Noise Problem. Both Comms Breaking Squelch

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