# How fast does an car's alternator spin?

Discussion in 'Off Topic' started by RU482, Aug 18, 2002.

1. ### RU482 Lifer

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i.e. how many RPM of the alternators shaft does it take for an alternator to being producing voltage? Figure a typical alternator from...say, a chevy lumina (off of the top of my head)

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2. ### PsychoAndy Lifer

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It would depend on engine RPM and the size of the core as well, wouldnt it?

-PAB

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3. ### Thegonagle Diamond Member

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Off the top of my head, perhaps 1600-1800 RPM for 12-14 volts. It's going to make a higher voltage at a higher RPM, of course, which is in turn regulated back down to about 14 volts. I'm making this guess based on the relative size of the drive and the driven pulleys, and a typical engine idle speed. AFAIK, an automotive alternator needs voltage from a battery to energize the rotor magnets until it starts rotating fast enough to energize the rotor on its own.

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4. ### OrganizedChaos Diamond Member

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it shouldn't have to spin very fast at all to produce a voltage, if you want to charge anything with it though it will have to spin quite fast. upwards of 5000 rpm. keep in mind an alternator needs power to make power on the inside its just a big spinning electomagnet.

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5. ### RU482 Lifer

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Hmm, so in a typical car, how much current does the battery provide TO the alternator vs how much current does the alternator provide to the battery?

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6. ### notfred Lifer

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note engine's RPM, note diameter of crank pulley. Note diameter of alternator pulley.

Should be simple calculation with a little bit of pi and some long division after that.

#6

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You are way low. Most alternators will not put out any appreciable amperage until they hit about 1000 RPM. Highest amperage ratings are usually found in the 5-6000 RPM range. ever notice how increadibly small the pully is on your alternator? Like the size of a silver dollar. Alternators are usually made to spin at 3x engine speed. 2.5 would be minimum.

Without an aftermarket stator and windings, RPM over 6000 will not generate any additional amperage.

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Current to an alternator is less than 5 amps. Surplus from the alternator goes from 65 amps on up. Most cars now have 100 amp alternators stock. I have a 220 amp alternator to power my onboard welder. 400+ Amps are possable with aftermarket windings and stators for RV's and such.

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9. ### RU482 Lifer

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interesting....just thinking of trying a little homebrew windmill experiment with some parts I've got laying around.

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