Match: 
#4
Message: 
#17363
Date: 
Mar 23, 2004
From: 
"Robert L. Nuckolls, III" <bob.nuckolls(at)cox.net>
Subject: 
Re: Dancing Ammeter



>
>
>My flying Lancair 320 has 517 hrs. TT.  Within the last few hours of
>flying, the ammeter has begun a rhythmic wiggle at about 1.5 cycles per
>second with about a 10 amp amplitude.  Occasionally the amplitude will
>go full scale with no apparent triggering event.  Adding load just moves
>the range of the wiggle further up the scale.  The frequency of the
>wiggle is independent of engine RPM.  It's the same at idle as at full
>tilt boogie.  I have pulled the alternator (generic automotive type,
>externally regulated) and had it tested.  No problems.  I have checked,
>cleaned and tightened all connections in the
>alternator/regulator/battery circuits.  I'm now down to either the
>regulator or the meter being bad.  The meter is one of those combined
>ammeter/voltmeter jobbies where it normally shows amps and you push a
>button for volts.  The ammeter is shunted from the B lead, which goes
>through a 35-amp breaker to the main bus.  The voltage indication is
>rock steady and agrees with the digital voltage shown in my EDM-700.
>The battery (Odyssey PC925) seems to be being charged normally.  The
>regulator is a generic automotive type and is the original installation
>(517 hrs.).  It is mounted on the engine side of the firewall in a very
>difficult place to get to (I didn't build my plane).  Changing it is
>going to be a major pain.  That's why I wanted to get y'all's feedback
>before attacking the regulator.
>
>Any suggestions for further troubleshooting?  I have a digital
>multimeter but don't know how to use it to diagnose this situation.

   Dancing ammeters are almost always due to increasing resistance
   of wiring and components between the bus and the regulator's "bus"
   terminal.

   Hundreds of Cessna's and Pipers do this every year. Most get "fixed"
   by replacing the regulator or perhaps one of the suspect components like
   the split rocker master switch. Here's a post from some last year:
>I felt pretty certain that if you recommended hooking the regulator right 
>to the back of the alternator, and then followed the appropriate 
>connection you knew things would settle down nicely.  Well, they did just 
>that!  The ammeter was rock steady to the charging side, and the voltage 
>read a solid 14.4


>Now the follow up to get these same results once the voltage regulator is 
>reinstalled, and not hanging off the alternator.  The other thing I noted 
>was that when hooked up in this manner the alternator side of the split 
>master was not working, but I'm sure you knew that would happen.
>
>Let me know what's next, and thanks!

    Okay, this experiment was important to tell
    us that the components were okay and that you
    didn't have a flaky regulator or bouncing brushes
    in the alternator.

    I'd start at the bus (did I ask whether you're
    using fuseblocks or breakers?) and check to see
    that you have good terminals, at least 20AWG wire
    all the way to the regulator's "A/S" terminals.
    If the regulator is mounted on the firewall,
    grounding isn't an issue for the regulator . . .
    and seldom does grounding affect stability . . .
    only voltage setting.

    Also, you mentioned that the "alternator side
    of the split master was not working" . . . I'd
    bet that MOST of your circuit resistance is
    happening in that switch. I have a plastic
    bag full of perfectly good looking split-rocker
    switches that were sent to me after putting
    in a new one cured a bouncy ammeter complaint.

    This has occurred in countless certified ships
    and a few OBAM aircraft. This doesn't mean that
    the split-rocker is necessarily a "bad" product
    (it's made by Carling and uses the same guts
    as the S700 series toggle switches B&C sells).

    Regulators are sensitive to small amounts of
    resistance in the lines between bus and regulator.
    I had one builder who mounted his regulator within
    a few inches of the bus, tied the A/S terminals
    directly to the breaker with short, single,
    solid wire and put his alternator control switch
    in series with the field wire. He added crowbar
    ov protection to the breaker and ended up with
    a combination that would probably be stable over
    the lifetime of the airplane.

    In older production Cessnas, I think I counted
    20 some odd crimps, connections and spring-pressure
    maintained metal-metal contacts between bus and
    regulator. As all of these joints age, they add
    resistance to the circuit. At some point in time,
    the system becomes unstable with symptoms you
    have observed.

    Thousands of spam-can owners have paid out $millions$
    to ignorant mechanics who replaced EVERYTHING BUT
    aged/compromised wiring before finally renewing the
    bus-to-regulator components. In many cases, owners
    have reported that replacing only the spilt-rocker
    "fixed" the problem. Indeed this single component
    can be a major contributor of total loop resistance.

    But consider that if NEW loop resistance was on
    the order of 50 milliohms and had climbed to
    100 milliohms with the switch contributing
    25 ohms of de-stabilizing resistance. Replacing
    the switch drops total down to 75 milliohms and
    the regulator is happy again . . . but not for
    as long as it would be when replacing ALL
    sources of age/service related resistance
    in the bus-to-regulator pathway.

    The obvious, elegant solution in original
    design is to incorporate a regulator that separates
    voltage sense wires from field current supply
    wires. The LR-3 does just that. Any new regulators
    I design will have separate sense wires too.

    Does this suggest that the OBAM community
    should rip out all their three-terminal
    switchers and bolt on the LR-3? Not at all.
    The automotive style regulators have for the
    most part given good value but they DO have
    special characteristics that only one mechanic
    in 1000 understands. In the spam-can world,
    ignorance is shoveled out at $thousands$ per
    non-idea, in the OBAM aircraft world, we've managed to
    keep those costs MUCH lower . . . and much of
    it happens right here on the AeroElectric
    List.

    Bob . . . 



End Msg: 
#4 

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