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 1 Using Arduino / Project Guidance / Re: Alternator sizing for energy harvest (hydro)? on: September 23, 2014, 04:07:49 pm Nice guys........ It is supposed to be a permanent installation so the engineering level is high. Battery is a good idea but out of the question. It is inside so no sunlight for cells. Regarding pressure drop, no limit but i dont want to be ridiculous. It is in a production facility so there is a LARGE pump connected (but unfortunately i cant pull power from a socket). But, when i do the math (pressure drop multiplied with volume flow) the available power becomes high quite quick. It is in SI-units so the pressure is in Pascal (100.000 to the Bar). Just to put it in perspektive - If 1/10 of a bar (10.000Pa or roughly 1m loss of head)  could be introduced, you could theoretically harvest 138W (which is about 70 times more then i need) if i am not mistaking. Regarding the gearbox i was planning on using a belt drive with a ratio. I may need to make it in several steps, but there is a very high efficiency associated with belt drives - 95%ish.Regarding the diameter of the pipe, it will vary but on the pilot project a 200mm.    Quote from: Kawgomoo on September 23, 2014, 12:13:30 pmyou will need to see 4000-6000rpm at the alternator shaft for good output.I guess that depends on the size of the alternator. The small air-core motors from eg Maxon, have a no load speed of tens of thousands RPM, but down to 1200RPM can also be found in their catalogue.The reason i would introduce a venturi is due to the equation i showed earlier: P=0,5*rho*A*v^3 because of the term v^3 so i would perhaps crank that up by reducing the cross section hereby increasing the velocity and reducing the pressure.just a thought. Thx alt
 2 Using Arduino / Project Guidance / Re: Alternator sizing for energy harvest (hydro)? on: September 23, 2014, 11:19:56 am I try to keep it short so you dont have to spend too much time reading but it does not seem to be working out for me ;-)
 3 Using Arduino / Project Guidance / Re: Alternator sizing for energy harvest (hydro)? on: September 23, 2014, 11:17:14 am ..............................@PaulS: Okay okay okay, here is the entire story - i guess i have to be happy since people seem to be paying attention to the small details ;-) It is not a creek but flow in a pipe - i know there is more or less constant flow but form time to time the flow is stopped in order to maintain the pipes and the area they deliver water to. It is not for long but long enough for me to need a battery. Initially i have to design the harvester/logger for this system, but over time it will be implemented on other pipes with other velocities and dimensions. I have already been looking at the propeller design, but the velocity is so slow (in the order of 0.5m/s) and the pitch of the propeller will be about 20 deg. I dont think is is very efficient to get lower then that - therefore the 50RPM. I know that i might need to implement a diffuser but i would rather not due to several factors.Regarding the DC motor jremington, then i know about the relationship you are describing but i would rather use a BLDC due to maintenance (brushes) and noise (spikes from the commutator) in the system. Regarding the energy needed i have calculated that it will be in the region of 2Wh/24h (on average 0,083W because the system is in sleep mode when not transmitting wirelessly)Correct me if i am wrong but i was planning to use the dc/dc converter to boost my voltage to the desired in order to charge. Stepping up the voltage will reduce the amps but that is how it has to be. I know that the available  power in a stream of water is P=0,5*density_of_water*A*v^3=1,3WCp is Betz limit = 59%, v= velocity of mediumIt can be shown that the maximum theoretical power one is able to extract is dictated by Betz limit - therefore it would be 0,8WBUT (not yelling)The thing is that the calculation is for flow in an open canal - here we have water in a duct. The harvested energy will be the pressure drop over the propeller multiplied with the volume flow [m^3/sek]. But since i don't know what the pressure drop will be and i cant measure it i am a bit lost.
 4 Using Arduino / Project Guidance / Alternator sizing for energy harvest (hydro)? on: September 23, 2014, 08:52:20 am Hey forum.This might be a silly question but here it goes.I am designing a small energy harvester/measuring unit. A propeller is placed in a stream of water and will act as a energy harvester.  It should be able to generate power enough from the flow to send data twice a minute through an XbeeThe propeller will be rotating at about 50RPM - this is obviously not changeable. Hardware:I will be using an arduino Fio - it seems to have a charging circuit and Xbee is easily mounted. I will add a 1000mAh battery to have some power in reserve, should the water stop flowing. Xbee pro to send dataThe general approach i thought about taking was to connect a rectifier circuit to the alternator to get a DC voltage. Then to a DC/DC converter that regulates the voltage to > 3.7V (compatible with Arduino Fio) and then directly into the Arduino Fio since it will take care of charging the battery. Question 1) How does that plan sound???  Question 2) HOW DO I CHOOSE THE ALTERNATOR????I am aware that i can use a BLDC motor, but how do i determine how large it has to be? It would be great if i could use a fan from a PC - knowing that the flow is almost constantly such that i will have 50RPM on my propeller i dont need to charge in bursts but rather a lower constant charge. But   Question 3) I obviously need some kind of gearing since the low RPM wont give me any voltage from the alternator. But how many RPM will i be aiming for - would i want to aim for half the no load speed of my motor/alternator as the speed/power characteristic plot of a motor indicates?   Any help is greatly appreciated.Bear
 5 Using Arduino / Motors, Mechanics, and Power / Re: Torque control aka current control of brushed DC on: November 08, 2013, 02:23:29 pm Hey @ MarkT. It (of cause) helped with the RC filter (nice), now i am down to about 1/2 the noise (0.014V) how do you think that will work? There is one funny (read terrible) but interesting response of the system. If i apply a steady low output from the arduino the spring will tighten but after a few seconds the motor begins to make a little audible noise. Thats not the problem, but once in a while it is as if the motor looses power for an instant - the duration of the "hole" is very short, only enough to make the motor rotate opposite the pulling direction for a few (15-20) degrees, then the motor reenergizes. My first guess was that it was the motor board is going into overcurrent protection (a feature it has to not burn out) due to the varying and very low resistance at certain rotor-positions, and the very low inductance of this kind of motor. I have looked at the PWM on a oscilloscope and it does not seem to change when the motor looses power. So i have a motor that seems to get a constant PWM voltage but occasionally looses power for (less than) a split second. ????!!!!!!????? Anyone?I am running PWM @ 32kHzThanks
 6 Using Arduino / Motors, Mechanics, and Power / Re: Stopping a Motor with a Current Sensor... on: November 07, 2013, 04:51:27 pm Yes that is possible, no problem at all.The only thing you have to keep in mind is that placing the sensor there you will sense both the motor and the drivers consumption. // Bear
 7 Using Arduino / Motors, Mechanics, and Power / Re: Stopping a Motor with a Current Sensor... on: November 07, 2013, 03:13:19 pm Well - its not me on the video, but i guess it is quite straight forward. First off you need something to measure Amps. This can be done by measuring the voltage drop over a resistor or (if you prefer to spend a bit of \$\$) you can find a hall-sensor solution on ebay if you search on hall sensor current.......or something like that. you can also find it on any electronics shop or website. If you need to run the motor in both directions you will need a Bi-directional sensor.Implementing it is very simple and the datasheet will tell you how to wire it up. Be sure to get one that takes 5V (or 3.3V)  as input if you are using an Arduino. In the datasheet you will find a rating like 40mv/V which means that for every amp flowing through the sensor, its output voltage will increase by 40mV. Very straight forward. The thing about motors is that they draw more current when you try to stop or slow down the rotation. This is because the BEMF (the voltage produced by a motor when it spins) becomes less as the speed is reduced. Conclusively you (as you wrote yourself) will be able to see if the motor has reached its stop if the Amps goes up. So your program will consist of a analogRead(ampSensor); compare that to the maximum allowable current and then either continue or stop. hope it helped. // BearYou can always post your code here if you want to have it checked. cheers
 8 Using Arduino / Motors, Mechanics, and Power / Re: Stopping a Motor with a Current Sensor... on: November 07, 2013, 11:53:02 am Like this?