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Mike Wills wrote:
Al,
That wasnt from me, that was from the weblink Kelly sent. I simply quoted it. I dont know if it is true or false, but I do know that at 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 car ignition switches though I dont know if its necessary or just a 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 - …..ensure that the field is de-energized while
cranking the engine.
And I will respectfully disagree with that – 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’t put out
their rated current until they get spinning about 1500 – 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 protect on-board electronics. Maybe it is there to protect the alternator.
Al is correct. The alternator output is limited by idle speed and 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 the strength of the rotor's magnetic field. The rotation speed is directly coupled to the engine speed, which doesn't amount to much during cranking. The field strength is tied to how much current the regulator is pumping through the rotor windings. To get that 10A/13.4V, the regulator is going to have to open the rotor current completely.
The regulator will be pumping multiple amps through the rotor windings and getting no appreciable output for it's efforts. The current is wasted, doing nothing but heating the regulator circuitry and rotor windings while reducing power to run the starter.
Why would you leave the alt on the line, when it is so easy to disable it? Pulling all the other electrical stuff off the line during starting isn't necessarily done to protect one device vs another. From an engineering perspective, it just makes for a simpler system. There are unknown states that simply get removed from the equation. If you're having to turn out a new car model every year, and don't want to 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 airplane, the necessary testing regimen is already in place. If everything works when turned on before start, go fly it. If not, put in some more switches...then go fly it.
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