Best Alternator For DIY Generator
Car alternators all basically do the same thing, and they all kind of look the same. So which one do you choose for a DIY generator project?
When re-purposing an alternator for a homemade generator, there are four factors you should be aware of:
1) An automotive alternator is not built for 100% duty cycle. Once they reach their peak output, they cannot sustain it without overheating and failing. This is not a fault of the unit, they're just not designed to do so. For use a passenger car or light-duty truck, they don't need to sustain peak output for very long.
2) A car alternator is designed to put out charging voltage and amperage at high rpm's. In an average passenger car, this figure could range from 3,000 to 6,000 rpm, which is much higher than a typical lawn mower engine spins at. This is why pulley diameters are important.
3) The greater the amperage rating of the alternator, the more heat it can generate. If you're needing to run your generator for hours, this will be a factor.
4) An automotive alternator needs to be "excited" to generate power. Although there are several ways to excite an alternator, this is usually done with a 12-volt battery.
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Delco Remy 10SI Alternator
For a DIY generator powered by a small engine, the most popular alternator is a Delco Remy 10SI unit. First appearing on Chevy Corvettes in 1969, it was fitted to literally millions of GM passenger cars and light trucks throughout the 1970's and early 1980's. They are easy to fabricate mounting brackets for, as well as easy to wire up.
There are several variations of the 10SI, and all of them have a built-in voltage regulator. The most common of type of 10SI alternator is the 7127.
The number '7127' is an industry part number, and often sold with various suffixes; example: 7127-3, 7127-12, etc. To name one specific application for this alternator, out of perhaps a hundred automotive and light truck applications, would be a 1975 Chevy Impala with a 350ci engine.
NOTE: 10SI is a Delco Remy model number designation, not a part number. The AC Delco part number is 334-2114.
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Delco Remy 12SI Alternator
The model 12SI was introduced with 1983 GM models, and it continued into 1988 with some applications. They have significantly greater output than 10SI units.
10SI Alternator (left) and 12SI Alternator (right)
The 7294 alternator is the most popular of the 12SI series, and has dozens of automotive applications. One specific automotive application would be a 1985 Chevy Impala with a 305ci engine.
10SI vs 12SI Alternator
A 7127 alt (above left) is a 10SI type and is rated at 40 to 65 amps.
A 7294 alt (above right) is a 12SI type and is rated at 94 amps.
At idle (about 1700 RPM on a small engine) a 7294 unit rated at 94 amps is able to put out about 20 amps. If it's revved up to full throttle (about 3600 RPM) it will put out about 70 amps. Output on a 7127 unit is considerably less.
7127 Alternator (left) and 7294 Alternator (right)
Visually, differences between the two are, the 7294 has a larger, more robust cooling fan. Mounting boss dimensions of the 10SI and 12SI are the same.
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Alternator Cooling
Alternators were designed to help cool themselves. Air gets pulled in from the back, and exits through the front.
An alternator will function in either direction. Cooling can be an issue, so notice which direction the fan blades face. The fan will still function either way, but at reduced efficiency.
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New vs Reman vs Used
Alternators can be purchased online (new), at an auto parts store (new or rebuilt) or at a junkyard (used).
Note: Remanufactured units ("reman") are sold on an exchange basis. If you don't have an old unit to trade in, they'll cost a little extra.
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Conclusion
For a home-built generator, you want an alternator that's small, lightweight, and easy to mount and wire. A 7127 alternator has all those qualifications, plus it's the cheapest and most plentiful out there. They are easy to take apart, easy to get replacement parts for, and have proven to be very reliable.
Shop: 7127 Alternator
A 12SI unit, like a 7294, has a greater output than a 7127 and it's the same physical size. But unless you can find a good used one, it'll cost nearly twice the price.
Shop: 7294 Alternator
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Wiring Harness
Along with the alternator, you'll need an alternator wire harness, also referred to as a pigtail. It's a two-wire molded connector/harness that plugs into the back casing.
Shop: GM Alternator Pigtail
You could skip the pigtail connector and just shove spade connectors into the back of the alternator, but the connector is just a few dollars. Not only does it look neater, it will simplify wiring.
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Wiring
The following wiring diagram is from the the EpiCenter website:
CAUTION: Do not wire the alternator unless you are sure you know what you are doing. If you make a mistake in the selection of the alternator or wiring diagram, you run a very high risk of damaging your battery, electronic devices, or causing personal injury.
CAUTION: Without a battery hooked up while it's operating, you can damage the internal voltage regulator while running at normal RPMs.
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DIY Wind Power Generator
GM 10SI and 12SI alternators can also be re-purposed for a wind-powered generator. When converted to wind power use, they are often updated to permanent magnets.
The Leece-Neville alternator is also popular for wind-power use.
Learn More: DIY Solar Forum
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