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John Slade

"Load Dump" Damage to Alternators with Built-in Regulators

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The following was posted on the Aeroelectric list this morning. It is important for anyone installing an automotive alternator. In general, I'd suggest that subscribing the Bob Nuckoll's list and getting his book is an essential part of the learning curve if you plan to wire you're own airplane.

__________________________________________________________________

--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <bob.nuckolls@cox.net>

 

As promised, I've been looking into this problem and I've crafted

a white paper on the topic. Here is the text of the paper . . .

 

-------------------------------------------

"Load Dump" Damage to Alternators with Built-in Regulators

 

Bob Nuckolls

1 February 2004

 

>AeroElectric-List message posted by: "Ned Thomas" <315@cox.net>

>

>For what it is worth, I had an internally regulated alternator

>on my RV6A. I had an overvoltage occur and had no way to

>shut it off except land and turn off the engine. When

>I smelled the battery acid cooking out I was quite concerned.

>I was able to land before ruining the battery but even tho

>I immediately turned off the master when I found the

>voltmeter reading high, I did find that one of my strobes

>had fried. After this happened I installed the OV protection

>recommended by Bob. In the event you do encounter an OV

>situation you must be able to isolate the alternator.

>I was lucky, the battery could have blown up...

 

>----- Original Message -----

>From: "Clay R" <clayr_55@yahoo.com>

 

>Now I see the following warning on Vans web site on

>the alternator page. (I think this was added this week)

 

>Warning!

 

>The internally regulated 60 ampere alternator should not

>be used with overvoltage protection systems. If you

>open the charging circuit while it is in operation,

>it will destroy the regulator.

 

>AeroElectric-List message posted by: "Steve Sampson"

<SSampson.SLN21@london.edu>

 

Clay - is anyone looking into this for you? It sounds

>like plenty of people are getting blown alternators

>after putting the B&C stuff on.

 

BACKGROUND

 

(1) The "B&C stuff" is only a collection of parts described in documents

described in the AeroElectric Connection. Let us take care as to how the

phenomenon is described with respect to implied cause and effect. It's not

B&C's architecture but AeroElectric Connection architecture . . . B&C only

sells the parts to implement it.

 

(2) For years and since day-one of my participation in B&C's development

and marketing of alternators, we have preached the doctrine of externally

regulated alternators. Examples of this philosophy are found throughout

early writings and particularly in chapters on alternators and regulators

in the 'Connection. A simple inspection of B&C's offerings from the

beginning will show that only externally regulated alternators are offered.

 

(3) There has been a lot of interest in adapting internally regulated

alternators to aircraft because they are so readily available and cheap.

Further, they've produced an excellent track record of reliability on cars

. . . it seems a shame not exploit that characteristic in aircraft.

 

(4) The challenge for adapting internally regulated alternators to

airplanes has always been making them behave like externally regulated

alternators and generators before them. On the instrument panel there is a

switch labeled ALT OFF/ON. One expects that operation of this switch will

produce the same result whether you're sitting in a 1965 C-172 or a 2004 RV-8.

 

(5) Most alternators with built in regulators, once given the ON command

via the rear-connected control wire will indeed come alive . . . but since

this wire was originally intended only as a means for the EFI controller on

a car to delay onset of engine loads after starting, there was no

requirement for being able to turn the alternator OFF via this same wire.

So, the vast majority of automotive take-offs cannot be turned OFF by

removing +14v from the control wire via panel mounted switch. This

condition was experienced by Mr. Thomas in the anecdote cited earlier.

 

(6) While the probability of regulator failure in cars is exceedingly low,

it is not zero. We've heard anecdotal stories of unhappy, high-dollar

events taking place in airplanes after failure of an internally regulated

alternator.

 

(7) With the goal of addressing a desire in the marketplace to utilize

off-the-car technology, figure Z-24 was developed to address both

controllability and overvoltage issues with the lowest practical parts

count and without modifying the alternator.

 

RECENT HISTORY

 

It seems that there have been a rash of failures of internally regulated

alternators installed per Figure Z-24. Let's review the inner workings and

shortcomings of the modern, internally regulated alternators. We know that

all alternators run best with a battery connected across their output. The

battery is an excellent filter for the noise inherent on DC power generated

by rectified 3-phase AC power. The battery also provides a flywheel effect

. . . a kind of electrical inertia that damps out the frisky nature of an

alternator's ability to quicky respond to and control its own output.

 

A phenomenon labeled by the automotive industry as "load dump" speaks to a

characteristic native to the physics of alternator performance. Its

existence has been known since the beginnings of alternator use in

vehicles . . . but it was only a concern after a proliferation of solid

state electronics for fuel injection systems, ignition systems, anti-lock

brakes, etc. Should an alternator producing a lot of power be suddenly

disconnected from the load, it may generate what would be properly called a

surge of voltage exceeding bus voltage by several times. If the load dump

is limited to shedding of normal system loads, the battery's electrical

inertia will be in place to smooth over the event. However, if the

disconnection includes the battery, no mitigating electrical-mass is

present to capture a significant energy transient. In aviation parlance,

a "load dump" is rapid shedding of normal system loads. The scenario we are

discussing might be more appropriately called a "battery dump".

 

Unlike relatively low energy spikes characteristic of switching transients

on inductive loads, a battery dump event is longer and carries a lot more

energy. In the spring of 1998, there was a romance in the OBAM aircraft

community with products called transient voltage suppressors (TVS). A

school of thought suggested that the electrical system be sprinkled with

these little critters to ward off effects of any gremlins of the

overvoltage persuasion which may be lurking about the system. The suggested

technique was to install a TVS on the power feeder for each vulnerable

accessory.

 

There was an extensive discussion thread. You may review published excerpts

of that discussion at:

 

http://aeroelectric.com/articles/spike.pdf

 

This was before we began to consider and refine any notions of using

internally regulated alternators in OBAM aircraft.

 

In that thread, I suggested it was much better to (1) identify and mitigate

such hazards at their source and/or (2) design accessories to be immune to

such hazards. For decades, DO-160 has been an effective guideline for

development of robust victims while MIL-STD-704 outlined design goals for

output quality of power generation equipment.

 

Here are but a few of hundreds of relevant documents on the phenomenon to

be found on the web . . .

 

http://www.sto-p.com/pfp/pfp-transients.htm

 

http://www.onsemi.com/pub/Collateral/CS3341-D.PDF

 

http://www.audifans.com/archives/1998/08/msg00929.html

 

. . . do a Google search on "load dump" and "alternator" for a wealth of

useful expansion of the topic.

 

WHAT'S HAPPENING?

 

The original discussions 4 years ago focused on the need to protect system

accessories from the effects of alternator behavior. In the cases before us

now, the victim is NOT airframe system accessories being hammered by a

skittish alternator. These are cases where the alternator is killing itself

. . . or more accurately, killing its own voltage regulator.

 

If you look over the specifications for modern, solid state regulator

chips, you'll find references to protection against load dump conditions

built right onto the chip. I believe what we're observing now is a

shortcoming of relatively mature automotive take-off alternators with

regulators that do not enjoy this kind of protection.

 

Referring to the group of block diagrams in this document, note that I've

illustrated 4 configurations of installation architecture for alternators

with built in regulators.

 

(download

 

http://www.aeroelectric.com/articles/Alternator_Failures.pdf

 

to get the illustration)

 

(1) The first diagram is captioned "AUTOMOTIVE" and it illustrates the

relationship between battery and alternator in virtually all automotive

applications. The battery is ALWAYS connected to the alternator. System

loads are controlled via panel switches and/or ignition switch but portions

of the power distribution system are always hot, even when the vehicle is

parked. Not desirable on airplanes.

 

(2) The second configuration is "FIGURE Z-24" referring to an architecture

described in the AeroElectric Connection to accommodate two hard-and-fast

design goals for using an alternator in an airplane: (a) absolute control

of the alternator operation from the cockpit irrespective of flight

condition and (b) protection against the very rare but potentially

hazardous and expensive overvoltage condition.

 

(3) The third configuration describes an ill-conceived recommendation

suggesting deletion of the alternator disconnect and wiring the alternator

to the airplane a-la-automotive. Note that while this configuration

prevents the pilot from switching an alternator off while in operation, it

does not prevent the battery from being taken off line. If we

disconnect the alternator from the system while leaving the battery on as

allowed with Figure Z-24, only the alternator is at-risk for self destruction.

 

When you leave the alternator connected to the system and shut off the

battery master, there is still risk of a battery-dump transient. While

system loads will soak up some transient energy and mitigate amplitude and

duration of the event, now the whole system is subjected to the transient.

 

In airplanes like the Baron and Bonanza where alternators and battery

master switches are separate, non-interlocked controls, switches can be

manipulated in a manner that will produce the same "battery dump" effect

that we're discussing. So the potential for this effect is not new nor is

it unique to the nifty little alternators so popular in the OBAM aircraft

community.

 

(4) The fourth configuration illustrates an experiment to be conducted

which may prove the usefulness of a technique intended to tame the dragon.

 

A PROPOSED GAME PLAN

 

(1) If you have Figure Z-24 installed and you're already flying or nearly

ready to fly, don't change anything. Although you may never need the

protections Z-24 offers, I don't recommend you go flying without it. It is

EASY to prevent battery dump damage to the alternator by controlling

sequence of operation for the switches.

 

[a] Battery master is the first switch to come on before cranking the

engine and it should stay on until after engine shutdown.

 

The alternator control switch may be turned on before cranking the

engine but it's probably better to leave it off until after the engine is

running.

 

[c] At the end of the flight, shut the engine down before first turning

off the alternator . . . .

 

[d] . . . followed by turning of the battery master switch.

 

Following suggestions in any of the Z-figures in the 'Connection will

provide you with interlocked battery master and alternator control switches

wired so as to prevent an alternator from remaining on-line with the

battery disconnected.

 

The battery dump transient is generated by the disconnection of the battery

from the alternator b-lead terminal while the alternator is working hard.

It can't be working hard if the engine is not running. The warning

published by Van's is accurate as far as it goes but misses important

points with respect to absolute operational control of the alternator from

the cockpit and overvoltage protection. So, if you don't diddle with the

switch while the engine is running, your alternator is not at risk for

battery dump damage and you retain both operational control and overvoltage

protection.

 

(2) If you have yet to select an alternator but need to do it soon, you

cannot go wrong with installing an alternator designed for aircraft

service. Alternators using external regulation are easily managed for both

operational control and overvoltage protection by simply opening the field

lead. This activity does not generate the battery dump transient we're

discussing.

 

(3) I'm planning to test an alternator with built in regulation on a test

stand using a fat TVS device connected as shown in the last block diagram.

I'll be making measurements of worst-case transient energies and making

sure that the diode we select is adequately sized to the task.

 

NOTE

 

If anyone out there remembers the zener diode

that Pelican Aviation used to stick on the back

side of their alternators (some STC'd no less!)

while calling it "overvoltage protection" please

recall that this was neither ov protection nor

was it a practical solution to the problem

before us now.

 

After the bench testing studies are complete, I'll be looking for

volunteers who are already flying Figure Z-24 alternator control schemes. I

will supply a pair of TVS diodes for installation on your airplane. You

will be asked to conduct a series of battery dump simulations. After the

simulations, you'll be asked to install the second diode and return the

first one to me for inspection.

 

Once we've done the repeatable experiment to demonstrate suitability of the

"fix", this paper will be updated to publish the results and Figure Z-24

will be updated appropriately. In the mean time, I'll supply a copy of this

paper to Van's in with the hope that it will clarify the issues and

forestall some poorly founded modifications to electrical systems in the

OBAM aircraft community. Please feel free to circulate this document for

both its informative value and potential for critical review.

 

As a closing note to this document, I ran across this paper:

 

http://www.st.com/stonline/books/pdf/docs/4345.pdf

 

. . . written by the folks at SGS-Thompson on battery dump mitigation. This

paper describes a proposed technique for building battery dump management

right into the alternator . . . what a concept! Whether or not this

capability will be offered in automotive products suited to airplanes soon

is hard to predict. In the meantime, it's a no-brainer to make the

alternators we have work quite nicely.

 

The bottom line folks . . . I believe there is good value in the use of

internally regulated alternators on airplanes. However, it's important that

we make decisions based upon good science that helps us understand and

accommodate their unique characteristics.

 

Bob . . .

 

 

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Okay. I'm sold. I'll be subscribing tonight.


Evan Kisbey

Cozy Mk IV plans # 1114

"There may not be any stupid questions, but I've seen LOTS of curious idiots..."

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Couldn't agree more. Although the odds of an alternator running away are low, they are non-zero, and there needs to be a way to shut them down in flight.

 

My take is to use an automatic system like the B&C in combination with overcurrent protection. I've seen quite a few airplanes with neither!

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My LongEZ has been flying for 15 years with a B&C regulator and for 10 years with a B&C alternator and I have never had a single problem, trying to save money using a automotive alternator might end up costing more in the long run and I bet the B&C alternators are lighter too.

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