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Old 07-10-2010, 04:05 AM   #1
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I need help with a simple solar powered pump circuit design


I have got a nice little project on the go to water my vegetable garden or "orto" as it is called here in Lombardy, Italy. I am going to pump water 10 meters from the stream to a series of interconnecting water butts that have brims 2 meters above the level of the stream. I will put a float microswitch on the brim of the main barrel. I have bought an adequately powerful Oase Nautilus 140 12 volt pump that consumes 17watts. This is going to be powered by 2 Maplins solar panels that produce 24 watts in total at 12 volts.
The circuit I think is pretty simple i.e. wire the panels up in parrallel, direct feed to solar panels with interruption by a floating microswitch to prevent barrel overflow. From my schoolboy knowledge of physics I think I need to wire in parallel with the pump a capacitor, what I would like to know is what type and value of capacitor would be correct.
I look forward to hearing from someone somewhere who will straight away know the answer ;-) ... Thanks John

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Old 07-10-2010, 04:06 AM   #2
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I need help with a simple solar powered pump circuit design


3 threads on same topic merged
Please only start one thread on an issue
Thanks
Whoops, sorry I pressed the upload button accidentally
& uploaded the question three times


Last edited by John in Milan; 07-11-2010 at 09:38 AM. Reason: User Error
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Old 07-10-2010, 11:48 AM   #3
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I need help with a simple solar powered pump circuit design


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I need to wire in parallel with the pump a capacitor, what I would like to know is what type and value of capacitor would be correct
A cap to store energy for when the sun isn't shining? It'd have to be huge.
A 1 farad supercap could run your pump for about 2 seconds.

Last edited by Yoyizit; 07-11-2010 at 09:11 AM.
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Old 07-11-2010, 07:34 AM   #4
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I need help with a simple solar powered pump circuit design


Typically one solar panel isn't enough as they only produce VERY small amounts of DC voltage. in a typical residential installation we do here, solar panels are wired in series in roughly 2 - 3 arrays of 14 or 15 panels to achieve about 380 to 450vdc. The arrays are then brought into a solar inverter like this one http://www.solren.com/pvi3000.html . this converts the solar energy into usable energy for you to use which will typically provide you with about 5.5kw of power, with this array setup. you also need to be very careful you don't mix solar panels with other brands, as they can be negatively or positively grounded depending on the manufacturer. Also panels to need to be properly grounded because panels that are not WILL catch fire if a problem occurs and a shot circuit can't be detected.

You might be surprised solar panels still put out voltage even if the sun isn't out, roughly 20% and about 5% at night. UV rays still come though the clouds and are still present at night :P

Last edited by The Deez; 07-11-2010 at 07:38 AM.
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Old 07-12-2010, 01:18 PM   #5
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I need help with a simple solar powered pump circuit design


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Originally Posted by John in Milan View Post
I have got a nice little project on the go to water my vegetable garden or "orto" as it is called here in Lombardy, Italy. I am going to pump water 10 meters from the stream to a series of interconnecting water butts that have brims 2 meters above the level of the stream. I will put a float microswitch on the brim of the main barrel. I have bought an adequately powerful Oase Nautilus 140 12 volt pump that consumes 17watts. This is going to be powered by 2 Maplins solar panels that produce 24 watts in total at 12 volts.
The circuit I think is pretty simple i.e. wire the panels up in parrallel, direct feed to solar panels with interruption by a floating microswitch to prevent barrel overflow. From my schoolboy knowledge of physics I think I need to wire in parallel with the pump a capacitor, what I would like to know is what type and value of capacitor would be correct.
I look forward to hearing from someone somewhere who will straight away know the answer ;-) ... Thanks John
Your system will work. You do not need a capacitor. The pump will only run when the sun is shining, and will only pump as much water as the available sunlight will allow. However, this is the easiest and simplest way to wire a solar pump and is a common way to do it for solar well systems. If you wanted the pump to run at night or on cloudy days, you would need to use a battery (not a capacitor, which cannot store enough energy). You would also need to use a "solar charge controller" between the solar panels and the battery to prevent damage to the battery. This is not too hard to do, but is probably unnecessary. As long as your pump can supply enough water while the sun is shining and your water storage holds enough water for the night, you do not need a battery.

If you are not getting enough water on cloudy days, you can add more solar panels in parallel to increase the available current when the sun is not bright.
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Old 07-13-2010, 04:22 AM   #6
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I need help with a simple solar powered pump circuit design


Thanks for your definitive answer saying that I don't need a capacitor, I sort of thought that, but wanted to know that the pump would not be damaged by running a low power levels. The solar pump is working brilliantly and all of my water butts are full of water. Thanks again.
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Old 07-13-2010, 10:01 AM   #7
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Thanks for your definitive answer saying that I don't need a capacitor, I sort of thought that, but wanted to know that the pump would not be damaged by running a low power levels. The solar pump is working brilliantly and all of my water butts are full of water. Thanks again.
Low voltage to motors that are under load may damage them but perhaps not at these power levels.
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Old 07-14-2010, 03:14 AM   #8
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I need help with a simple solar powered pump circuit design


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Low voltage to motors that are under load may damage them but perhaps not at these power levels.
Now now, you know enough to think this through and see why it's not possible here.

First, this really only applies to AC induction motors. Induction motors try to act as constant-power loads. Reducing voltage without changing the mechanical load will result in increased current draw to maintain constant mechanical power. Losses in the motor scale with the square of current, so efficiency rapidly drops and current increases further to maintain constant output. The increase in current is proportionally larger than the decrease in voltage, so total power increases (a natural consequence of maintaining constant output at lower efficiency). The motor overheats due to the increased I^2*R losses.

DC motors aren't subject to this because their speed is proportional to voltage. Decreasing voltage decreases speed, which decreases mechanical loading, thus avoiding this runaway condition.

Even more importantly, in order for the motor to overheat it would have to have MORE POWER available. This is not the condition that results when solar panels are only partially producing. Available current decreases along with voltage. If the motor can handle operating in full sun, then it can surely handle anything less than full sun.
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Old 07-14-2010, 09:09 AM   #9
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I need help with a simple solar powered pump circuit design


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Now now, you know enough to think this through and see why it's not possible here.
Any motor under rated load with rated voltage generates a back EMF that governs current flow. Given no change in load, a decrease in voltage = increase in current.
No?

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Old 07-14-2010, 02:58 PM   #10
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Any motor under rated load with rated voltage generates a back EMF that governs current flow. Given no change in load, a decrease in voltage = increase in current.
No?
Yes, but no. If it were possible to maintain a constant mechanical load, then this would be true. This assumption relies on the operating characteristics of AC induction motors though, and doesn't hold for DC motors. AC induction motors operate at a speed determined by frequency (and modified slightly by slip), not voltage. Therefore, decreasing voltage to an induction motor does not reduce the rotational speed (again, with the exception of increased slip). The mechanical output (product of torque and speed) remains the same because the speed doesn't change. Back-EMF drops, and current rises. Effectively, induction motors have a negative resistance and function as nearly constant-power loads.

Now consider a DC motor. Speed is determined by a combination of torque and applied voltage (see the KV constant for the motor). As voltage is decreased, speed decreases proportionally, unlike for an induction motor. With any type of "dumb" load like a fan or pump, the torque also decreases (often as the square of speed). Therefore, mechanical load scales at least linearly with voltage, and usually as the second or third power of voltage. Thus, "given no change in load" is an inherently invalid assumption for a DC motor with varying voltage. DC motors present a normal positive resistance, though sometimes a nonlinear one.

DC motor speed controls work by decreasing voltage to the motor (usually PWM though). That's how you operate DC motors. As a dependent variable, current usually scales either linearly or as the square of voltage, depending on the mechanical characteristics of the motor's load.

This is easily verifiable by experiment. Connect a 12VDC fan to a 5V supply, and measure the current. Then try 12V.

Of course, in the OP's application it's not even possible for a load to draw increased current when the supply voltage decreases. The supply voltage decreases when there's not enough sun, and available current decreases then, too.
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Old 07-14-2010, 07:28 PM   #11
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I need help with a simple solar powered pump circuit design


Been out of school too long. . .

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