converting car alt. to 120v ac

You would need to rewind the stator, chuck out the diodes and the voltage regulator, make your own voltage regulator. Then you need to drive it at the right speed and finally you have to deal with the

Obviously you're gonna have to open up the alternator. Do you know anything about alternators? Tap two of the stator windings' three outputs to get your single phase ac. And you have to take the diode pack's dc output (this is the alternator's output, normally) and connect it directly to the field (bypassing the voltage regulator). If you don't need tight regulation of your ac power and the splitter's engine has some abilitly to respond to load changes by adjusting its torque, you might not need any electronics.

Not to mention the frequency.

Graham

just bypass the regulator, which controls the feed to the field. And I guess dont use the rectifier, if you want ac. DC will run many ac loads though.

This will of course give you an unregulated generator, which wont give any voltage regulation. And the frequency will be high, too high for some loads, but fine for many.

NT

How about removing the diodes and adding a suitable transformer?

Leon

Yes,by accident. My regulator circuit regulated the wrong way and when we touched the accelerator, the whole board went bang!!!!!! The IC missed its hat,and any trace of chip and wire kind of disappeared. My college was splattered all over with pieces of electrolytic capacitor.

Something has always bothered me about this, maye some can explain it....

I know a car alternator can put out about 100 Amps at 12Volts as it is designed to by regulating the field winding which is on the rotor. This is about 1200 Watts... Ok no problem for an alternator that size...

I also know you can "full field" an alternator and it will put out a higher voltage...

OK, so lets say we full field an alternator and it puts out say 50 Volts. The stator wires can still handle 100 Amps output , but since its at 50 Volts...this is now 5000 Watts. I realize it will take a lot more torque to turn the shaft but suppose we have the torque... that still seems like a lot of power to get out such a small alternator...

Is the size required of an alternator detemrined by the current it can supply, not the power... can we get more and more power out of an alternator of a given size by raising the output voltage (given the insulation holds) ?

What am I missing?

Mark

Wrong voltage, wrong frequency, wrong number of phases. Your best bet (if its 120V, 60 Hz you're after) is to use it to power an inverter.

The effects of the series reactance of the alternator windings. The device might be able to put out 50V at no load, but, for a given value of field current (and RPM) the terminal voltage will drop off as load increases. At full field, the alternator will put out significantly less than he no-load voltage times the full load amps.

An automotive alternator is a three-phase configuration, so each winding only handles roughly 1/3 the DC output current. So if you could rewind a 100 amp it for single phase output you might get 12 x 3 x 1.414 = 50 VAC at 33 amps, or about 1500 watts. However, the alternator is designed to provide at least 12 VDC at idle speed, say 600 RPM, and will probably put out 6 times that at 3600 RPM,or 300 VAC. The magnetics will probably limit it to something lower than that, but it might be possible to get 120 VAC at 30 amps = 3600 watts. However, it's probably something like 400 Hz, so you couldn't use it for motor loads, unless you want them to spin *really* fast!

I haven't thought this out completely, but it's worth discussing.

Paul E. Schoen

Think about the fact that the regulator probably makes the alternator go full field pretty much anyway, when the output has a very heavy load on it -- your hypothetical 100 amps. It's already full field at 100 amps and you're only getting 15 volts -- not 50.

You are forgetting that an automotive alternator is essentially a current source.

As built, if you remove the diodes, they produce 3-phase AC. The voltage will be a function of the load and the field current.

...Jim Thompson

I think we can agree Mark won't get an alternator to put out five times as much power just from bypassing the voltage regulator.

The alternator WILL put out the power... until it starts to melt ;-)

...Jim Thompson

That is the point of my question...why would it melt...?

The stator is designed to deliver 100 Amps. When you full field it and the voltage is say 50 Volts, if the current is still 100 Amps the I^2R losses are still the same so it wouldn't get any hotter compared to when it delivers 100 Amps at 12 Volts.

Mark

Good point.

...Jim Thompson

I think you'll find the on-load voltage drops though on account of the alternator impedance.

Graham

Yep that would work but he still needs speed regulation and a 3 phase transformer.

A 3-phase transfomer still needs a 3-phase load !

Graham