X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Fri, 16 Jun 2006 18:08:59 -0400 Message-ID: X-Original-Return-Path: Received: from smtp110.sbc.mail.mud.yahoo.com ([68.142.198.209] verified) by logan.com (CommuniGate Pro SMTP 5.0.9) with SMTP id 1180884 for lml@lancaironline.net; Fri, 16 Jun 2006 16:30:23 -0400 Received-SPF: none receiver=logan.com; client-ip=68.142.198.209; envelope-from=elippse@sbcglobal.net Received: (qmail 93387 invoked from network); 16 Jun 2006 20:29:37 -0000 Received: from unknown (HELO Computerroom) (elippse@sbcglobal.net@75.15.122.1 with login) by smtp110.sbc.mail.mud.yahoo.com with SMTP; 16 Jun 2006 20:29:36 -0000 X-Original-Message-ID: <000501c69183$987d0030$017a0f4b@Computerroom> From: "Paul Lipps" X-Original-To: "Marv Kaye" Subject: Avionics transient X-Original-Date: Fri, 16 Jun 2006 13:29:32 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0002_01C69148.E6FC8720" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2869 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2869 This is a multi-part message in MIME format. ------=_NextPart_000_0002_01C69148.E6FC8720 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable I have a friend who accidently left his master switch on while at a = picnic at a small airport. When he tried to start the engine - nothing! = He got a hand prop, then immediately took off for the short 10 mile = flight back to his home airport. He noticed that his 35A alternator = breaker on his O-320's Nissan alternator had popped, so he pushed it in, = and it popped right back out. This happened several times. After he got = back he asked me what I thought happened. It finally dawned on me that = once he took off and the rpm came up that the alternator was putting out = so much current into the almost dead battery that each time he restored = the breaker, the same thing happened, since the battery wasn't getting = charged, and actually was discharging some from his minimal avionics and = the field supply. When he was taxying at his departure airport, there = wasn't enough rpm to give the high current demand of the low battery to = pop the breaker. Ive seen CB's that will just pop with no overload; = remember, they are temperature-activated devices, and their ratings = change with ambient temperature as well as the time-current product of = an overload. Another scenario would be if you were checking the operation of a = back-up alternator in flight. Some people have installed a second = alternator connected to their main buss but with its regulator adjusted = to a lower voltage to pick up the load if the main charging system = fails. So if someone decides to kill the main alternator by pulling its = circuit-breaker in flight to check his secondary system, the lower buss = voltage will still cause the main regulator to pump max voltage into the = field to get its alternator output up. 'Remember the circuit that they = used to install, and maybe still do, to disconnect their truck or car = alternator from the battery and use it to drive power-tools on a job = site by putting a higher current on the field? They would adjust the = field supply to give them 120V! A voltage transient in excess of 100V = onto the buss can do a lot of damage in the few milliseconds that it = takes for the filtered OV circuit to short and put a load on the field = breaker, which then takes a few ms to open, which then takes a few ms of = the alternator field and stator windings to discharge. Do you have a = diode or TVS across the field to slow field decay to reduce the field's = high inductance transients back into the wiring. WOW! That will add to = the field's L/R decay time-constant! Then you have the fact that the = battery isn't known for having a low impedance to short-duration = transients. Then, too, there is the wiring inductance between the = alternator and the battery. Are your loads connected in between the = alternator and the battery, where they may experience the inductive = transient voltage drops along the wires? Guys; if all you want to do is argue whether it can happen or not, = putting forth all kinds of scientific facts and scenarios, just = disregard what I've written as the ravings of a lunatic. But I still ask = you to make sure that if your alternator breaker does pop, for whatever = reason, pull the field breaker or turn it off at a split master before = restoring the alternator breaker. OK? ------=_NextPart_000_0002_01C69148.E6FC8720 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
    I have a friend who = accidently left his=20 master switch on while at a picnic at a small airport. When he = tried to=20 start the engine - nothing! He got a hand prop, then = immediately took off=20 for the short 10 mile flight back to his home airport. He noticed that = his=20 35A alternator breaker on his O-320's Nissan alternator had = popped, so=20 he pushed it in, and it popped right back out. This happened several = times.=20 After he got back he asked me what I thought happened. It finally dawned = on me=20 that once he took off and the rpm came up that the alternator was = putting out so=20 much current into the almost dead battery that each time he restored the = breaker, the same thing happened, since the battery wasn't getting = charged, and=20 actually was discharging some from his minimal avionics and the field = supply.=20 When he was taxying at his departure airport, there wasn't enough rpm to = give=20 the high current demand of the low battery to pop the breaker. Ive seen = CB's=20 that will just pop with no overload; remember, they are = temperature-activated=20 devices, and their ratings change with ambient temperature as well as = the=20 time-current product of an overload.
    Another scenario would be if = you were=20 checking the operation of a back-up alternator in flight. Some people = have=20 installed a second alternator connected to their main buss but with its=20 regulator adjusted to a lower voltage to pick up the load if the main = charging=20 system fails. So if someone decides to kill the main alternator by = pulling its=20 circuit-breaker in flight to check his secondary system, the lower buss = voltage=20 will still cause the main regulator to pump max voltage into the field = to get=20 its alternator output up. 'Remember the circuit that they used to = install, and=20 maybe still do, to disconnect their truck or car alternator from the = battery and=20 use it to drive power-tools on a job site by putting a higher = current on=20 the field? They would adjust the field supply to give them 120V! A = voltage=20 transient in excess of 100V onto the buss can do a lot of damage in the = few=20 milliseconds that it takes for the filtered OV circuit to short and put = a load=20 on the field breaker, which then takes a few ms to open, which then = takes a few=20 ms of the alternator field and stator windings to discharge. Do you = have a=20 diode or TVS across the field to slow field decay to reduce the field's = high=20 inductance transients back into the wiring. WOW! That will add to = the=20 field's L/R decay time-constant! Then you have the fact that = the=20 battery isn't known for having a low impedance to short-duration = transients.=20 Then, too, there is the wiring inductance between the alternator and the = battery. Are your loads connected in between the alternator and the = battery,=20 where they may experience the inductive transient voltage drops = along the=20 wires?
    Guys; if all you want to do = is argue=20 whether it can happen or not, putting forth all kinds of scientific = facts and=20 scenarios, just disregard what I've written as the ravings of a = lunatic.=20 But I still ask you to make sure that if your alternator breaker = does pop,=20 for whatever reason, pull the field breaker or turn it off at a = split=20 master before restoring the alternator breaker. = OK?
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