Date: Fri, 09 Sep 2005 22:46:14 -0400
From: Finn Lassen <finn.lassen@verizon.net>
Subject: Re: [FlyRotary] Re: Overvoltage control (help Ed A)
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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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 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.

The contactor can stick closed (on) or open (off), and of course uses 
continuous power to stay closed and adds weight and cost.

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 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...

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'm having a feeling of deja vu. Haven't we discussed this already?

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".