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.