The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Fri, 27 Sep 2002 12:47:45 +0100
Organization: (Posted via) GTS Netcom.
NNTP-Posting-Date: Fri, 27 Sep 2002 11:41:26 +0000 (UTC)
X-Newsreader: Forte Agent 1.8/32.548
>The diagram above does not match the OVP/VR/Alternator wiring found in
>Cessnas and Pipers.
>The biggest errors are:
>1. The VR does NOT do "remote sensing". The "sensing" is done via
> the same wire that connects the VR to the main bus through
> the "field" switch. There is no separate "sense" input to the VR.
>2. One of the field slip ring brushes is grounded to the alternator
> frame, hence the VR modulates the field current by breaking
> the path between the field switch and the non-grounded brush.
> Only a few MOPAR alternators used low side switching as depicted
> in your diagram. I've never seen one of these on an aircraft.
I realise the above will vary. However I don't think it matters to the
issue in question, because the VR has to *somehow* sense the
alternator output, and has to *somehow* control the field current.
Unless I am missing something, the exact topology should not matter.
>In automobiles, the "field switch" is a set of contacts in the ignition
>switch, which is closed in the RUN position, and is automatically
>OPEN while the key is in the START position, effectively shutting
>down the entire charging system during cranking.
>In airplanes, the
>pilot gets to decide if he/she cranks with the alternator field
>switch open or closed!
>The best reason for cranking with the alternator field OPEN is that
>by so doing, you save 2 to 3A of load on the battery from the
>time you turn on the master, until the engine is running. Further,
>cranking with the field open makes a few more ft-lbs of starting motor
>torque available for cranking the engine. These effects are minimal
>on warm days, and with a near-new battery. They can help a lot during
>cold weather starts.
The above is exactly what I thought.
>Cranking with the field breaker/switch closed will not generate any
>significant transients. What generates the biggest transients is either
>the starter motor itself, or the starter solenoid. I have recorded
>transients of > 400V across the main bus in Cessnas that came out of
>the factory without the proper spike suppressor diodes...
That would suggest the problem is associated purely with starting and
not shutdown. Where are the spike suppressors? A diode across the
starter relay will do something but I suppose one could fit a beefy
VDR across the bus somewhere...
How long were the 400V transients?
There is an ISO spec for permissible vehicle supply transients, for
12V and 24V systems. I did design some equipment to that spec some
years ago, and IIRC it allows for about +/-150V transients on a 12V
system; presumably twice that on a 24V system.
>Cranking with the alternator field open protects the OVP/VR from said
>transients, just like having all of the avionics off protects the avionics
>There is no transient generated while the engine is stopped with the idle-
>cuttoff. It doesnt matter if the avionics/alternator are left on while
>the engine stops. However, an unsupressed master relay solenoid can also
>put out a 400V spike, so it is a good idea to open the alternator field
>switch, and turn off all the avionics before turing off the master.
>In Cessna with a split master switch, this happens automatically, because
>the ALT half of the Master switch is mechanically interlocked with the
>The key to eliminating spike-induced damage to avionics and/or the OVP/VR
>is having said equipment off line at the instant the spikes are generated,
>and also by installing "catch" diodes across the starter and master solenoids,
>as well as having a "transorb" (MOV or Zener) across the main bus...
Is there a US STC for these devices? Or are any "FAA-certified"? I am
in the UK and cannot do anything although if there is a STC the UK CAA
respects that (in trivial mods like this).
>owner Skylane '1MM
> Pacer '00Z
Peter (only a BSc EE :)
Return address is invalid to help stop junk mail.
E-mail replies to zX80@digiYserve.com but remove the X and the Y.
Please do NOT copy usenet posts to email - it is NOT necessary.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup