steve(at)tomasara.com
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 Posted: Mon Apr 08, 2013 9:05 am Post subject: Charging System Debug steps and success... |
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Greetings All,
From time-to-time people have posted assorted problems, and gotten
answers to, charging system issues. Many of these are of the wandering
or oscillating voltage nature.
As my charging system (LongEz, externally regulator "ford-style"
charging with regulator and battery in the nose) developed an
oscillating attitude I had the "opportunity" to collect, review and then
apply the information Bob has generously provided and thought an
overview might prove useful for the next guy down the road. I'll start
with key summary points for those who don't have the current need or
desire to read through the details.
My symptoms were an oscillating (2 - 6 Hz maybe?) charging voltage
visible with the old (WesTach) panel meter. It was sufficient to pop
the over-voltage crowbar once. It might also be related to an transient
undervoltage alarm I got once.
After isolating the problem, I fixed it by replacing the store-bought
regulator connector (which had pig-tails, the bad connection was on the
factory-made regulator +V crimp) with good-quality Fast-ons direct to
the regulator. This dropped the alternator B+ to regulator V+
resistance from 528mOhms to ~140mOhms which fixed the problem. btw: the
140 ohms, which is still higher than I would like, seems to be roughly
evenly shared by the fuse-link, breaker and switch (and associated wiring).
The details and references for "the next guy":
The debug strategy was to start by checking the resistance (using
techniques appropriate for milliohm measurments), without doing any
disconnecting of the current loop path from alternator B+ to the
regulator and then back from the regulator ground (i.e. case) to the
alternator ground (i.e. case). My plan was to keep dividing the problem
in half to minimize debug time (i.e. binary search). I wanted to do as
much testing as I could without taking anything apart as I didn't want
to inadvertently change the problem (i.e. wiggle something and have the
problem go away...)
In the case that no problems were uncovered via the milliohm
measurements, I then planned to follow Bob's charging component problem
isolation technique which is in chapter 3 of the Aeroelectric connection
book. This involves making measurements while the engine is running but
I didn't need go to that step and I certainly didn't want to start
there. For lots of charging system background and debug information,
search Bob's site (www.aeroelectric.com) for "charging" and ignore all
the stuff on plug-in-the-wall battery chargers.
Bob has written articles and has product related to making good milliohm
measurements using an applied current. I prefer having the current
source (whether a bench supply or other similar to Bob's milliohm
probes) separate rather than combining it with the volt-meter probes.
Also, as Bob has covered in his articles, you don't need a
regulated-to-a-known-value current source. It suffices to have an
unregulated current source (like a D-cell, in a pinch) if you are using
a second meter to measure the current. For more information on accurate
milliohm measurements, search Bob's site for "milliohm".
I conceptually divided the loop in half (ground side vs power side) and
picked the ground side first. I put a current source (bench supply with
an accurate current limit set to 1A (and set the voltage limit to be 2V
though I didn't expect it to ever run in voltage mode regulation))
between the alternator case and the regulator case (which, on my LongEz
is the length of the entire aircraft). I then measured the voltage
(using separate wires) between the two cases and got 33.9mV which, at
one amp, means 33.9mOhms. This is well below the ~200mOhms threshold
area of concern (per Bob's annotations contained in
"Know_Your_Charging_System.pdf"). Therefore the problem is in the V+ side.
I repeated this on the V+ side which is a little bit trickier to measure
as the "master" progressive switch needs to be in the "Alternator on"
position which can add more currents to deal with (though, it turns out,
not much). With the switch on, I measured the voltage from alternator
B+ to regulator V+ both with the current source connected, and not
connected. The value I care about is the connected value minus the not
connected value. (though it still applies, I won't repeat this detail
from now on). I measured 528mV (=528mOhms) which is well above the
200mOhm area of concern. So I've identified one (but maybe only the
first?, it turned out to be only) problem.
I then listed all (well, sort of, I didn't list both side of each
wire...) of the connections between Alternator B+ and regulator V+. For
me, starting from B+: Near side of load-meter shunt, near side of ANL,
near side of fuse-link, near side of over-voltage breaker, nearside of
alternator switch, V+ at the regulator.
I kept the current source running between B+ on the alternator and
regulator V+ (no reason to move it) and picked the point in the middle
(near side ANL), B+ to near side ANL was only 11mV (=11mOhms). I
continued the divide (roughly in half) and conquer approach (continuing
to measure from B+ to my point of interest) which led me quickly to the
crimp at the regulator.
In the future I *might* start by measuring any connections I didn't
personally make (of which there are very very few) before switching to
the more disciplined binary search approach. In this case I would have
gotten lucky but, in truth, it would not have saved much time. The time
was spent going through Bob's articles and developing a plan. The
execution of the plan went very fast.
Repair and run-up test showed success.
Specific Bob articles I found helpful (in addition to those on milliohm
measurement):
Know_Your_Charging_System.pdf
03_Alternator_12A2.pdf (i.e. the current chapter 3 in the aeroelectric
connection, start at page 3-7 for debug info)
Happy debugging!
Steve Stearns
Boulder/Longmont, Colorado
Restoring (since 1/07) and flying again (8/11!): N45FC O235 Longeze
Cothern/Friling CF1 (~1000 Hrs)
Flying (since 9/86): N43732 A65 Taylorcraft BC12D
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