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. Solar panels are interesting in that there are not simply voltage sources. 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. 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 will be particularly bad news for a water pump, where the mechanical load is always present on the motor, and will resist shaft acceleration. Instead, the solar cells will drop into current limit, and the starting torque will be much less than normal. 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. Divide 12 by this resistance to get the nominal start current.
Measure the resistance of the motor with no shaft rotation. 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. Most likely, you simply will not provide sufficient current to get the motor running up to speed.
This is typically much greater than the nominal operating current. The problem is starting current, also known as stall current. 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.Įven with 2 panels in parallel, you probably will not be able to run your pump. 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. 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. As speed increases, torque increases in proportion to the square of the speed as the pump builds up pressure. The rotor of the pump is just stirring the water a little, not producing any pressure or flow. The only resistance to rotation is the bearing friction in the pump plus the bearing and brush friction in the motor. With a centrifugal pump, very little torque is required to get it moving. If the power supply limits the current, the torque that the motor can produce is limited by about the same percentage. That current is determined by the applied voltage and the winding resistance. The motor's stall current is the current that it draws at zero speed regardless of the torque that it is loaded with. Two 17.2 Vmp panels with a 12 volt regulator would also be good You could connect the motor directly and let the speed vary as the sun moves and the clearness of the day changes. 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. 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. 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 that is expensive and will only provide operation under ideal conditions. You might be able to buy a converter, that would convert 6.4 A at 17.2 V to 9 A at 12 V. As Mark states in a comment, the solar panel is not large enough.
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