X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from relay01.roc.ny.frontiernet.net ([66.133.182.164] verified) by logan.com (CommuniGate Pro SMTP 5.0c2) with ESMTP id 717755 for flyrotary@lancaironline.net; Sat, 10 Sep 2005 15:49:40 -0400 Received-SPF: pass receiver=logan.com; client-ip=66.133.182.164; envelope-from=canarder@frontiernet.net Received: from filter04.roc.ny.frontiernet.net (filter04.roc.ny.frontiernet.net [66.133.183.71]) by relay01.roc.ny.frontiernet.net (Postfix) with ESMTP id 8B1843640A3 for ; Sat, 10 Sep 2005 19:48:56 +0000 (UTC) Received: from relay01.roc.ny.frontiernet.net ([66.133.182.164]) by filter04.roc.ny.frontiernet.net (filter04.roc.ny.frontiernet.net [66.133.183.71]) (amavisd-new, port 10024) with LMTP id 02732-01-90 for ; Sat, 10 Sep 2005 19:48:56 +0000 (UTC) Received: from [127.0.0.1] (unknown [70.98.128.118]) by relay01.roc.ny.frontiernet.net (Postfix) with ESMTP id DD22F3640BA for ; Sat, 10 Sep 2005 19:48:55 +0000 (UTC) Message-ID: <4323389F.80401@frontiernet.net> Date: Sat, 10 Sep 2005 14:48:47 -0500 From: Jim Sower User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7) Gecko/20040514 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Rotary motors in aircraft Subject: Re: [FlyRotary] Re: Overvoltage control (help Ed A) References: In-Reply-To: Content-Type: multipart/alternative; boundary="------------080908030807020209070807" X-Antivirus: avast! (VPS 0536-5, 09/09/2005), Outbound message X-Antivirus-Status: Clean X-Virus-Scanned: by amavisd-new-2.3.2 (20050629) at filter04.roc.ny.frontiernet.net This is a multi-part message in MIME format. --------------080908030807020209070807 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Finn Lassen wrote: > This sentence doesn't make sense: "If my crowbar overvoltage opens the > B+ circuit..." > Crowbar shorts a circuit to ground. A relay or contactor allows a > circuit to open. > The relay could be in line with the wire to the field winding > (handling just a few amps) or in line with the B+ wire (output from > alternator). > > The crowbar typically shorts the field lead to ground, popping its CB > or blowing its fuse. > The "field" lead actually controls the B+ relay. Short it to ground, > blow the fuse and the relay opens isolating the alternator. > The problem with all this is that these safeguards introduce failure > modes of their own: > The crowbar can be triggered by a brief spike and short the voltage to > the field winding. > True. If you design it that way. I've heard of no "false positives" > around the system I use. > The contactor can stick closed (on) or open (off), and of course uses > continuous power to stay closed and adds weight and cost. > True. But I've never heard of that happening (starter relays - yes; > master relays - NO) > If one or all of the main rectifier diodes in the alternator shorts > out, you will have an alternating current (voltage) feeding the > battery and other loads. I sure hope that the "overvoltage" detector > will recognize this and kill the voltage to the field winding or > disconnect the wire from the B+ terminal. I'm not sure that ANY OV > protection devices will detect failed diodes. Which one have I missed? > > I suspect that the regulators in our alternators are very reliable, > providing they are adequately cooled. That would be one advantage for > having a external regulator: it can be mounted in a cooler location. > On the other hand, I don't know how reliable those "cheap" ford > regulators are... Looking at the automotive industry in general, I'm > confident that they design their regulators to perform adequately > inside the alternator. > > It sure would be nice with some real world info from an alternator > repair shop as to what are the most common failure modes of the > alternators we use. I could quote you song and verse back when I was > in the business. Things might have changed in the past 25 years, but > not a whole lot I would suspect. I am pretty sure that rebuild shops > just field strip the units and rebuild. They're not in the data > gathering business, so they don't spend valuable time exploring > failure modes. They know about them, but I've never run into one that > had any better statistical data than I could pull out of my ass from > my experience 30 years ago. > > I'm having a feeling of deja vu. Haven't we discussed this already? > Yup. No definitive answers then either IIRC. > > Finn > > Jim Sower wrote: > >> If my crowbar overvoltage opens the B+ circuit, there's NO current >> flowing out of the alternator. Absent current, there can be no heat >> generated. The field can go to max output, there will be high >> voltage at the B+ terminal, but no heat generated. Sounds to me like >> a bogus concern. I think the key issue is "... Nuckolls expressed >> dislike ..." and no further support is "needed". > > > > > -- > Homepage: http://www.flyrotary.com/ > Archive and UnSub: http://mail.lancaironline.net/lists/flyrotary/ > > --------------080908030807020209070807 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit Finn Lassen wrote:
This sentence doesn't make sense: "If my crowbar overvoltage opens the B+ circuit..."
Crowbar shorts a circuit to ground. A relay or contactor allows a circuit to open.
The relay could be in line with the wire to the field winding (handling just a few amps) or in line with the B+ wire (output from alternator).

The crowbar typically shorts the field lead to ground, popping its CB or blowing its fuse.
The "field" lead actually controls the B+ relay.  Short it to ground, blow the fuse and the relay opens isolating the alternator.
The problem with all this is that these safeguards introduce failure modes of their own:
The crowbar can be triggered by a brief spike and short the voltage to the field winding.
True.  If you design it that way.  I've heard of no "false positives" around the system I use.
The contactor can stick closed (on) or open (off), and of course uses continuous power to stay closed and adds weight and cost.
True.  But I've never heard of that happening (starter relays - yes; master relays - NO)
If one or all of the main rectifier diodes in the alternator shorts out, you will have an alternating current (voltage) feeding the battery and other loads. I sure hope that the "overvoltage" detector will recognize this and kill the voltage to the field winding or disconnect the wire from the B+ terminal.   I'm not sure that ANY OV protection devices will detect failed diodes.  Which one have I missed?

I suspect that the regulators in our alternators are very reliable, providing they are adequately cooled. That would be one advantage for having a external regulator: it can be mounted in a cooler location. On the other hand, I don't know how reliable those "cheap" ford regulators are... Looking at the automotive industry in general, I'm confident that they design their regulators to perform adequately inside the alternator. 

It sure would be nice with some real world info from an alternator repair shop as to what are the most common failure modes of the alternators we use.   I could quote you song and verse back when I was in the business.  Things might have changed in the past 25 years, but not a whole lot I would suspect.  I am pretty sure that rebuild shops just field strip the units and rebuild.  They're not in the data gathering business, so they don't spend valuable time exploring failure modes.  They know about them, but I've never run into one that had any better statistical data than I could pull out of my ass from my experience 30 years ago.

I'm having a feeling of deja vu. Haven't we discussed this already?  Yup.  No definitive answers then either IIRC.

Finn

Jim Sower wrote:

If my crowbar overvoltage opens the B+ circuit, there's NO current flowing out of the alternator.  Absent current, there can be no heat generated.  The field can go to max output, there will be high voltage at the B+ terminal, but no heat generated.  Sounds to me like a bogus concern.  I think the key issue is "... Nuckolls expressed dislike ..." and no further support is "needed".



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