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It's identical to the way a genset works - the more amps/watts, the more HP needed...
then why was a GM single wire alt the same for the 6cyl and the V8's even up to 454's and 502's? they were putting out the same amps across the board and all used the same model number.
Because all of those motors have vastly more HP than is necessary to max out those alternators. Car alternators are quite small, few can put out more than 1kW, and virtually none can put out more than 2kW. That means that they would need 5-10 HP *MAX*, even with lots of inefficiencies and losses. Cars don't take a whole lot of electricity.
In those cases, the limits are the magnetic fields in the alternator, the copper losses, and the ability of the alternator to cool itself. As you increase the size of the generator and decrease the size of the motor, eventually the motor would be the limiting factor. You generally don't want that to be a factor.
The more electrical power you want to generate, the more mechanical power you have to put into it. Fundamental law of the universe. If you can find a way around that, you will quite literally become the wealthiest individual in the world.
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why does HPhave any effect on the output of an alt? if the motor turns the correctspeed the alt doesnt put that much, if any, strain on the motor.
It is only easy to spin when electrical load is light. As demand increases, the mechanical forces needed to turn the alternator increase. Sounds like magic, but again, it's a fundamental law of the Universe. As Lenz first described it, it is called Lenz's law.
Take a brushed DC motor, and spin it. Nice and easy. Now short the wires together, and spin it again. You have just seen Lenz's law in action!