ASK THE EXPERTS: Car Alternator for Wind Turbine?


I’m thinking about using an automobile alternator for a homebuilt wind generator. Will this work?

Ron Johnson • Albuquerque, New Mexico

Hello Ron, A car alternator is a bad choice for a wind generator. The efficiency in normal use is never more than about 60 percent. The bearings are too small to reliably support large blades (more than about 1.5 meters diameter). It is designed to be lightweight and robust, and to withstand running at very high rpm. At low rpm it produces nothing, and low rpm is where wind generators spend the majority of their time running.

If you use a car alternator in a wind turbine, the speed problem can be addressed in one of several unsatisfactory ways:

  • Use a small blade area so that the short blades can spin at high rpm. This means that you cannot catch much wind, and even so, you will need a high wind speed to get the necessary rpm. It will also take a lot of wind to produce high enough power to excite the magnetic field and actually have energy to spare.
  • Use gearing to increase the rpm. This involves extra cost, extra losses, extra unreliability, and overall ugly and clumsy engineering.
  • Rewind the coils to work at lower speed. This means more turns of thinner wire in each coil. This reduces the cut-in rpm, but also increases the losses in the coils themselves, limiting the power output and further reducing the already low efficiency.

A car alternator’s rotor needs to be powered to excite the magnetic field. The field has to be at a maximum to get output at the lowest speed. This represents a constant power loss of 30 to 40 watts during operation. You will also have to remove and bypass the internal regulator. The internal regulator in the alternator is not suitable for charging a deep-cycle battery via a long wire run.

While it is cheap and attractive at first look, the car alternator is more trouble than it is worth. It is better to build a purpose-built alternator for a wind turbine.

Hugh Piggott • Scoraig Wind Electric

Comments (7)

baylensman's picture

What if a "generator" from an older vehicle or tractor that is designed to operate at a lower RPM and a lesser HP engine was used. Not to generate direct power but say to charge a set of back up batteries for when the grid shuts down. Maybe enough for lighting and fans?

Hugh Piggott's picture

hi Baylensman,
Back in the 1970s when I started doing this I did manage to find low rpm vehicle generators and built many machines based on them. I used jeep generators that weighed over 50 pounds each. I rewired the field coils so I could run them at lower rpm at 12V instead of the 24V they were designed to produce. It was a workable solution with recycled technology and it powered a couple of dozen homes for ten or twenty years. But when magnets got cheaper and I learned to build simple alternators with them I was very pleased and I never looked back.
The problems were twofold. The brushes and commutators needed frequent attention. but the worst problem was the power consumed by the field coils. Low winds were when we needed power the most. PV was too expensive for us in those days. And the field coils used up all the power in low winds.
Nowadays we can have all the power in low winds because the magnetism is permanent. It's easy to make an alternator using the axial flux approach. So I don't see any role for vehicle alternators or generators. They are too small, too inefficient, too fast-running.

goodall280's picture

you actually can add magnets to the alternator to make it a permanent voltage or you can build a turbine with a pully on the back with belt chain etc connecting to alternator. if your pully on turbine has say a 17" inch circumfrance and your alternator is a 1" your ratio is now 17:1 so lets say your turbine turns 3 times in one sec we times 17 by 3 then times by 60 to fig out what it would be in rpm which would be 3060 revolutions per min (rpm) your average alternator ranges from 500-1500 rpm and buy adding belts or chains you remove the strain on the bearings lets face it if the cant handle belts they wouldn't use it on a vehicle then all you would need is batteries and a powerful enough inverter many people are now using these types of power sources in camps where there are no power lines you can also make an easy to build power sourse with a old powerline wire spool for example and build a water turbine unit in a stream you can look some of these ideas that people have used on youtube

Ian Woofenden's picture
Most smaller residential wind turbines today are direct drive, because it reduces losses and costs -- KISS. As soon as you introduce gearing of any sort, you're introducing a loss. That might be acceptable if there's enough energy and money to play with, but the physics and the marketplace tells us that it's not the ideal scheme.

I'd suggest caution with leaning too heavily on YouTube videos, which may come from serious and long-term users, or maybe just from hobbyists or experimenters. Wind electricity isn't easy to implement, even when you have good knowledge.

Ian Woofenden, Home Power senior editor.
Michael Welch's picture
Some of that is true and doable. Belts work for car engines because of the large amount of power available from an internal combustion engine to overcome their high resistance. But if you want something that actually works and lasts, explore other options. Building your own turbine is doable, though. Check out "Wind Turbine Workshop," a book by Hugh Piggott.
hhanafi's picture

Hi, I think if you use a verical turbine you may avoid the bearing problem. The rpm problem may be solved by using a 1:2 or more belt assembly. Again this is only a rough thought that needs more research.


Ian Woofenden's picture

Vertical-axis wind turbines (VAWTs) have not shown themselves to be reliable in the marketplace or in the wind. Many uninformed people seem to think they are an answer to wind energy problems, but generations of real-world wind energy designers and users know otherwise. VAWTs' successes are generally for venture capital seekers and salespeople, not for the end users who want clean and reliable energy.

There's a grain of truth in the comment above, since VAWTs tend to run at lower rpm. But they also have significantly lower efficiency and reliability, so are not a good tradeoff if your interest is in reliable energy generation.

Ian Woofenden, Home Power senior editor

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