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alec(at)alecmyers.com
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 Posted: Wed Jul 22, 2026 5:31 am Post subject: Galloping alternator - Piper Warrior |
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Hi Bob et al
I have been reading Aeroelectric resources on the cause of the galloping alternator issue, and how to fix. I just want to verify that the circuit where a too-high resistance will cause an oscialltion is this one (yellow):
And that a differential diagnosis is that the problem goes away on temporary connection of the red wire - have I understood that correctly?
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alec(at)alecmyers.com
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| Posted: Wed Jul 22, 2026 10:03 am Post subject: Galloping alternator - Piper Warrior |
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Another question for Bob, about the galloping alternator issue.
The aircraft alternator field coils have (typically) resistance of 4 ohms and inductance 0.25 henrys. That gives a time constant of 62 milliseconds, or equivalently a 90 degree phase shift at 2.55Hz. Values are of course approximate but order-of magnitude is useful.
If the oscillation is about 3 Hz (from memory) there’s a second parasitic pole at about 3.5Hz. This means the phase shift at 3Hz has hit 180° below the frequency at which the magnitude of the loop gain is below 1. Hence the oscillation.
I understand that a field circuit resistance of 200 milliohms is likely to be troublesome.
An RC circuit with a R value of 200 milliohms needs a C value of 0.225 Farads. Which is enormous. So - the question - is it the chemistry in the aircraft battery that’s acting as a capacitor with that sort of value?
On Jul 22, 2026, at 09:30, Alec Myers <alec(at)alecmyers.com> wrote:
Hi Bob et al
I have been reading Aeroelectric resources on the cause of the galloping alternator issue, and how to fix. I just want to verify that the circuit where a too-high resistance will cause an oscialltion is this one (yellow):
<Screenshot 2026-07-22 at 09.24.57.png>
And that a differential diagnosis is that the problem goes away on temporary connection of the red wire - have I understood that correctly?
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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