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Date: Tue, 01 Sep 2009 09:31:48 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: FW: More Charging Circuit Info
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Mike Wills wrote:
> Al,
> That wasnt from me, that was from the weblink Kelly sent. I simply=20
> quoted it. I dont know if it is true or false, but I do know that at=20
> least one of my previous cars opened the alternator field any time the =

> starter was engaged. It is my understanding that this is typical of=20
> car ignition switches though I dont know if its necessary or just a=20
> holdover.
>
>     ----- Original Message -----
>     *From:* Al Gietzen <mailto:ALVentures@cox.net>
>     *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net=
>
>     *Sent:* Monday, August 31, 2009 12:18 PM
>     *Subject:* [FlyRotary] Re: FW: More Charging Circuit Info
>
>     From Mike - =85..ensure that the field is de-energized while
>     cranking the engine.
>
>     And I will respectfully disagree with that =96 at least until
>     somebody can prove me wrong. The output current is a function of
>     rpm as well as what the voltage regulator is demanding. From what
>     I can find, I conclude that alternators typically won=92t put out
>     their rated current until they get spinning about 1500 =96 2000 rpm=
,
>     or more (which is usually about double engine rpm). So there is no
>     risk of damaging the rectifier diodes at cranking speed.
>
>
Consider this: Turning the alt off during cranking isn't a feature to=20
protect on-board electronics. Maybe it is there to protect the alternator=
=2E

Al is correct. The alternator output is limited by idle speed and=20
generally can't output enough energy to do any damage at cranking speeds.=


But say for instance the regulator is demanding 10A at 13.4V. The output =

is determined by the combination of how fast the rotor is spinning, and=20
the strength of the rotor's magnetic field. The rotation speed is=20
directly coupled to the engine speed, which doesn't amount to much=20
during cranking. The field strength is tied to how much current the=20
regulator is pumping through the rotor windings. To get that 10A/13.4V,=20
the regulator is going to have to open the rotor current completely.

The regulator will be pumping multiple amps through the rotor windings=20
and getting no appreciable output for it's efforts. The current is=20
wasted, doing nothing but heating the regulator circuitry and rotor=20
windings while reducing power to run the starter.

Why would you leave the alt on the line, when it is so easy to disable=20
it? Pulling all the other electrical stuff off the line during starting=20
isn't necessarily done to protect one device vs another. From an=20
engineering perspective, it just makes for a simpler system. There are=20
unknown states that simply get removed from the equation. If you're=20
having to turn out a new car model every year, and don't want to=20
implement a testing regimen for each car, pulling everything off line is =

just the sane thing to do. For a pampered, one-off project like an=20
airplane, the necessary testing regimen is already in place. If=20
everything works when turned on before start, go fly it. If not, put in=20
some more switches...then go fly it.