the capacitor does not hve sufficient time to get around the fully charged state and resist the current, same thing goesnwith frequency the more the frequency the less the capacitive reactance, Thats why when capacitors are connected in series with an ac source are used usualy as high pass filters and a low pass filter when connected in parallel. Thats all i can say about capacitors in ac at the moment ifnyou have further inquiries feel free to pm. Last edited by flame_bringer; 11th Jul 2012 at 18:07.
 11th Jul 2012, 20:09 #3 (permalink) Join Date: Jul 2006 Location: UK Posts: 12 Hi, Thank you for your replies. The capacitance/capacitor idea was just an assumption I made based in the fact that capacitors can store charge. So going down the alternator vs dc generator route, how does each system charge a battery and how does the principles of operation differ between the two techniques? I am trying to research this but it's a bit confusing. Thank you again for your help.
 11th Jul 2012, 21:15 #4 (permalink) Join Date: Dec 2007 Location: devon Age: 74 Posts: 283 As I understand it, and I stand to be corrected, an alternator on a car produces AC which is rectified by the diode pack mentioned into DC to feed the battery etc. This system gives a better charge rate at lower revs than the old DC dynamos.
 11th Jul 2012, 22:15 #5 (permalink) Join Date: Apr 2007 Location: in the shed Age: 58 Posts: 94 first of all we must know more of the initial task, as it is its like saying is an apple better than an orange a dc charging system in an older type motor car works just fine ie the system recharges the batt after eng start so the batts ready for the next start ----- got that? there is no sutch thing as an ac charging system, you can have an alternator,like a modern type of motor car where the pwrs made in a alternator (ac) then turned into dc (lots of miracles) then passed to batt --does that make sense?? i have a bunch of old motorbikes that have dc systems and they all work just fine, you have to remember they are products of their time and if you do not expect too much you will not be dissapointed. scuuse the spling its the whisk you know!! gs.
 11th Jul 2012, 23:05 #6 (permalink) Join Date: Nov 2010 Location: Scandinavia Posts: 330 Look what happens when you google "discuss the principals of alternating current and direct current in a charging system"..... http://autoclub.atilim.edu.tr/dokuma.../Chapter08.pdf Not aircraft specific but it's probably a good place to start. Last edited by Avionker; 11th Jul 2012 at 23:11.
 12th Jul 2012, 06:43 #7 (permalink) Join Date: Feb 2005 Location: Past the rabbit proof fence Posts: 212 It wouldn't pertain to pulse and trickle charging of batteries would it? Nicad batteries are usually charged with a pulse/reflex charger which can supply a full charge in a greatly reduced time. Lead acid batteries are usually charged with the constant current method over a long time period.
 12th Jul 2012, 06:44 #8 (permalink) Join Date: Apr 2008 Location: Cilboldentune, Britannia Posts: 513 WRT light aircraft: Early aircraft were usually fitted with 'dynamos' which generate DC and don't need any rectification; they have a separate voltage regulator usually mounted on the firewall. Although generating DC there is still some 'noise' in the system and capacitors are fitted to reduce problems with radios. Later aircraft were fitted with alternators which have an internal bridge diode rectifying system to produce DC and unlike a car have a separate voltage regulator, again usually mounted on the firewall. A capacitor is fitted between the alternator output and ground this is to smooth out any remaining AC and to stop radio interference. This is often not enough and a 'choke' is also fitted in the output line. A capacitor passes AC and blocks DC is 'wot' I was taught.
 15th Jul 2012, 09:24 #9 (permalink) Join Date: Jul 2006 Location: UK Posts: 12 Thanks all for your replies! One more question, then I'll stop It's says in the PDF document from the link provided Avionker that "The battery cannot be recharged with alternating current." Can somebody please explain the fundamental reason why a battery can't be charged from AC? Many thanks.
 15th Jul 2012, 10:16 #10 (permalink) Join Date: Sep 2009 Location: bahrain Age: 24 Posts: 341 Because it will discharge in one cycle and charge in the other so nothing will be gained, In order to charge up you have to apply current in the reverse direction of the battery discharging to reverse the chemical reaction inside it and hence get it charged. Last edited by flame_bringer; 15th Jul 2012 at 10:17.
 15th Jul 2012, 10:28 #11 (permalink) Join Date: Nov 2010 Location: Scandinavia Posts: 330 Quite simply, a battery is a DC power source. When a battery is discharging a chemical reaction occurs between the electrodes and the electrolyte. In order to recharge the battery current must flow in the opposite direction in order to reverse the chemical reaction. fb beat me to it I see... Last edited by Avionker; 15th Jul 2012 at 10:29.
 15th Jul 2012, 11:04 #12 (permalink) Join Date: Jul 2006 Location: UK Posts: 12 That's great! Thanks for all your help...and patience!

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