Scott Fratcher explains how to make an emergency welder with a high-output
It's possible to weld with nothing but an alternator driven by a small
engine, and although it's not recommended as a day-to-day activity,
it's useful in an emergency.
What if you were out at a remote island and broke a chain plate or
your windlass mount cracked? This type of thing happens all the time
with cruising boats.
Here's how it's done. Start with at least a 100amp alternator (120amp
is perfect - Delco's CS144 alternator is great for this application).
You also need:
* A 110V 60W (or more) light bulb and socket;
* A few cables you can weld through (No. 4 or larger);
* A welding electrode (stinger);
* A small supply of 2.5mm welding rod (6013);
* A variable resister capable of passing four amps continuous. It should
be a rugged variable resister, at least 50mm across.
To use your alternator as a welder you have to attach the welding cables
to the back of the alternator. The ground lead is attached to the negative
pole of the alternator and the positive lead connects to the welding
electrode (or stinger as it's often called).
At the same time, wire the light bulb across the alternator's positive
and negative output leads. This is important as the light will glow
bright any time the welder is not burning a rod.
This light is what takes up the extra current and voltage when the
welding is stopped. When welding, the voltage drops to about 20V and
the light goes out. It kind of works backward from what you would expect.
Light on at rest, light out while welding.
The variable resister is wired in line from the positive of a battery,
through the variable resister and into the positive field brush, through
the field and back to the negative of the battery. The boat's original
alternator regulator is completely disconnected from the alternator.
The alternator should be a P-type (externally regulated). In other
words, one field brush should be grounded and the second brush should
need 12V positive to produce current (almost every high-output alternator
is wired as a P-type externally regulated).
When ready to weld, hook up the cables and light bulb as described
above, start the engine, turn off the battery switch connecting the
alternator to the battery*. Gently turn up the variable resister. The
light should begin to glow. Turn the light up bright, but not maximum
and get ready to weld.
That's all there is to it.
* Normally turning off the battery switch with engine running would
ruin the diodes of the alternator. This is because, while the alternator
is producing power, the stator or outside of the alternator has an electrical
field formed. When you disconnect the load (battery), that electrical
field has to go somewhere. It looks for a path to ground. It finds the
shortest path through the diode, thus ruining it along the way. The
light bulb run in parallel with the welding leads prevents this disaster
from happening and gives the electrical field somewhere to go.
Tip 1: If you find yourself in an emergency and just need to weld but
don't have a variable resister, you can use a 12V light bulb wired in
place of the variable resister to power the field. Put two bulbs in
parallel or even three in parallel till you get the right resistance
to strike a welding arc. This is kind of crude, but it will work in
Tip 2: Don't try to back feed a battery switch as a simple method of
switching between battery charge and welding. The switch will arc inside
and weld itself together. I went through three battery switches before
I worked out why this happens.
Tip 3: Make sure you use a good 110V light bulb (you can't use a 240V
bulb) and a secure electrical connection to the alternator. If the light
becomes disconnected or the bulb burns up, the diodes in the alternator
will immediately fail. This is why it's important to carry a spare set
of alternator diodes (even better is a complete spare alternator) if
you're planning on using this “bush” welding technique.
Tip- Copy this article and keep it with some spare welding rod on the
boat in the unlikely event it's ever needed.
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