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I'm relatively new to energy production using alternators or reversible motors but have been asked to find the most efficient method of producing energy from a small underwater turbine.

The only information I have to go on is that the overall blade diameter will be 1m tip to tip and max output will be 150 RPM (direct). [The torque can be altered with the blade design.]

I've found an alternator that will produce 9V, 4A @ 150rpm but this doesn't sound all that impressive.

I'm therefore in search of a reversible to DC motor that I can spin at 150 rpm and compare the amount of power produced. If I get less than roughly 35 watts (equivalent power) I'll know the original alternator is the better method for my requirements.

Does this sound like I'm asking the right question or am I doing this arse backwards?

Help!

Have you looked into gearing up or picking a different alternator? Some Ebay stores have alternators by a company called Hornet power systems that are available in different voltage output windings. Depending on your load and operating RPM this might make sense. I can't vouch for these as I have only seem them advertised. They look to be permanent magnet retrofits of new General Motors alternator parts.

You could also perhaps gear the unit up with 5:1 pulley/belt ratio or something like that. Just make sure that the mechanics are sound.

You might also look into impedance matching. Servo motors, for instance, are capable of driving heavy torques at low speeds (just look at the torque constant of the motor). If used as a generator you would expect similar for power output. It is just that to get this full current output your load resistance has to be very low (like when you "brake" the motor by shorting the windings). I wonder if you could run the 150 RPM equivelent three phase voltage through a transformer. This would provide a low impedance load to the generator phases and a high voltage output to your load. Just a thought, I haven't really done the analysis on it. If you use a 3 phase brushless motor and appropriate transformer, you should be able to have near full power output of the thing at any reasonable speed. Having said all this, if you pick the appropriate generator you don't have to screw around but this all depends on your budget, what parts are available to you, etc.

Just be careful of the specs, most generator plots I've seen for hobbyist type applications only give the output curve for one load (usually a 12V battery) vs. RPM. This makes it really difficult to do analysis on! You really need a graph of different load impedances.

~KEJR