Noise Problem. Both Comms Breaking Squelch

Lancair IV-P Flyer wrote:
On Mar 31, 5:59 pm, MikeMl wrote:
Lancair IV-P Flyer wrote:
On Mar 29, 4:45 am, "
Steve,

I do not have any specific Lancair experience, however, I know about
Cessnas. The new glass-panel Cessnas have an alternator controller which
includes an overvoltage detector which incorporates an electronic
"crowbar" circuit. It purposely overloads and trips the Alternator Field
Breaker if a momentary bus voltage transient is detected. Could your
Lancair have a similar system?

On the radios unsquelching, is it possible that the radios are just
overly voltage sensitive? With the alternator on-line, the bus voltage
should be nominally 28.5V. With the alternator off-line, the bus voltage
will quickly sag to about 24V. Some radios do better than others at not
having their squelch threshold change when the input power changes that
much.

MikeM

Mike,

The solid state voltage regulators used in many experimental category
aircraft do in fact have "crowbar" type protection as well as over
voltage protection. As I understand the operation of the crowbar trip
from a simple over voltage trip is if the crowbar opens the circuit,
it can only be reset by turning off the alternator switch. Once that
occurs, you can reset the breaker, turn the alternator switch back on
and the alternator will be back in business.

No, what you are describing is how the 1976 to 1986 steam-gauge Cessnas
worked. They have a latching relay in the OverVoltageProtection module
which has to be reset by momentarily turning off the ALT side of the
Master Switch.

The glass panel Cessnas (post 2003?) actually have an electronic crowbar
which artificially creates an overload current which blows the
Alternator Field Breaker when an overvoltage is detected. The Breaker
must be physically reset in order to bring the Alternator back on line.
A momentary spike lasting only milliseconds will trip the Field breaker;
a truly stupid system! This system has an AD against it; spurious
tripping, what else. Many new Cessna owners have had issues with this,
and have spent money replacing things like alternators when the root
cause was an OV circuit which was too sensitive to short duration
transients caused by things like an inductive gear pump motor turning off!

My breaker trips are
always resettable without resetting requirement of the alternator
switch. So, I think something is shorting to ground that shouldn't
be. I am thinking the main alternator may still be the culprit or
possibly the battery, which is an AGM type, may be shorting as it
heats up. I am going to try pulling the field breaker and sense
breaker of the backup alternator to see if the problems occur during
the isolation. If so, then running from the backup system and pulling
the breakers of the main system. If the failures only occur on the
main system I am going to replace the alternator with a completely
different unit. If both tests show voltage excursions, I am going to
replace the battery and see if that isn't the culprit. There is
really not much left to try.

Thanks for your help.
Steve

The only faults (other than the possibility we are discussing above)
that would cause the Field Breaker to trip (in order of likelyhood) is:
rubbed through insulation on wiring between the breaker and the
alternator field, a short between a wire shield and its center
conductor, a loose brush holder inside alternator, or a fault inside the
Voltage Regulator. You have already looked at wiring and replaced the
alternator.

Since your trips occur many minutes into a flight, ask yourself what
happens to the bus voltage as the alternator recharges the battery after
the startup? The higher the bus voltage climbs, the closer it gets to
the OVP's trip voltage, biasing it ever closer to its trip point. I'll
take a bet that you have an oversensitive OVP. The battery has nothing
to do with this; the VR should keep the voltage on the battery at no
higher than 28.8V.

Find out what voltage your OVP trips at. I would do the following ground
test: Isolate the OVP/VR from the Field Breaker (just open it if it is
the pullable kind). Connect a 3A, 28V regulated, current-limited
dual-metered adjustable Power Supply to the wire downstream from the
Field Breaker. Start with the supply set to about 24V with a
short-circuit current set to 3A; you should see about 1.5A flowing which
is going into the alternator field winding.

Slowly increase the supply voltage; at about 28.5V, you should see the
supply current suddenly drop to a few tens of mA. That is the cutout
voltage at which the VR regulates the bus voltage (by turning off the
field excitation). Slowly decrease the supply voltage until the field
current jumps up again; that is the cutin voltage, and might be a few
tens of mV lower than the cutout voltage (hysteresis).

Now increase the supply voltage above the cutout voltage and watch what
happens. Ideally, the OverVoltage trip should be at 32V or higher.
Observe what happens when the OVP fires. Does it act like a short
circuit to the supply? (i.e. did the supply go into current limiting and
stay there until you disconnect a wire?). What you are looking for is at
what voltage did the trip occur, and would the trip have blown the field
breaker?

MikeM (PhD EE, retired)
Skylane, Pacer.

MikeM (PhD EE, retired)
Skylane, Pacer.
I had a 172M that gave random overvoltage tripout problems too. First
we spun the alternator with an electric motor on the gorund & looked
at the output voltage with an oscilloscope. Sure enough it was
tripping out at well less than 16 V.

Maybe these units drift? We replaced it with new, but noted that
there is an exposed tweekpot on the OV sensor that probably adjusts
the trip point.

On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote:

The glass panel Cessnas (post 2003?) actually have an electronic crowbar
which artificially creates an overload current which blows the
Alternator Field Breaker when an overvoltage is detected. The Breaker
must be physically reset in order to bring the Alternator back on line.
A momentary spike lasting only milliseconds will trip the Field breaker;
a truly stupid system! This system has an AD against it; spurious
tripping, what else. Many new Cessna owners have had issues with this,
and have spent money replacing things like alternators when the root
cause was an OV circuit which was too sensitive to short duration
transients caused by things like an inductive gear pump motor turning off!

Do you have an AD number for that?

Peter Clark wrote:
On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote:


Do you have an AD number for that?

I don't own a 182T. Now that you mention it, it was a Cessna Service
Bulletin that required replacement of the ACU. It was discussed
extensively on the Cessna Pilot's Assoc. forums.

On Tue, 01 Apr 2008 15:47:49 -0600, MikeMl wrote:

Peter Clark wrote:
On Tue, 01 Apr 2008 08:56:27 -0600, MikeMl wrote:


Do you have an AD number for that?

I don't own a 182T. Now that you mention it, it was a Cessna Service
Bulletin that required replacement of the ACU. It was discussed
extensively on the Cessna Pilot's Assoc. forums.

I'd like to know the SB number for that. The only SB I can find that
references alternators is SB06-24-03 which is an alternator
replacement for manufacturing defects in Kellly Areospace alternators
made from 8/95 to 8/04.

On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote:
Would anyone have an idea about the cause of this?

Problem:
Intermittent noise (sounds just like a squelch volume test) on both
communication radios. Lasts from 15 seconds to two minutes. Will
occur at any RPM including idle. Changing frequencies eliminates the
noise but the noise can and most likely will come back shortly on that
same frequency.

I have a back up alternator on this aircraft and when I have the main
alternator turned off to check the operation of the backup alternator,
the noise never seems to occur. Also, when the noise manifests, if I
turn off the alternator the noise always goes away. I have done this
several times on the ground and it has been 100%. The voltage
regulator has just been replaced and the noise is still present. The
alternator has been rebuilt three times in the last 30 hours of
operation. The alternator shop is tired of hearing from me. This
squelch break noise is new however in the last 15 hours of
operation.

The alternator has been having so much trouble because we have been
trying to locate an alternator field circuit breaker popping issue.
Typical scenario is the aircraft must fly for at least 40 minutes
before the breaker will pop. Usually, it pops in conjunction with a
small static crack in the headset and a voltage excursion of one to
two volts. This most often happens in level flight. This same
voltage excursion has been noted at least once when the main
alternator is turned off and running on the backup alternator.

All connections in the alternator/breaker/battery/ground circuit have
been cleaned, tightened and cable ends replaced when suspect. All to
no avail. This is a 24 volt single battery system. Running the 35
amp hydraulic pump to actuate flaps or gear will on occasion trigger a
field alternator breaker pop but many times it won't either.

Thanks in advance for any light you might be able to shine on this
problem.

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.

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

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

Steve

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

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.

Peter wrote:
MikeMl wrote

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.

I am an electronics engineer (35 years' design experience) and can't
believe anybody would do something so stupid in an aeroplane.

Overvoltage crowbars are used on switching power supplies which have
instant acting short circuit protection features and whose output
power is limited by the magnetic components anyway.

But on an aeroplane you have a very powerful alternator and more to
the point you have thermal circuit breakers which take a while to
trip. They are not like the magnetic ones in one's house which trip
really fast. The thermal ones have to heat up first.

If one was going to do an overvoltage protector for an aeroplane, the
way to do it is to put something in series with the alternator field
winding (i.e. in series with the existing voltage regulator) which
goes open circuit when the bus voltage reaches say 32V. That will kill
the alternator output very fast.

I think the problem with that approach is that the
huge inductive surge of the field collapsing would
add to the already present overvoltage.

Crowbars are used in military and spacecraft design
where a positive zero-volts shutdown is needed.