Will This Work? Rough AC Welder/Weldernator.

For what I think is your alternator a bit on the light side 45 Amps per Phase?

A 4.5mm rod (steel) needs 100 Amps plus.

I guess, it will be difficult to get a good tack voltage (open voltage) to start welding. My Miller lets me know once in a while that it supplies 30 volts plus.

OK, roughly multiply 24V by 90A and that's the max power you will get from whatever transformer you use, if you need 400Amps, your voltage will be around 5.4Volts, consider the primary to secondary turns ratio, but most important the transformer KVA rating.

times 1.7 ? (3 Phase)

Sorry, you're right. My mistake, but still he won't get more than the alternator's rated watts.

So, help me understand, your final output is planned as three phase? How do you in envision connection of these phases to the rod/workpiece?

Easiest way will be to rectify the phases again and work without transformer. Or he can use a 3 phase to single transformer.

His power, however will be too limited I guess. There are 120 Amps, 3 Phase alternators in some buses with AC.

I know professional skippers that weld straight from their batteries on board.

Rectifying could work, just as batteries can work. It's just that the initial description notes doing away with rectification.

Trying to stick weld with 3 phase would be pretty tricky. Even if he gets to single phase the arc will extinguish as the voltage passes through zero at frequencies I'd expect from the setup described , unless something is done to maintain it, like adding a high frequency component.

Once he goes through a 3ph/ single phase transformer, he is working between 2000 Hz and 3000 Hz, considered the tops. You will also notice that there will be no resulting zero crossings. This technique is used by Lastec, Cloos and others in Europe.

Hmmmm, a single phase at 2 to 3 kHz with no zero crossing? Certainly sounds like something other than just a special transformer is used.....something a lot like rectifying enough to provide sufficient DC bias to keep it always the same polarity. AC is going to cross zero.

Considered not rectifying (at the end), the zero crossing, Yes.

But a DC welder runs a lot smoother.

You can safely forget all about that 3 phase transformer thing. The requirements of a power source suitable for stick or TIG welding is just an implementation of a constant current source, and that can only be done by smart field current management with the addition of enough inductance in the rectified (welding) output, in order to minimize the necessary complexity level of that field current control circuit. Now the coils of a 90A alternator unmodified are capable of more than 200A with your duty cycle, providing the RPM is kept at a normal range for a car alternator, no less than 15000 RPM that is. But the OEM rectifiers are totally unsuitable for your application, not only for their low current but also their low breakdown voltage, usually they will avalanche at no more than 3 times the alternator rated voltage. Too low. And remember, mainstream silicon diodes are NOT fast enough for that application, at that speed a common alternator outputs about 750 HZ, you need to replace them with high current, high voltage Schottky's or ultrafast high current silicon diodes (with much higher losses) All that can be easily implemented, the difficulty is that system voltage while welding drops too low for proper alternator field powering, so you must also use some kind of power storage, BIG capacitors or better a battery for that dead time. If all this doesn't discourage you, by all means, go on. S.M.

My arc welder works fine from a 2kW petrol generator that cost me £170, maybe less if you can find a used one. How much are you going to spend building your own version?

I share the opinion of other posters here, that besides the kick to keep yourself busy building one, all other options seem more considerable.

For real lightweight I would opt for a generator and a welding inverter. Some are less than 2 pounds.

Not possible. The 24V x 90Amp cannot provide the amperage/power required to do what you are attempting to do at.

Power In = Power Out

If you increase the output voltage to 40-50 volts the current maximum value will be a ratio of 24/40 or 24/50 x 90 amps.