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