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I have a 12 volt DC 9 amp water pump that I would like to power from a 110 watt 12 volt (17.2 volt at peak) 6.4 amp solar PV panel. I only want to run the pump when the sun is out, it's circulating water through water heating solar panels.

I'm not an electrical engineer so I'm hoping to find a easy and inexpensive way to connect these two in a way that won't burn out the pump.

I've read online about using a battery and charge controller, I'm not opposed to this idea but I really only want it to run when the sun is shining bright and heating the panels.

I've also read about using a "linear current booster", this seems like it would work, but the ones I found cost over $200.

I've tried connecting the solar panel direct to the pump, but that of course did not work.

So... does any one have any ideas? Photos of the labels of pump and panel below:

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    \$\begingroup\$ The pump draws more power than the panel produces. You would need two panels. \$\endgroup\$ – Mark Jun 18 '16 at 18:53
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    \$\begingroup\$ Since you only need to power the pump, and not trying to squeeze every single electron from the panel(s), a simple 12 volt switching regulator should protect the pump when the voltage is over 12 volts. If the regulator has under-voltage lockout, it would also not try to run the pump when the voltage was too low. \$\endgroup\$ – Mark Jun 18 '16 at 18:57
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As Mark states in a comment, the solar panel is not large enough. You might be able to buy a converter, that would convert 6.4 A at 17.2 V to 9 A at 12 V. However that is expensive and will only provide operation under ideal conditions. If you buy a second panel, the two working together should be able to power the pump even when the panel is not perfectly lined up with the sun on a perfectly clear day. However, if you do have perfect conditions sometimes, the panels would drive the motor above the rated speed, try to produce more flow then the pump is designed for and overload the motor. It would be better to buy a panel that has a Vmp rating that is only slightly above the motor's full-speed voltage rating and an Imp rating that is comfortably above the motor's full-load current rating. You could connect the motor directly and let the speed vary as the sun moves and the clearness of the day changes.

Two 17.2 Vmp panels with a 12 volt regulator would also be good

Stall Current

The motor's stall current is the current that it draws at zero speed regardless of the torque that it is loaded with. That current is determined by the applied voltage and the winding resistance. If the power supply limits the current, the torque that the motor can produce is limited by about the same percentage. With a centrifugal pump, very little torque is required to get it moving. The only resistance to rotation is the bearing friction in the pump plus the bearing and brush friction in the motor. The rotor of the pump is just stirring the water a little, not producing any pressure or flow. As speed increases, torque increases in proportion to the square of the speed as the pump builds up pressure. If the current is limited to 50%, the pump can accelerate until the pump requires about 50% of rated torque or about 0.5 X 0.5 = 0.25 or 25% of rated speed. If more current is available, the motor will draw more and accelerate faster, but it is totally unnecessary to make more current available than is required to run the pump at full speed. The friction torque at at standstill may be a little more than friction with the shaft turning, but no where near the total torque at full load.

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Even with 2 panels in parallel, you probably will not be able to run your pump. The problem is starting current, also known as stall current. This is typically much greater than the nominal operating current. Most likely, you simply will not provide sufficient current to get the motor running up to speed.

If you have a meter (and you really, really need a meter if you're going to mess around with this stuff) try the following. Measure the resistance of the motor with no shaft rotation. Divide 12 by this resistance to get the nominal start current. While it's true that, in normal operation, this peak current is only required for a very brief period, as the current will decrease with shaft speed, something on this order is needed for that brief period, and solar cells have no energy storage mechanism to provide that spike of current. Instead, the solar cells will drop into current limit, and the starting torque will be much less than normal. This will be particularly bad news for a water pump, where the mechanical load is always present on the motor, and will resist shaft acceleration.

While I doubt that you can do it, you'd be best off by providing a clutch between the motor and the pump mechanism. This would allow the motor to get up to speed with no load, then use the mechanical inertia of the motor armature to help get the pump going when the clutch is engaged.

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  • \$\begingroup\$ yeah, or use the panels to charge a battery and use the battery to provide the starting current. \$\endgroup\$ – Jasen Jun 19 '16 at 0:26
  • \$\begingroup\$ "the mechanical load is always present on the motor, and will resist shaft acceleration" Not true for a centrifugal pump which this is, judging from the picture. \$\endgroup\$ – Charles Cowie Jun 19 '16 at 13:58
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Solar panels are interesting in that there are not simply voltage sources. For example, in full sun your panel puts out 17 volts, but attach it directly to a 12 volt battery and the voltage it supplies will drop to 12 volts and the current it supplies will be based on the current that the panel is capabe of producing at 12 volts and possibly also affected by the 12 volt battery as well.

So it is likely that if you simply attached your motor to the panel it would run at or around the 12 volts needed, but your motor requires a current that is much higher than the panel can produce to overcome the stall factor and even to keep the motor going, the panel is just a little too small.

The easiest solution might be to get a 12 volt lead acid battery, hook the solar panel to it. Also hook the motor, or pump to the battery. The battery can supply the stall curent to the motor and keep things going when a cloud goes overhead. You would just need to keep the panel on the battery for an hour or two before or after you turn on the pump!

If you did it this way, and monitored the voltage of your bsttery, you could do it without a solar charge controller and save money. Just watch you battery for over and under charge by measuring voltage - the numbers you need are everywhere on the internet. If you don't want to monitor it yourself, a cheap solar charge controller for a lead acid battery for under a 10 amp load should b 15$ or less. Good luck.

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  • \$\begingroup\$ The source does not need to supply the normal full-voltage stall current. See the Stall Current revision to my answer. \$\endgroup\$ – Charles Cowie Jun 19 '16 at 14:02

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