Help my understanding of DC generators please

[james weaver]

Hello, I am currently working on a engineering project for school and have been tasked with researching the basics of electrical theory. We are building a small, portable hydroelectric generator for the purposes of charging a cell phone or other small electrical devices. I already have the motor set aside which will be a 12V, 60RPM DC motor.
https://www.amazon.com/dp/B072N84JX7/?tag=pfamazon01-20

I'm afraid I do not know much about electrical engineering but have been reading/watching videos on the basics of how generators work and their anatomy, however I still have some unanswered questions I am hoping you guys could help me with.

1) According to the link, this motor has a rated current of 0.06 amps, and I believe most phones draw in the range of 0.5 to 1.5 amps. Does this mean that it will be insufficient to charge at all or that it will simply take longer?

2) If the generator drops below the 60 RPM that it's rated for, will it still supply a charge to the battery? And what happens if it goes above the RPM? I'm still a little confused on how voltage, current and rpm are related quantitatively. Any resources would be helpful.

3) Do you think the 60 RPM motor would be the best option or should I choose something higher?

4) Would this setup be a feasible option for charging a backup battery? Let's say I just want to leave it running all day when I'm out camping, is there a way to calculate a rough estimate of how long it would take to charge a typical 12V battery?

I know these questions are probably overly generalized but I'm not sure where else to start. Any help appreciated, thanks.

 

[Drakkith]

1) According to the link, this motor has a rated current of 0.06 amps, and I believe most phones draw in the range of 0.5 to 1.5 amps. Does this mean that it will be insufficient to charge at all or that it will simply take longer?

Voltage determines whether the battery can be charged at all, current determines how fast the battery can charge. If your phone battery needs 4.3V and you only give it 3V, then it's not charging no matter how long you leave it connected or what the supplied amperage is.

 

[berkeman]

We are building a small, portable hydroelectric generator for the purposes of charging a cell phone or other small electrical devices.

Can you post the full project requirements statement? How much water "head" are you guaranteed for this power conversion? How much flow rate can you count on?

When you have a situation where the input voltage can vary above and below the target output voltage, you use a special kind of power converter circuit to do that conversion. Do a Google search on Buck/Boost DC-DC converters.

 

[DaveE]

I would suggest giving up on the phone battery for now. There is some extra complexity in the Li-Ion battery charging interface usually. You're probably not really charging the phone battery, but supplying power to a charge regulator inside the phone which does the actual charging. Plus phones are expensive and people get mad if you break them (or even take them away for 5 minutes, LOL). How about just charging some NiMH batteries, which are cheap and safer than Li-Ion batteries, fire-wise. The voltage required is lower than a phone, or you could put two in series for higher voltage if you like. Honestly, if I was doing this for real I would probably not have a battery at all, I'd substitute resistors (or an electronic load) to have more control for collecting performance data.

 

[james weaver]

I would, however the project is already set. Fortunately, my professor said that it is alright if the final product is insufficient for practical uses as long as it can be shown by thorough analysis.

 

[james weaver]

Can you post the full project requirements statement? How much water "head" are you guaranteed for this power conversion? How much flow rate can you count on?

When you have a situation where the input voltage can vary above and below the target output voltage, you use a special kind of power converter circuit to do that conversion. Do a Google search on Buck/Boost DC-DC converters.

The system would be placed in a shallow creek or small river. I've attached a conceptual drawing that should help.

 

[berkeman]

There are existing products for this application. Can you post links to those products and address what their design decisions were?

 

[james weaver]

This is pretty much the exact same thing we are trying to build


I think I have a better idea now on what I want. I would like to derive an expression for the net torque on the shaft as a function of the velocity of the water. The dimensions and mass of the fan blades are software generated and are precisely known ( i have the part in inventor). From there, I can work towards a more general function the relates the water velocity to output voltage and current. I have an understanding of classical mechanics so I think I could figure it out, but would probably need a little help.

 

[anorlunda]

It sounds like the analysis is the important part of your project. This passage from WIkipedia can help you to get started. As it says, Power is a function of the hydraulic head and volumetric flow rate. DC power is voltage times current. After you calculate the power P, any pair of V and I such that V*I=P works.

https://en.wikipedia.org/wiki/Hydropower#Calculating_the_amount_of_available_power

 

[james weaver]

Ok thank you this helps. Here's one more question that I can't find a straightforward answer to:

When the manufacturer give the rated torque and current for a motor (in this case it is 6.5kg-cm and 0.06 amps), is that with the assumption that it is operating at the specified rpm? (in this case it's 60 rpm)

 

[anorlunda]

Ok thank you this helps. Here's one more question that I can't find a straightforward answer to:

When the manufacturer give the rated torque and current for a motor (in this case it is 6.5kg-cm and 0.06 amps), is that with the assumption that it is operating at the specified rpm? (in this case it's 60 rpm)

That thing you linked is advertised as a motor, not a generator. It has a reduction gear that you don't need. It says Reduction Ratio: 1:57.7, which is very high.

That reduction gear may lose most of the power. It is not clear on which side of the gear they quote the torque and the 60 RPM. It it 60 RPM on one side and 60/57.7 RPM on the other, or is it 60*57.7 on one side and 60 on the other?

Why are you asking about torque anyhow? Power is the useful number to have. RPM can also be important.

 

[james weaver]

Yes i realized when I tried to move the shaft with my fingers that the reduction ratio was way to high. What do you think a good rpm would be for a hydroelectric generator? Maybe 300?

The reason I am interested in torque is because I want to derive a function that relates the velocity of a moving stream to the generator rpm, although maybe a different approach would be better.

 

[anorlunda]

The reason I am interested in torque is because I want to derive a function that relates the velocity of a moving stream to the generator rpm, although maybe a different approach would be better.

The equations you need are in post #9. It includes flow rate, not velocity.

Here's a completely engineered solution. We don't have your complete project requirements, so using a garden hose as your water source may be allowed.

If you bought it, it might come with a data sheet that gives the parameters needed to make an analysis. For example, a similar unit's ad includes this curve.

Here's a generator that comes with several gears that allow you to experiment with speeds. It is meant for wind generators, but you could adapt to hydro.

A search for "12V 10W DC generator" will reveal very many other products offered. I don't think you understand that the turbine is more difficult to design and make than the generator. A propeller mounted in moving water is a poor choice. A water wheel is more realistic.

This video shows a simple DIY micro hydro project using a water wheel, and a used car alternator. Note how they used pulleys and belts to make the water wheel RPM different than the generator RPM.

For general advice, it is a mistake to select components first. Take some time looking at other successful micro hydro DIY projects first. Do this search, and watch several videos.

https://www.youtube.com/results?search_query=DC+micro+hydro+DIY

 

[james weaver]

Ok, thank you. I guess what I am really trying to figure out is if there is a way to make a rough calculation of how long it would take to fully charge a typical cell phone battery (say ~4000 mAh) as a function of flow rate given the specs of my generator (may have to pick one with a higher rpm). I have derived an expression for the number of coulumbs as a function of flow rate and time and have attached it. Could someone please let me know if it is correct and how I can use it to estimate charge time? Thank you.