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  #1  
Old 12-25-2006, 10:27 AM
vlittle's Avatar
vlittle vlittle is offline
 
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Location: Canada
Posts: 1,442
Default Fluctuating alternator output

This is a repost from another thread that I thought would be of wider interest:

...

Today, however, I modified my my external regulator setup which uses the Van's 35A alternator with their external regulator. This setup is prone to oscillation due to voltage drops and resistance in the field circuit wiring.

(see last paragraphs of this Aeroelectric ariticle, excerpted below:


"Another common victim of resistance pollution is the
alternator’s voltage regulator. Consider the wiring and
all the components in the wiring between the ship’s main
bus and the voltage regulator on many certified airplanes.
In some of the older Cessnas, it’s not uncommon to
travel through a circuit breaker, field side of a DC Master
switch, overvoltage relay and then out to the regulator.
The production harness includes several AMP Mate-n-
Lock connectors with pins that are NOT gold plated.
Over time, every crimped joint, mated pin, clamped up
ring terminal, switch contact and relay contact increases
it’s resistance with age. No one device goes up a lot. I
think I counted something like 16 assembled connections
in the alternator’s field power path on one particular
Cessna.

An alternator’s field draws about three amps maximum.
Let’s suppose that the field circuit resistance is
something on the order of 100 milliohms. A 3-amp
current draw will induce a 300 millivolt drop in the
pathway from bus to regulator. This can have two
profound effects. First, the regulator thinks the bus is
running 300 millivolts lower than true and jacks up the
alternator’s output to compensate for it. This means that
as the alternator’s load is increased, field current goes up
and the regulator’s perceptions of true bus voltage are
increasingly in error.

Normally, when you load a device or system with heavier
current, you expect resistance associated with the system to
cause voltage to go down. However, when resistance is
included inside the control loop of the voltage regulator, an
increasing load on the system causes the alternator’s output
to rise instead of fall. This is called a “negative resistance”
condition. If it progresses far enough with some regulators,
the system will break into oscillation.
The problem manifests itself as a wiggling ammeter or
perhaps agitated panel lights. More than one airplane
exhibiting these symptoms has suffered replacement of
alternators and/or regulators with no or temporary relief.
Sometimes the problem goes away when the mechanic
replaces the DC Power Master switch (usually the infamous
split-rocker). What the mechanic generally doesn’t know
is that regulator instability is caused by small build-ups in
most of the field supply line’s components. Replacing one
component may bring total resistance down to a stable
condition but the problem returns sooner than necessary
because all of the components continue to age and drive
circuit resistance up."

My fix was to wire a relay into the field circuit that connects the regulator directly to the alternator B lead when the ALT switch is turned on.



See the bottom of this web page for more information.

My flight test showed rock-solid voltage and no fluctuations. The cockpit lights were steady (before they were not). The voltage on my voltmeter reads 14.1 volts, about 0.1 volts below the regulator set point. This makes sense because the voltmeter is several connections and lengths of wire downstream of the main bus, and is measuring the voltage drops due to loads on the main bus.

Previously, I would see 14.3 to 14.5 volts on the voltmeter, depending on load and phase of the moon (resistance in the circuits and voltage drops across connections and the regulator was several feet of AWG20 downstream from the main bus as well). This would imply 14.6 or more volts on the battery... a tad high.

This problem was so severe at one time due to a Master Switch failure (resistive contacts), that I had overvoltage alarms in flight. Now, a resistive switch will have no effect (within reason).

Vern Little

Last edited by vlittle : 01-28-2011 at 07:05 PM.
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  #2  
Old 12-25-2006, 04:52 PM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
 
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Post Vern clarification

Vern Question:

I think I got it, and makes sense but just want to make sure.

+B is the output from the alternator, got that right. (I think)

I am a little confused about FIELDOUT & FIELDIN?

FIELDOUT = +A terminal on the voltage regulator?

FIELDIN = ALT switch, positive switched pwr, main buss fuse/CB protected (not shown)?

You don't show the actual field wiring, i.e., "F" field terminal on voltage regulator to the "F" field terminal on the alternator.

Question? You use a transient voltage suppressor P6KE18CA-E3? That has a clamp range 17.1 to 18.9 volts. Have you used those before. I am interested in it but kind of afraid of including it in my wiring, just from unfamiliarity. Do you know what the leakage was and can it handle 35 amps?

BATTERY CHARGE?
Question you say 14.6 volts is too high and that 14.1 volts (14.2 volts) is better. If you are running an Odyssey battery the manufacture says 14.2 volts is the bare minimum to get a full charge. May be you want to run the +A terminal (FIELDOUT) thru relay and direct to battery instead of +B terminal on alternator to keep the voltage up a little more towards 14.2 volts?

REGULATORS
Some VR's have a remote voltage sense which illuminates the hunting resistance issue. It is an extra wire that goes to the battery to avoid all the resistance of the main buss which may be further away. Others have have higher set points, adjustable voltage with digital control (not just solid state but digital control). What VR are you using Vern?

Here are some heavy duty digital control and adjustable VR's that might avoid this “negative resistance” condition.
http://195.125.241.148/cgi-win/product.exe?V1200
http://195.125.241.148/cgi-win/product.exe?V1700

Nice work, G Merry Christmas
__________________
George
Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

Last edited by gmcjetpilot : 12-25-2006 at 06:18 PM.
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  #3  
Old 12-25-2006, 09:59 PM
vlittle's Avatar
vlittle vlittle is offline
 
Join Date: Jan 2005
Location: Canada
Posts: 1,442
Default

Quote:
Originally Posted by gmcjetpilot
Vern Question:

I think I got it, and makes sense but just want to make sure.

+B is the output from the alternator, got that right. (I think)

I am a little confused about FIELDOUT & FIELDIN?

FIELDOUT = +A terminal on the voltage regulator?

FIELDIN = ALT switch, positive switched pwr, main buss fuse/CB protected (not shown)?

You don't show the actual field wiring, i.e., "F" field terminal on voltage regulator to the "F" field terminal on the alternator.

Question? You use a transient voltage suppressor P6KE18CA-E3? That has a clamp range 17.1 to 18.9 volts. Have you used those before. I am interested in it but kind of afraid of including it in my wiring, just from unfamiliarity. Do you know what the leakage was and can it handle 35 amps?

BATTERY CHARGE?
Question you say 14.6 volts is too high and that 14.1 volts (14.2 volts) is better. If you are running an Odyssey battery the manufacture says 14.2 volts is the bare minimum to get a full charge. May be you want to run the +A terminal (FIELDOUT) thru relay and direct to battery instead of +B terminal on alternator to keep the voltage up a little more towards 14.2 volts?

REGULATORS
Some VR's have a remote voltage sense which illuminates the hunting resistance issue. It is an extra wire that goes to the battery to avoid all the resistance of the main buss which may be further away. Others have have higher set points, adjustable voltage with digital control (not just solid state but digital control). What VR are you using Vern?

Here are some heavy duty digital control and adjustable VR's that might avoid this “negative resistance” condition.
http://195.125.241.148/cgi-win/product.exe?V1200
http://195.125.241.148/cgi-win/product.exe?V1700

Nice work, G Merry Christmas

George, you've got most everything correct above. I didn't swap to a voltage regulator with a remote voltage sense because it was mechanically more complicated than just adding the relay to the field circuit and using my current regulator. My fix costs around $5.00.

The battery charging voltage issue was more that it was fluctuating, than the absolute voltage. When I had a faulty (resistive) master switch, I saw over 15 volts. The 14.2 setting is good enough to allow for battery charging on long flights without worry about overcharging. This is adjustable on my regulator, which is the Transpo V8405A available from Van's.

The reason I didn't run the B+ input directly to the battery (which I considered) was to eliminate another live wire running around the engine compartment. Theoretically, connecting to the battery would give slightly better regulation, so it's your choice.

I'm using the P6KE18A in several places to protect avionics (note: don't use the bidirectional P6KE18CA). The diode clamps at a higher voltage than the primary overvoltage protection (OVM-14 from B&C). It perform secondary overvoltage protection to shut down the alternator. The leakage current is 5 uA at 15.3V. I've also used P6KE20A which have a 17.1V 5uA leakage rating.

These TVS diodes are rated for energy absorbtion, not just current limit (600 watts x 1 mSec). They can take a large current for very short periods of time, hopefully burning the fuselink. They are also designed to fail short... sacrificing the diode, but ensuring the fuse opens. This is why they are good secondary protection, but not primary protection.

This diode is not really required... it's just my 'belt and suspenders' approach to failure modes. With proper primary overvoltage protection, it's redundant and can safely be eliminated.

In summary... I would recommend the Ford Style voltage regulators for new designs, because of the remote sensing. However, in applications using the Van's regulator, my relay fix is simple, reliable and cheap... kind of like me.

Merry Christmas,
Vern

Last edited by vlittle : 12-27-2006 at 09:00 PM. Reason: updated comment
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  #4  
Old 12-26-2006, 03:53 AM
wv4i wv4i is offline
 
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Location: Palm Beach County, FL
Posts: 304
Default Vans 35a alt w/ ext VR

A good while back, had intermittent problem of going to zero output fm alternator. Checked belt tension first, even wired in a new plug to back of alternator, grounded shield of field coaxial shielded line at alt end only. Finally threw a new VR at it, fixed problem. While I was at it, wired in a breaker switch for field power. Oscillation?
__________________
Link McGarity
Wellington, FL (FD38)
RV6/N42GF bought flying, sold
RV6/N72AT bought flying, sold
B737-800/NxxxAN
Sonex TD w/Aerovee/N732SX bought flying
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  #5  
Old 12-26-2006, 08:14 AM
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vlittle vlittle is offline
 
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Posts: 1,442
Default

Link, read the excerpt from the first email. Oscillation is apparent by flickering cabin/instrument lights or noise in the intercom/radio system.

Here is a more complete schematic of how I wired the above circuit into my aircraft.

Vern




Last edited by vlittle : 01-28-2011 at 07:03 PM.
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