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I have a 24 V 100 W solar panel and a 24 V DC 50 W pump, which has a resistance of 2 Ω while at 0 rpm. The solar panel is directly connected to the pump motor.

The problem is: While the solar panel can drive the motor when it is running, it mostly fails to provide the break-away current the pump requires, so its motor more often than not fails to start turning.

Therefore I thought of a starter circuit, which stores charge in large caps and and fires (drives the pump) as soon as the voltage over the caps has reached a certain limit. Then the caps would provide more current and help starting the motor. When the sun is blocked by clouds etc., the pump stops and the circuit should "reset", so that the caps can be charged again.

I thought of triggering a thyristor at a certain voltage, but I fear it would only reset back to non-conduction at night, since the solar panel will provide some current even in low-sun conditions, but not enough to keep the pump running.

So, I think I need kind of a custom starter circuit that "fires" when a certain voltage has accumulated across some caps, and stops conducting when the current falls under a certain limit afterwards, so that the caps can be charged again.

Starting voltage and stopping current limits would need to be adjustable, because I have no means to really measure the minimum voltage/current behavior of the pump.

How do I get a solar powered DC pump motor to start reliably?

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    \$\begingroup\$ Are you connecting the panel directly to the pump? \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 20, 2018 at 15:22
  • \$\begingroup\$ Ask a specific question, you'll get better results \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 20, 2018 at 15:32
  • \$\begingroup\$ What guaranteed current does it need to begin turning? What is the minimum votage required to get it turning? What is the full-speed load current (worst case)? \$\endgroup\$
    – Andy aka
    Commented Jun 20, 2018 at 16:57
  • \$\begingroup\$ @Andyaka I do not know for sure, but it should be 24 V/2 Ω = 12 A. Full speed load current is somewhere around 2 A, as it is a 50 W motor. \$\endgroup\$
    – Jinxed
    Commented Jun 20, 2018 at 20:22
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    \$\begingroup\$ @laptop2d: What you have written was already in my question, but obviously not clear enough. I hope my question is more understandable after my edit now. \$\endgroup\$
    – Jinxed
    Commented Jun 20, 2018 at 20:27

2 Answers 2

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The problem is your solar panel can't source that much power with that much load, look at this curve:

enter image description here
Source: https://arstechnica.com/civis/viewtopic.php?f=26&t=1402629

If you draw more current from the cell you get less power. People have fixed this problem long ago, you need a MPPT tracker to regulate the voltage to 24V.

The second problem is power, the motor won't start unless you have enough starting current. Make sure you size your solar system (and MPPT tracker) to have enough power for the starting current. If you need 6A of starting current, you'll need 144W of power. If it's 12A you'll need more.

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  • \$\begingroup\$ I know that, which is why I asked for a starter circuit, not an explanation of how solar panel power output works. And to counter your argument: I already pointed out in my question, that at times the motor does start (full sun etc.pp.). \$\endgroup\$
    – Jinxed
    Commented Jun 20, 2018 at 21:13
  • \$\begingroup\$ I was just trying to answer your question: How do I get a solar powered DC pump motor to start reliably? The answer, you get a MPPT tracker, you cannot get your motor to start without an MPPT tracker, solar modules do not have enough source impedance and they never will. \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 22, 2018 at 17:14
  • \$\begingroup\$ My panel does sometimes start the motor, so "never will" does not apply. I just need a starter circuit, which makes use of caps, not a MPPT, since I do not need the panel to run at the MPP and do not want to use regular batteries. I just need a starter circuit in the aforementioned parameters; voltage and current limits I can adjust on my own, but I have some difficulty with designing a suitable circuit. \$\endgroup\$
    – Jinxed
    Commented Jun 25, 2018 at 19:51
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Find out how much time the motor takes to start. I'm guessing that it is significantly-less than a second but you need to find out.

Find out what the minimum voltage that the motor will start at. It will be less than the normal running voltage - perhaps significantly so.

Use the numbers above to calculate an estimate of the minimum amount of capacitance that you need to place in parallel with your solar panel. Remember that a current of 1 Amp will discharge a 1 Farad capacitor by 1 Volt in 1 second.

When you come up with the minimum amount of capacitance required, double that value (safety margin) and try it.

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  • \$\begingroup\$ And how do I trigger the caps then? I am lacking a proper circuit to turn the load on, when the caps are charged (voltage-triggered) and recharge the caps, i.e turn off the load (low-current-triggered). Any ideas? \$\endgroup\$
    – Jinxed
    Commented Jun 24, 2018 at 16:48

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