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Topic with a little difference

 
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khurst(at)taroom.qld.gov.
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PostPosted: Thu Feb 23, 2006 12:04 am    Post subject: Topic with a little difference Reply with quote

All,

We discussed a topic at work today which has baffled me for some time.
Ok, I admit it is not hard to baffle me but I don't know the answer and
I'm sure someone on this list will know so . . . . .

I'm told that when welding on a vehicle that has an alternator, the
diodes in the alternator can be damaged if the battery is not
disconnected first. Fact or fiction ?

If fact, explanations gratefully received thank you.

Cheers
Kingsley
Europa builder in Oz.


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nuckollsr(at)cox.net
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PostPosted: Thu Feb 23, 2006 4:33 am    Post subject: Topic with a little difference Reply with quote

At 06:01 PM 2/23/2006 +1000, you wrote:

Quote:

<khurst(at)taroom.qld.gov.au>

All,

We discussed a topic at work today which has baffled me for some time.
Ok, I admit it is not hard to baffle me but I don't know the answer and
I'm sure someone on this list will know so . . . . .

I'm told that when welding on a vehicle that has an alternator, the
diodes in the alternator can be damaged if the battery is not
disconnected first. Fact or fiction ?

If fact, explanations gratefully received thank you.

Actually, the battery is the best filter in the vehicle
for transients induced by an external source.
This idea is further discredited by the notion
that most system have multiple accessories that are less
robust than the system's alternator. If the threat were
serious, perhaps one would do well to remove all the fuses
from the fuse block, disconnect all potentially vulnerable
devices from the system, etc. etc.

I can deduce no physics by which serious stresses might be
induced into any of the system's components by welding
equipment.
Bob . . .
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Thu Feb 23, 2006 6:47 am    Post subject: Topic with a little difference Reply with quote

Kingsley Hurst wrote:

Quote:
I'm told that when welding on a vehicle that has an alternator, the
diodes in the alternator can be damaged if the battery is not
disconnected first. Fact or fiction ?

If fact, explanations gratefully received thank you.

I see Bob has replied and I think that he is right in general but I can
tell you what the rationale is about disconnecting the alternator.

Arc welding uses relatively high voltage (compared to your DC system)
and potentially high amperage (once the arc starts, voltage goes down
but current goes way up). Imagine you connect the ground from the arc
welder to one of the two pieces of metal to be welded. When you go to
strike the arc you may touch the other piece of metal before you get the
arc started. This impresses a rather large voltage between the two
pieces of metal. (This assumes that they are not clamped together well
or that there is something else, e.g. paint, preventing good contact
between the pieces of metal.)

Now imagine that the case of the alternator is somehow connected to the
first piece of metal and the ground of the battery is connected to the
other. (We are postulating a cracked frame in your car or some such.)
Now what you have done is to put the output of the welder in series with
the battery going to the alternator. Since the welder may be putting out
80V-100V open-circuit, this is more than enough to cause the diodes in
the alternator to go into reverse breakdown. The battery and welder then
can deliver more than enough current to destroy the diodes.

Could it happen? Yes. Is this likely to happen? No. Is it something you
need to worry about? Probably not. But the point is good: that arc
welder can put out a LOT of voltage before the arc actually starts. And
if your bonding fails, the other wiring in your airplane or car can
become the return for the current in the arc welder. It is probably
better safe than sorry to disconnect or remove things when arc welding
on your airplane.

--
Brian Lloyd 361 Catterline Way
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery


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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery
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nuckollsr(at)cox.net
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PostPosted: Thu Feb 23, 2006 7:41 am    Post subject: Topic with a little difference Reply with quote

At 06:45 AM 2/23/2006 -0800, you wrote:

Quote:


Kingsley Hurst wrote:

> I'm told that when welding on a vehicle that has an alternator, the
> diodes in the alternator can be damaged if the battery is not
> disconnected first. Fact or fiction ?
>
> If fact, explanations gratefully received thank you.

I see Bob has replied and I think that he is right in general but I can
tell you what the rationale is about disconnecting the alternator.

Arc welding uses relatively high voltage (compared to your DC system)
and potentially high amperage (once the arc starts, voltage goes down
but current goes way up). Imagine you connect the ground from the arc
welder to one of the two pieces of metal to be welded. When you go to
strike the arc you may touch the other piece of metal before you get the
arc started. This impresses a rather large voltage between the two
pieces of metal. (This assumes that they are not clamped together well
or that there is something else, e.g. paint, preventing good contact
between the pieces of metal.)

<snip>

Quote:
Could it happen? Yes. Is this likely to happen? No. Is it something you
need to worry about? Probably not. But the point is good: that arc
welder can put out a LOT of voltage before the arc actually starts. And
if your bonding fails, the other wiring in your airplane or car can
become the return for the current in the arc welder. It is probably
better safe than sorry to disconnect or remove things when arc welding
on your airplane.

Of course, this is a special case condition. I just worked
a relay failure in an airplane where the failure caused a
wire to become disconnected from the relay enclosure and
drop down into other portions of the circuitry. The 115 vac
400 Hz power on this wire was conducted into other rather
sensitive systems and caused a LOT of damage secondary to
the relay's initial failure. I don't expect to see this again
in my lifetime . . . especially after I keel-haul the relay
guys for some really dumb design errors.

The point to be made here is that if an alternator is at-risk
for the secondary fallout of a failure (welder's inattention
to proper grounding of his tool's electrical power) then
components of the entire system are equally at risk. Protecting
one's assets from this kind of event would call for disconnection
of everything in the system, not just the alternator.

Bob . . .


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khurst(at)taroom.qld.gov.
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PostPosted: Thu Feb 23, 2006 12:26 pm    Post subject: Topic with a little difference Reply with quote

Quote:
I'm told that when welding on a vehicle that has an alternator, the
diodes in the alternator can be damaged if the battery is not
disconnected first. Fact or fiction ?

Bob and Brian,

Thank you both for your considered opinions.

I was not clever enough to deduce for myself the scenario you described
Brian, but I am able to understand what you have said and agree the
chances of this happening should be very low. BTW, people who have told
me of this phenomenon were not welding cracked chassis so ???

I have always been very sceptical of the claim but as I said, I didn't
know so I am heartened to learn my scepticism had some foundation.

Once again, I sincerely appreciate your willingness to share your
knowledge with us.

Best regards
Kingsley in Oz

Do not archive


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rparigor(at)SUFFOLK.LIB.N
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PostPosted: Thu Feb 23, 2006 1:46 pm    Post subject: Topic with a little difference Reply with quote

"welding on a vehicle that has an alternator, the diodes in the alternator
can be damaged if the battery is not disconnected first. Fact or fiction
?"

For what it's worth, I have TIG welded on Automobiles here and there.
Chassis, exhaust and motor).

I never had a alternator failure, never disconnected battery. Did remove
battery a few times when welding near it, precaution in case some hydrogen
decided to follow my puddle round.

That said, when welding with DC, many TIGs have a high frequency AC start
to help easily establish an arc. Also when welding Aluminium you use AC
all the time.

Once in a great while when welding DC, especial in awkward positions, the
torch electrode can hit the welding rod, instantly stopping the arc and
insulating the rod from the grounded work area. Now for tacking purposes
don't have gloves in rod hand (left), and somehow for balance purposes
have right arm resting on grounded work area. Of course it all happens
when high frequency is happening.

OUCH!!!!!

Do that just once and at least you will rest arm on something semi
insulated. When this happens if you pull away and are less than immediate
on stopping electron flow, if the now welded rod onto the electrode has a
nice long extension of it. If you left a can of your favorite beverage on
the welding table, if it was carbonated, closed and sticky it "Will" get
punctured at this precise time.

This holds true for your electronics project that is laid out at the far
end of the welding table that could not possibly get hurt so far away (the
only free table area in shop).

Resting on work that you think is grounded to table will give you a jolt
as well especial if some other part of yuor body is better grounded.

When TIG welding, always consider just how far that rod can reach when you
get zapped and jerk away. Use shorter rod, or cover stuff that is
critical. It is good practice to always make a rounded bend with a needle
nose pliers on end of rod that can get you or others when welding in tight
awkward positions.

I forget exact voltages, think 28 or 30 for both DC and AC, AC hurts a lot
more.
Ron Parigoris


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