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Old 01-28-2010, 10:34 AM
frederic frederic is offline
The Junk Man
Join Date: Jun 2009
Location: New Jersey
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You can drive automotive alternators with a pull-start engine. You still need the battery to provide the field voltage as the alternator is "excited" while it speeds up. Once at a certain, minimum speed the alternator then charges the battery as it does in a car, truck or boat. If you remove the battery while the alternator is providing current, you can easily overheat and damage the alternator windings as well as the internal current regulator. The battery, and it's resistance, is integral to the current regulation circuit.

Automotive engines typically do about 2500-2700 rpms for highway speed, so alternators are pulleyed to about a 3:1 or 4:1 ratio. For a gasoline engine that produces it's peak horsepower close or at it's redline of 3500 RPM, you could gear things a little closer. For durability you'd want to increase the pulley diameters but keep the ratio the same, so that more belt surface touches the pulley. This will compensate a bit for the conditions that homemade systems of this kind must endure - like sitting in the freezing garage for six months allowing the belt to get a "memory" of it's shape.

Also know that temperature of the alternator windings and casing impacts the amount of current you can draw. The colder it is, the more you can draw. This is why automotive alternators have an integrated fan, and hope that the car is moving more than it is idling. I'll give you a tip - large case alternators generally can provide more current than small frame alternators earlier in the RPM range for a given temperature. If you can get a large frame alternator that's generally better however whatever you get at a junkyard, you can drive with a lawnmower engine no problem.