A car alternator makes electricity once its shaft turns at roughly 1,500-2,000 RPM or faster and its field winding is fed from a 12 V source, so a bare alternator turned slowly by hand, wind or water gives nothing without that excitation and a step-up drive. Wire the output to a battery through a fuse, never run it with the battery disconnected, and expect 700-1,120 W at 50-80 A when it is spinning fast with proper excitation.
The alternator is the simplest vehicle-to-electricity conversion, because it already charges a battery from a rotating shaft. Its difficulty is speed: it was built for an engine turning thousands of RPM, not for a wind turbine or a pedal.
The Alternator Needs Speed And A Field Current, And Hand-Cranking Supplies Neither
The parent article describes the alternator as an AC generator with internal rectification producing 12-14 V DC. Most alternators need an external 12 V supply to the field winding (the small connector) to generate at all, and most begin producing useful output at 1,500-2,000 RPM, reaching full output at 3,000-6,000 RPM. Alternators with permanent magnets (from some motorcycles, tractors and marine engines) need no external excitation and produce power at any speed, which makes them better for slow wind or water drives but less common. See Permanent Magnets and Alternator Design for the principle.
What You Need
- An alternator, with its mounting bracket.
- A drive source: an engine pulley, a wind turbine, a water wheel, a bicycle or a hand crank, and a pulley set to bring it to speed.
- A 12 V battery, for excitation and for storing the output.
- Heavy wire for the output, a fuse or breaker close to the battery, and a charge controller if the drive is variable; see Charge Controller.
- A rigid mount, belt and guard.
Method: Mount, Drive, Excite, Output
Step 1: Mount the alternator on a rigid bracket, with the pulley in line with the drive pulley.
Step 2: Connect a drive source through belts or chain: engine pulley, wind turbine, water wheel, bicycle or hand crank (parent article). Size the pulleys to reach the speed range above.
Step 3: Excitation. Connect the field wire, the small connector on the alternator, to a 12 V source. Without this, most alternators produce no output. Identify the terminals before connecting, since a wrong connection can destroy the regulator or rectifier.
Step 4: Output. Connect the output terminal, the large threaded stud, to the battery through a fuse for charging, or to a 12 V load. Connect the battery before spinning it.
Step 5: Spin up. Raise the speed gradually. Watch the battery voltage: it should rise above the resting battery voltage as the alternator takes over.
Step 6: Never disconnect the battery while spinning. The alternator then sees no load and can produce voltage spikes that damage its electronics.
Worked Example
Output power is volts times amps. At 14 V, 50 A gives 14 × 50 = 700 W and 80 A gives 14 × 80 = 1,120 W (parent article), reached only at speed and with excitation. A drive that is slow needs a step-up. Suppose, as an example, a turbine turns at 300 RPM and the alternator should run at 3,000 RPM. The ratio is 3,000 ÷ 300 = 10, so a 300 mm pulley on the turbine driving a 30 mm pulley on the alternator gives it, since 300 ÷ 30 = 10.
Common Mistakes
- Expecting output at hand-crank speed. The alternator is too slow to excite.
- Forgetting excitation. Nothing is generated.
- Running with no battery. Voltage spikes and heat.
- No fuse on the output. A short circuit then burns the wiring.
- Spinning it backwards. Many alternators have a fan that works in one direction only.
Safety
Spinning belts and high current
A belt or pulley at speed entangles clothing and hair; guard it and keep clear. The alternator’s output can supply 50 A or more, enough to heat wire red-hot if shorted, and the battery it charges gives off explosive hydrogen. Fuse the output close to the battery, keep sparks away from the battery, and see Battery & Electrical Safety. Wind and water drives can start turning without warning; lock them before working on the wiring.
What’s Next
- Learn next: Starter Motor Applications — the other electrical part worth pulling
- Combine with: Generator Coupling — connecting a turbine shaft to a generator