Alternator Output Question

17.5kW output? I'm pretty sure no alternator puts out that kind of power...

Alternators are usually 12-14V because that's what cars' electrical systems use.

In theory, you could rewind one with five or six times the number of turns, but then you'd have to use wire with 1/5 or 1/6th of the cross-sectional area, drastically increasing resistive losses. Higher resistance would either mean the thing fries from the heat generated at 250A, or even that it won't be able to generate 250A at all.

What do you want this for, anyway?

Actually, it's not really that complicated, but it will be big, heavy and expensive. An alternator (generator) at 70 V / 250 A works the same as one at 12 V (or 15 V, rather) and 100 A, only it's bigger. I guess you want 70 V rectified output (DC), which means the AC voltage will be around 50 V. Unfortunately, that's not a very common voltage. You might find a synchronous machine built for 110 V AC and 250 A, and use a lower excitation current to get around 50 V AC. That will however be a big machine, perhaps around 100 kg. Also pulling 250 A rectified DC will heat the windings more than 250 A sine wave, so better get something capable of a bit more. Even bigger. At that size, the machines are likely to be three-phase, so look for something like 110 V and 3x100 A 3-phase.

I was also contemplating the possibility to overvoltage a 28 V alternator from a truck (not US, they run 12 V, but the old world has 24 V). That includes a lot of risks though, and alternators for cars in general have bad efficiency. The biggest I saw were 180 A so you would need two.

A motor/generator shop might be able to rewind a 110 V machine to 50 V, so you get something smaller.

Finally, guessing you are contemplating some kind of series hybrid or large battery charger setup, you could let a 110 V machine power a DC motor controller (such as the Zilla). 110 V AC rectified is around 150 V, which should be within reach for many controllers. Even Kelly has one that takes up to 144 V (168 V). Then you can get any voltage you want from 0 - 150 V.

to clarify what you said: 110v alternators are available and a controller (man, that Zilla is expensive!(though, that's to be expected))will be able to regulate the correct voltage to the electric motor as well as control it...?

Not really. He's suggesting you use a motor controller on rectified mains power to charge the batteries. I think. But since it sounds like you're trying to make a series hybrid (gas engine powers electric and charges batteries) that's not going to help you...?

You could used a permanent magnet motor like it was an alternator. But, that's not cheap; an Etek-RT (about the only motor I can think of that would be suitable for this) will run you $500. You wouldn't need a controller to charge the batteries, though, since you could gear the engine to make the motor put out the right voltage.

And, yeah, the Zilla is quite pricey, but that's a performance controller. Unless you're building a sportscar, you won't need anything near that kind of power.