James Baldwin
Joined: 16 Jan 2006
Posts: 23
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 Posted: Mon Aug 18, 2008 11:54 pm Post subject: B&C alterator problem (Robert L. Nuckolls, III) |
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| Quote: | Bob and others -
>I have a newly purchased RV8 with a B&C 40 amp alternator and automotive
>style blade fuses. Good grounding with a firewall mounted ground bus
>which is then routed to the engine. The digital ammeter indicates a
>running load of 6-7 amps. The battery is a Concorde dry cell mounted on
>the firewall. On two separate occasions the 15 amp alternator field fuse
>has blown without any additional or noted transient loads being placed on
>the alternator. The builder had this happen to him also and he thought it
>was the battery so he replaced it. The new, year old Concorde battery has
>great cranking power so I am not looking at this as a potential
>problem. The only other problem I have noted is when using all of the
>lights, strobes, fuel pump and electric flaps the main 40 amp C/B popped
>once. Ignoring this as an overload my real problem appears to be the
>field fuse. Why does the 15 amp field fuse blow and what's the smartest
>way to diagnose this problem? Thank you. JBB
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The first problem is that the original builder did not
understand the physics of the system he/she was crafting
and chose not to study and understand why the z-figures
were designed the way they are.
A 40A breaker on a 40A alternator is UNDERSIZED. The
nameplate rating of an alternator is for worst-case
conditions. I.e. minimum rpm for full output and max
ambient operating temperature. Under less stressful
conditions, the alternator may deliver 15 to 25%
more current.
This says that the 60A breaker on the b-lead of 60A
alternators in 100,000 GA aircraft is DESIGNED to
nuisance trip. Suggest you replace the 40A breaker
with a 30-50A current limiter out on the firewall.
There should not be a field fuse with an LR series
regulator. These us crowbar OV protection and are
designed to work downstream of a breaker only. Is
the recommended 5A breaker also installed? IF so,
the wiring between the breaker and the bus needs
little if any protection. The z-figures recommend
a fusible link . . . exceedingly robust compared
to the fuse.
Now we need to figure out what's irritating the
OV protection system. Do I presume correctly that
the regulator also came from B&C (LR3 series?) or
is there some other combination of regulator/ov
in place?
I'll assume that since the field protection is being
nuisance tripped, there is some form of crowbar
ov protection in place. The ov protection may
be an older version that was sensitive to some
forms of normal bus transients. The designs at
both B&C and AeroElectric Connection were modified
to eliminate that condition . . . the system you're
working with may need to be updated.
This is not a condition unique to 'crowbar' ov
protection. EVERY ov protection system has some
form of comparator between bus voltage and
some stable reference. When a setpoint is exceeded,
it sends a signal to some form of disconnect
device (relay, transistor, scr, etc). There is
a dynamic component to ov protection too . . .
voltage excursions above the setpoint are allowed
as long as they do not exceed certain time
intervals. It's the fine tuning of the dynamic
sensitivity that makes some products prone to
nuisance trips. I've probably designed two dozen
or more OV protection systems in my career, I've
only had to re-tune two of them. In one case,
the transient condition generated by the aircraft
was greater than the Mil-STD-704 guidelines
to which the product was crafted.
Tell us what regulator/ov combination is installed.
If an LR3, what's the manufacturing date? If
an AEC crowbar module, what colors of wires does
it have. Older versions are red/black, newer
are orange/black.
Bob . . .
Bob -
Thank you for opening up the design issues I hadn't even considered. I will return home next week and provide the more detailed information you request. It is a B&C voltage protection device of some sort I have seen and he used a "Z" design but I'll have to get you the specifics. Thank you. JBB
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