Noise Problem. Both Comms Breaking Squelch

On Apr 2, 9:26 am, MikeMl wrote:
Lancair IV-P Flyer wrote:
On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote:
Mike,

The OV protection is part of the voltage regulator product. The one
we are using is an LR3C 24 volt from B&C Specialties. It is widely
used in the experimental market and has a bullet proof history of no
problems. Since we had tried everything else the company sent us a
replacement regulator to try just in case we had a problem. I flew
the airplane last week with the new regulator and saw no change in the
symptoms. So, I am pretty confident the OV protection is not causing
the problem.

And I am just as confident that it is!!!

According to this description:

http://www.bandcspecialty.com/QuickFacts_LR3C.pdf

the LR3C has exactly the type of crowbar circuit that I have been
describing! If it detects what it thinks is an "overvoltage" condition,
then it responds by firing its "protection crowbar", which instantly
overloads the aircraft's Field Breaker, causing it to overheat and trip,
which removes power from the LR3C, and therefore removes excitation from
the Alternator's field circuit, thereby taking the entire charging
system offline until the Field Breaker is reset.

I personally think this is a DUMB design that causes many more problems
than it prevents.

Regarding the field wire integrity, I hooked up a multimeter in series
to the field breaker and looked at the amperage to the breaker during
a flight. I was hoping for a building amperage which would have
indicated resistance building then I could have begun searching for
what was building resistance. But the multimeter amperage reading was
dead solid at 1.5 amps which is quite a cushion from the 5 amp
rating.

This is normal behaviour. Think of the alternator as a current
amplifier. Its output current is nominally about 25 times its field
current. In other words, it takes about 1A of Field Current to produce
25A of output current. In steady flight, many minutes after engine start
after the battery has recharged, the average electrical load in the
aircraft is somewhere around 20 to 40A, so the alternator has to produce
20 to 40A, meaning its Field Current will be 0.8 to 1.5A. Almost all of
the current that you measured at the Field Breaker is flowing through
the Alternator Field to ground. The LR3C regulator effectively
"regulates" the Field Current so that the Alternator output just matches
the electrical load.

The current that is tripping the Field Breaker is a momentary overload
cause by the LR3's crowbar that lasts only a few 10s of msec. It would
take a "peak-capture&hold" type of meter to display it!

A minor nit. If there was a "building resistance" in the field circuit,
that would reduce the field current thereby reducing the likelyhood that
the Field Breaker would trip. It takes an unplanned shunt path (fault)
to ground to increase the field current. The crowbar inside the LR3 is a
"shunt" path to ground when it fires.

Something is causing a voltage spike. I just have to find
it.

Yes, that is the root cause of your problem; its just not where you have
been looking.

You have one of three problems:

1. The OverVoltage detection level of the LR3 is set too low (too close
to the actual bus voltage, assuming that is correct). Solution, raise
the LR3's Overvoltage Threshold. My preference for a realistic
Overvoltage Threshold is 31V.

2. The Bus voltage really is climbing to unsafe levels. For your AGM
battery, the bus voltage should never get above 28.5V. Solution: adjust
the regulated bus voltage to 28.5V or lower.

3. There is an inductive load somewhere in the aircraft (flap motor,
gear pump motor, autopilot servo, trim servo) which during its normal
cycling puts a short duration inductive electrical transient voltage
spike onto the main bus. The spike is of sufficient energy that the OVP
circuit sees it, and reacts to it by firing its crowbar. Solution: find
the source of the spike and suppress it at its source, or make the LR3
less sensitive to short duration spikes, either by raising its
Overvoltage detection threshold, or by "filtering" its sensing input to
prevent it from "seeing" the short-duration spikes.

I am grateful for your help please forward any additional ideas you
may have on this.

I outlined a method of testing the Regulation voltage and the
Overvoltage Threshold voltage of the VR/OVP in situ (using a lab supply)
in my other post. The only thing new is that the LR3 is a "linear"
regulator, so you will see the Field Current decrease linearly between
about 28V and 28.4V, rather than exhibiting a bistable on-off behaviour.

MikeM

Mike,

Your explanations are really helping me through this problem. Thank
you very much for your continued involvement.

Using your theories, I contacted the president of B&C Specialties
today and asked him what the crowbar trip point is set at as the LR3C
is a sealed box. The only user adjustable item is a pot screw for
adjusting the voltage the regulator maintains. Bill, the president,
said the crowbar trip point is set at 32 volts.

You are absolutely correct about the voltage set point ideal is at
28.5 volts. I got that number from speaking with the Concorde battery
tech. We have the regulator set so that at high RPM we are getting
28.5 and at idle rpm we get 28.2 volts. This has been the set point
for several months.

We have a JPI 930 installed that gives us voltage, load amperage and
other useful information. Each of those have a user defined alarm
limit. The voltage alarm I have set is at 29.5 volts. Any time I
hear the telltale "crack" of static in the headset, I look to the JPI
or the voltage readout on the Davtron timer and can see the voltage
excursion. It is usually less than one volt. Whenever the voltage
excursion goes above the alarm point I see an alarm post as the
display color goes to red, and the word "alarm" appears. Most of the
time when we have a trip event on the field breaker it occurs without
the voltage spiking high enough to trigger the alarm set point of 29.5
volts.

Regarding the idea that something in the aircraft is triggering a
transient voltage spike onto the bus, this is the theory we have been
working on for the most part since August. After we got the
alternator fixed all of our attention was looking for the origin of
the transient. The flap and gear are run by the hydraulic pump which
is the highest draw in aircraft. The circuit breaker is a 35 amp size
and when actuating the flap or gear one can see up to 35 amp increase
in the load on the load meter shown on the JPI 930. There is a light
on the panel that comes on anytime the hydraulic pump is actuating
which it does on its own to maintain specified pressure. I have
looked closely when we begin to hear static and see voltage excursions
which are precursors to a trip event to see if they are accompanied by
a cycling of the hydraulic pump. I have never seen that to be the case
except on landing operations when the flap and gear are actuated. In
those occasions, when the field breaker trips it is directly in
response to hydraulic pump actuation.

Mike, given this information, what do you suggest I do next to try and
isolate the cause?

Thanks again, you have a better grasp on this issue than anyone I have
talked to since I began working on it 7 months ago.

Steve

Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors
etc would be ballasted by the battery - as long as it stays connected
to the main bus. But if there is anything intermittent in the master
contactor system, any inductive load will kick into whatever
electronics are down-circuit from the intermittent.

A classic example of this is if a master contactor should stutter
(like from a weak battery) when the starter is engaged, the collapsing
field from the starter could generate enormous voltage spikes which
could wipe out any electronics that are also turned on. This is
probably why we are told to turn off radios etc before starting.

nrp wrote:
Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors
etc would be ballasted by the battery - as long as it stays connected
to the main bus. But if there is anything intermittent in the master
contactor system, any inductive load will kick into whatever
electronics are down-circuit from the intermittent.

A classic example of this is if a master contactor should stutter
(like from a weak battery) when the starter is engaged, the collapsing
field from the starter could generate enormous voltage spikes which
could wipe out any electronics that are also turned on. This is
probably why we are told to turn off radios etc before starting.

May not be applicable, but I have a '65 Cessna that has a single master
switch (DPST). It does connect the alternator and pull in the master
solenoid. I had ammeter swings and they would get so bad as to shut down
the system. I guess it was making the OVR trip. I found quite by
accident that it was the bad (design) master switch on the alternator
side. Replaced with a really good (designed) switch and I have had no
more problems.

--

Regards, Ross
C-172F 180HP
KSWI

Lancair IV-P Flyer wrote:
On Apr 2, 9:26 am, MikeMl wrote:
....
Mike, given this information, what do you suggest I do next to try and
isolate the cause?

Do the static test on the LR3C using the lab power supply I outlined in
the earlier post. You can do it with the LR3C either in or out of the
aircraft! If doing it on the bench, use a 28V 1A lamp as a substitute
for the alternator field.