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If an alternator is rated at 100A/13.5V then that's 1350Watts.

Assuming an external regulator and a external rectifier what's to stop that same alternator outputting 28A/48V.

Everywhere I have read, the requirement seems to be to add more coils to the stator to increase voltage. Manufacturers that require external regulators have different products for 12, 24, 48v alternators.

What stops ohms law from applying to a alternator? What else is coming into play that requires the alternator to be designed differently for different voltages?

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An automotive alternator is designed such that it can supply at least 14 volts to charge the 12V battery even at idle RPM, usually 600-900 RPM. The pulleys may be sized to spin the alternator at as much as twice that speed, or 1200-1800 RPM. The field coil in the rotor is supplied with DC from the regulator, and for idle speeds, the full 12 volts from the battery may be supplied. As RPM increases, voltage will increase proportionally, so the regulator may supply only 1 or 2 volts at maximum engine speed, typically about 10 times idle speed, or 6000-9000 RPM.

If the regulator is disconnected, and 12 volts is supplied to to the field, the output voltage can be as high as 120-140 VAC (three phase). The rectified output would then be as high as 200 VDC. The rectifiers in the alternator may not be able to handle such high voltage, and could be damaged or destroyed. But the alternator could be modified to bypass the diodes to obtain the high voltage when it spins at 12,000 RPM or more.

I found an article that explains this in detail:

http://gfretwell.com/electrical/Alternator%20Secrets.htm

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  • \$\begingroup\$ Curious to know what the breakdown voltage of the actual windings would be. I concur that the diodes would likely fail first, but if you take them out of the picture, there's going to be some point where the insulation on the windings gives out. \$\endgroup\$
    – JustJeff
    Apr 14 at 15:04
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The alternator has to output that voltage at a certain (minimum) RPM. Ignoring voltage drop due to resistance, that means you need more turns to get more voltage at the same RPM, all other things being equal (such as rotor flux and number of poles). This follows directly from Faraday’s Law.

It would likely be possible to get 24V from a 12V alternator if you could gear it up by 2:1 and use an appropriate regulator, however mechanical limitations may cause issues at maximum engine RPM. In fact with the regulator maxed out you may be able to get more than 100V from a 12V alternator at high engine RPM.

Note that the regulator works by reducing the flux from a design maximum.

Note that you could use a 48V alternator at 12V over a normal RPM range with an appropriate regulator, however the alternator would have about 1/4 the maximum output current of a similarly-sized alternator designed exclusively for 12v so it would not be cost-effective.

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