need advice..Motor/turbine

[faomari]

Hi i want to build an udnerwater turbine that senses waterflow at a specified time, and charges up a chargable battery when not bieing used for waterflow measuring. I went to a car wreck today to get a cheap radiotor fan for a car. The guy who works there told me that if i use the fan for underwater flow measurement, it will overload and it wont work, as the fan is electrical not mechinical. Can anyone tell me if this is correct and if it is then what is exactly happening for it to over load.

Also another thing, i need a motor to charge my battery, dont know much about motors, but what i want is a motor that charges a battery and also when not being used for charging(i.e no load), it could take relatively accurate measurement for water flow, so i need a motor that it easy to spin(small internal friction) yet capable of charging. any ideads of what type of motor i should use.

thanks.

 

[stevez]

Some things to consider -

It sounds like you want a motor to serve as a generator. Some electric motors can serve as generators, some can't. Don't ignore the use of a generator or alternator if you can find one that suits your needs.

Some motors are constructed in a way that allows them to operate under water though most are not constructed that way. Submersible well pumps and sump pumps are constructed to operate submerged. Consider a low end marine pump.

You'll want to charge the battery at some rate which means you'll need enough power to do that. The power delivered to the generator shaft by the propeller or turbine will have to exceed that power requirement to account for efficiency. That suggests that the propeller/turbine will have to turn at the right speed (or you need speed reducer/increaser) and provide sufficient torque. The prop/turbine power (speed/torque) is determined by water velocity.

You might want to work thru some of this stuff even if only with rough estimates to have some confidence that the energy balance is in your favor. In short, the power you need to charge your battery might exceed the capacity of the prop, generator or other part of the system.