Most of their time wind generators rotate at low rotations. Car alternators would not generate anything at lower speed of wind turbines. Therefore, using a car alternator in wind generator is really a poor option.
- The efficiency of alternator would never reach more than about 60 %. The bearings are very small and can not support large blades reliably (about 1.5 meters diameter). The car alternator has been designed as lightweight and robust being its functioning at very high rpm.
- Although a small blade can be used to make the alternator spin at high rpm. But smaller blades would catch less wind. You would need a high wind speed to obtain necessary speed of rotation.
- Gears with pulleys or other methods can be used to boost rpm. But a lot of power is wasted through friction. This would result in additional cost, further losses, added unreliability, and a horribly and awkwardly made project.
- There is another possibility of rewinding the alternator to run at lower speed. But this also means that there would be more turns of thin wire in each coil. In this method cut-in rotations are decreased, but losses in the coils are also increased, thus you would get decreased power output and further reducing the already low efficiency.
- Car or truck alternator is an electromagnetic device. It means that separate power is required by the alternator to make the magnetic field for its electromagnetic coils internally through brushes and slip rings.