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Date: Mon, 23 May 2005 21:38:44 -0500
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Amps consumed data
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WALTER B KERR wrote:

>Finally got around to getting batteries in the clamp on amp meter and a
>run with the cowl off and someone to clamp the feed line between the
>battery and alternator. The alternator was pumping 19.3 amps after
>turning the alternator field on immediately after the start with
>everything on in the airplane. When I switched the alternator off it went
>to 0 despite some folks thinking it would continue to produce using its
>on voltage to power the field. This does not happen in my case where
>there is only one wire of the 3 connected to the plug. It is a battery
>supply line via a switched CB to the B lead on the alternator. One of the
>things I wish to get input on at Charlie's(really Tupper's) this weekend
>is advice on hooking up a voltage crowbar to keep the alternator from
>burning up my EC2 filter or whatever else. It seems like an easy thing to
>do since shutting off this one line stops the output cold. 
>
>After running a few minutes to get the battery charged back measured the
>amps going to the bus with everything off except engine stuff and it was
>6 amps at 1400 rpm and 10 amps at 4500rpm. So my 1 hour on the battery
>only flight at 5000 rpm probably consumed most of the amps available in
>the PC625 battery.
>
>Still would be interesting to run a test on the ground from 11 volts till
>engine dies , but want to clear this with Tracy first.
>
>Bernie
>
 Are you saying that you are running an internally regulated, 'one-wire' 
alternator & you were measuring current, not voltage? If so, the current 
measurement dropping to zero when you break the B-lead doesn't mean the 
alternator isn't 'putting out.' There won't be any current flowing 
because the circuit is no longer complete, but the alternator can still 
be producing voltage potential at the B-lead terminal. The only way to 
tell is to have a volt meter hooked directly to the B-lead terminal when 
you open the B-lead. Same idea as an electrical outlet in your house: 
120 VAC al the time but no current until you plug something in & turn it on.

The aeroelectric list hand-wringing about 'load dump' should only be 
relevant if you lose the connection to the *battery*. If the regulator 
fails & allows unregulated output from the alternator then the B-lead 
overvoltage disconnect relay described in the 'Connection' is what 
prevents the high voltage from damaging the a/c electronics. By the time 
you get to this point, 'load dump' is meaningless because the result of 
load dump (overvoltage) has already happened & preventing load dump is 
no longer an issue.

We'll talk when you get here on Friday.

Charlie