Today's Message Index:
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1. 06:31 AM - Galloping alternator - Piper Warrior (Alec Myers)
2. 11:03 AM - Re: Galloping alternator - Piper Warrior (Alec Myers)
Message 1
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| Subject: | Galloping alternator - Piper Warrior |
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?
Message 2
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| Subject: | Re: Galloping alternator - Piper Warrior |
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) theres 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
thats acting as a capacitor with that sort of value?
On Jul 22, 2026, at 09:30, Alec Myers <alec@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?
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