nuckolls.bob(at)aeroelect
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 Posted: Mon Apr 08, 2013 6:46 pm Post subject: Charging System Debug steps and success... |
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At 12:04 PM 4/8/2013, you wrote:
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.
Yes. The "galloping ammeter" syndrome is almost
exclusive to alternator/regulator systems wherein
the wire with duties to sense bus voltage ALSO
carries alternator field current. This is a VERY
common configuration in tens of millions of cars
and tens of thousands of airplanes. A typical at-risk
system uses the legacy FORD regulator featured in
many of my writings and schematics.
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).
Of course the ideal supply loop resistance is
zero ohms. However a Bode plot of the closed-loop
response of the alternator-regulator-aircraft
system usually shows that several hundred mOhms
can be tolerated before control loop stability
margin becomes risky.
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!
The phenomenon we're exploring is the twin brother
of a 'ground loop'. In a ground loop, a potential
noise current shares a conduction path with a potential
victim (audio system, radio, etc). In our 'buss-loop',
a noisy current (field supply) shares a pathway with
a potential victim (voltage regulator).
Operating currents for the alternator field can
be anywhere between a few hundred milliamps (very
light load, high rpm) to 3 or 4 amps (low rpm,
high, load). The number and kind of accessories
turned on at the time have a second order effect
on control loop stability.
If you have 500 milliohms of field supply resistance
feeding a moderately loaded alternator current of
say 2 amps, then the 2 amp x 0.5 ohms or 1 volt
'modulation' of voltage at the regulator's
A/S terminals presents as a 1 volt error in the desired
regulation set point. This is not a guarantee for
unstable operation but the risks are very high.
Wire resistance in the rest of the system has very
low to zero risk for setting up the same condition.
This problem came to light early on for single engine
Cessnas. I addition to conductors that ran from
bus to regulator, there were ohmic joints, (crimped
pins, engaged pins, fast-ons, switch saddles, switch
contacts, etc). I forget the exact number but I think
I recall something like 19 ohmic joints in the Cessnas,
far more than any other TC aircraft.
About once a year I get an email from a TC Cessna
owner asking about fixes for the 'galloping ammeter".
Quite often, replacing one item in the constellation of
system components provides relief . . . but it is temporary.
My advice is to start at the bus and replace EVERYTHING
with ohmic joints starting with breaker, master switch
and it's fast-on terminals, and crimped on pins in all
the Mate-n-Lok connectors.
This 'shotgun' approach will reduce total path resistance
to a value close to factory original values. Assuming
the modern regulators have similar stability models,
this approach should set the system up for another 40
years of stable operation.
Steve, thank you so much for the validation of the
simple-ideas offered as to root cause and remedy for
this unique phenomenon. Good work my friend!
Bob . . .
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