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How to select the DC generator?

  1. Dec 3, 2011 #1

    I have designed a water turbine which weighs about 5.3 Kgs and it rotates at about 150 rpm from a head of 3 mtrs. The water is guided by a single 25.4 mm dia pvc pipe. I can achieve better rpm upto 200 rpm by increasing the flow. My plan is to generate steady 12 Volts DC for rural application in hilly terrain. The turbine is cylindrical in nature and the shaft extends on either side of the cylinder. It is my desire to connect a DC generator on both the sides of the shaft. Following are my questions:

    1. How to select the DC generator?
    2. How to calculate the torque?
    3. Is the speed and torque of this turbine sufficient to produce 12 Volts DC for 750 Watts load?
    4. Since the application will be in variable flow terrain, how should my selection of generator be?
    5. What other home work is to be done from my end for better results.
    6. Also, can this application be increased to 24 Volts DC with higher torqe, with same rpm by increasing the weight of the turbine (since I hv constraints on generating higher rpm.)

    I will be highly obliged with any help from your end.

    Thank you,

  2. jcsd
  3. Dec 3, 2011 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    Welcome to PF.

    1. consider building one or driving the shaft of an electric motor while you are still experimenting. Then get used to how manufacturers characterize their products.

    2. one way is to put your turbine in it's working position, how it will be used, and hold the shaft with a torque wrench. What you actually want is it's moment of inertia ... which you can find by applying a torque and measuring the acceleration (eg. wind a cord around the axle, one end attached to a weight: drop the weight, time it's drop and rise etc.)

    It is best not to calculate things if you can measure them - the universe knows more physics than we do.

    3. No way to tell.
    In a 3m drop the water gains 29.4J/kg so, for 750W you need it to pass more than 25kg per second.

    By itself, the water gains about 8m/s, that translates into 32kg/s so there is enough energy supplied... you just need to be able to extract 78% of it :) The high speeds suggest not, but attaching a generator will slow things down considerably.

    Of course, if you are putting this in a stream or something you have a bigger drop available - you really want to know how much you have to slow the water down in order to get the needed energy, then measure the speed of the stream.

    4. this is the same questions as 1. Have a look at what the commercial people use for the same sort of situation. You seem to have a reasonable idea - you want to know how much the generator will resist being turned.

    5. what you are doing looks cool - you just have some experimenting to go.

    6. Yes and no. A heavier turbine just needs more energy to accelerate it, and will likely have more losses. Consider - a very heavy turbine won't turn at all... successfully removing almost all the KE from the water but not allowing you to extract any of it to do useful work. However, a big torque can run a heftier generator. How much you generate depends on the water flow and how good your generator is.

    You are finding out that building a turbine is the easy bit :)

    I've done something similar - I diverted part of a stream through a hose-pipe on to a water-wheel (converted bicycle wheel - nice big torque) which was driving the shaft of an electric drill. I got enough to run car-indicator light bulb. But you know how stiff electric drills are right?
  4. Dec 3, 2011 #3
    Modern 12 volt car alternators put out 12 VDC at 50 to 70 amp. Truck and bus alternators put out higher current.
    Use a car alternator and design your turbine for the RPM required by the alternator.
    A 1 inch diameter pipe and a 10 foot head of water doesn't seem to be enough energy to obtain 750 watt. (My assumption without doing any calculations)
  5. Dec 4, 2011 #4
    Many thanks for the reply, specially to Simon Bridge. I will certainly incorporate all the suggestions & get back to the Forum at the earliest. I guessed that 750 Watts was a tall order & therefore I have decided to increase the flow to obtain better torque & perhaps rpm too. In other words, I want to stick to 12 Volts DC generation & not try anything silly! By the way, friends, this is the first time I am ever interacting through a Forum & I am really exited about it. Actually I am eying to build this model turbine since 30 years but business commitment & other related family priorities kept aside innovative ideas/experiments. I am now retired from active business since both my children are well settled! Hence this letter (& all the crazy things I wanted to do!). By the way, I am 60 years old!

    Can I share my drawings & calculations to the Forum for a more useful discussion on this subject?

    Thanking you all once again,

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