What you have not said is what the voltage at the battery is when the
aircraft is running and the alternator is charging. This voltage
should be 13.9 to 14.2 volts at the battery at about 70F degrees. The
trip point on most over voltage limiters is set at 16.0 volts.
Both of these points needs to be measured with a simple bench setup
using a small light bulb for the alternator field load and a regulated
variable voltage power supply, Increase the voltage to the regulator
until the field light goes out. This is the voltage the regulator is
set to regulate at. Continue to increase the voltage until the over
voltage relay opens. This is the voltage the OVR is set to. Do not
exceed 17 volts.
Until you know these two voltages you are guessing and trying to fix
the problem with the hope and poke method.

A bad diode in the alternator can cause the over voltage to trip due
to the excessive voltage ripple. A bad diode in the alternator will
also limit your maximum output current to less than 2/3 of rated
current and may be as low as 1/3 if it is shorted.

Turn on all of your lights and other loads on and measure the current
from the alternator. Measure both the AC and DC currents. If the AC
current is more than 15% of the DC current when you are generating at
least 50% of rated current you have a bad alternator diode or stator
winding.

Make damn sure you do NOT have any stainless steel washers between the
battery post and the battery cables.

Do not use stainless parts in electrical circuits because they are
high resistance. The film that forms on stainless parts to keep them
from rusting can be very high resistance.

John

frerichsatrodotcom

On Fri, 09 Jul 2004 22:03:23 GMT, Chris Kennedy
wrote:

My turn in the barrel with alternator issues.

I have a PA-24 250 with an InterAv 50A alternator conversion installed
that has a chronic but transient issue that appears to be associated
with regulation. Observed symptoms include:

- Overvoltage relay kicking the alternator offline when the gear motor
shuts down when running the gear down (but not when running the gear up).

- Overvoltage relay kicking the alternator offline with demand spikes
(like hitting all the landing lights at the same time)

- Transient discharge indications on the EI volt/ammeter when running
the strobes.

I've replaced the spike guard cap, rebuilt the wiring harness, made sure
that the grounds are clean and that there are no ground loops. Measured
resistance between the regulator output and the field terminal on the
alternator is less than one ohm. The battery is new and the ship has
copper cables.

This issue isn't new; it's been observed with the aircraft since we
purchased it and we're still trying to track it down. The issue with
running the gear down is both the most common and the most disturbing,
since the last thing I want to do is take all the avionics offline in
order to recycle the alternator while swimming through the marine layer.
I'd considered putting MOVs on the gear motor, but that doesn't
address the real problem, which is that the regulator seems slow to
respond to changing load, causing overshoots when heavy loads drop off
and droops when loads come on.

So, two questions for the group:

- Anyone have a clue regarding what in the world is going on here and
- Does anyone know of an alternative to the InterAv conversion for the
O-540? Even my hangar mate with the '68 Cherokee has 60A alternator...

Thanks,
Chris.