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How to use a highoutput alternator as emergency welder DIY

How to use a highoutput alternator as emergency welder DIY

by yachtwork » Sun Jan 03, 2010 8:49 pm

Greetings - This article describes how to convert a alternator into a stick welder for emergency use. When done welding (if your lucky) the alternator can be used to charge batteries again. I should say for years we tried many different methods to weld on board including one of those aircraft starter motors. We also tried inverter welders run from a genset and a small mig machine aboard. This system has the advantage of not needing to carry much except the small amount of parts listed as a "just in case".

I posted the whole article, and the original can be seen here with the diagrams-


Emergency welding with a high output alternator
How to turn most any alternator in an emergency welding machine

By: Scott Fratcher - Marine Engineer/Captain

Scott Fratcher explains how to make an emergency welder with a high-output alternator

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 an emergency.

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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Joined: Mon May 12, 2008 5:46 am
Location: New Zealand

Re: How to use a highoutput alternator as emergency welder DIY

by Louis Riel » Tue Jan 05, 2010 9:59 pm

What do you mean by" Emergency" I have been using that setup for welding for many years now. As 100 amp alternators are $35 in autowreckers, they can be considered disposable.I use a 65 watt car headlight bulb for the resistor, and put a toggle switch in to turn the field off when not welding. I had to go for a 9 inch pulley on the engine to get her going fast enough.
I run my angle grinder , and other 120 volt electrical tools off the output of the alternator. I control amperage with the RPM. She is the smoothest DC welder I have ever used.
A friend cruising Mexico is aways telling yachties who have trouble with their generators " Bolt a 10 hp briggs and an alternator to a pieced of plywood . It will weld, run your power tools and charge your batteries, for a fraction the price of a generator and if she craps out , you can buy a new alternator in an auto wreckers for $35."
Louis Riel
Posts: 945
Joined: Mon Nov 27, 2006 6:14 pm
Location: BC

Re: How to use a highoutput alternator as emergency welder D

by brentswain » Tue Sep 12, 2017 3:24 pm

The 65 watt headlight bulb goes between the starting battery and the field on the welder ,so you don't burn out the field windings.
A 50 watt, 120 volt house light bulb on the outputs, makes the diodes which come with the alternator, last much longer.
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Joined: Tue Aug 29, 2017 4:55 pm

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