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I have a 12VDC high-head low-flow well pump. It's been running satisfactorily from a couple of 12V solar panels for about 6 months. For most of that time I had it connected to a small timer controller to enforce a 2hr on/30mins off duty cycle in accordance with the manufacturers recommendations. Due to laziness/disorganisation I never got round to weather-proofing the controller and when the rainy season started it get soaked and broke. Since then I've had the panels connected directly to the motor via a float switch.

The other day we noticed that our water tank wasn't filling correctly and it turns out that although the pump is still running when the sun comes out, it's very weak and can no longer lift the water high enough. On measuring the current flowing with a clamp meter, I'm only seeing 0.1-0.2A flowing despite the fact that the pump is rated at 8.0A max.

Things I've checked/tried:

  • The pipe isn't blocked - and it behaves the same with the pipe disconnected
  • Connected directly to a known-good 12V battery, it behaves the same
  • I disassembled the pump. There were no obvious blockages; the bearings seem fine; there's no visible evidence of the coils burning out; the brushes look fine although there was quite a lot of carbon dust from the brushes (sealed motor unit) which I've now cleaned off
  • I tried measuring the resistance of the various sections of the motor coils, there didn't seem to be any that were obviously higher/open circuit
  • No excess resistance in the power cord/ internal connections

Given that it worked for this application before and I've tried it with multiple power sources it seems safe to conclude that something has failed with the motor; and this wouldn't be entirely surprising given that I've been running it beyond the manufacturer's specifications for several months now. But before I give up and buy a new pump (and timer unit!) I was keen to try and pin down exactly what was wrong to avoid wasting money in case there was something else amiss, or, even better, something that I could credibly fix myself.

What else should I check? What other failure modes to these motors have?

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2 Answers 2

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Try new brushes before you buy a new motor.

You report a large amount of carbon dust so the brushes may have worn down to a point where they no longer make effective contact with the commutator. Typically they will be spring-loaded. As they wear down the springs extend and exert less force on the brushes, leading to a higher resistance contact. As you have a low voltage supply trying to push a large current through the motor you can't afford any significant voltage drop across the brush-commutator interface.

Your motor manufacturer may document a minimum length for the brushes in which case you can check that. Otherwise just try some new brushes. You don't seem to have any other obvious motor faults. Basically brushes are a wear item for a brushed motor and you should expect to have to replace them periodically.

I've successfully repaired a non-working motor by replacing the brushes. Other than being shorter than their replacements the old brushes looked in perfect condition - they just didn't work.

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  • \$\begingroup\$ That's interesting, I hadn't considered that possibility. The brushes look fine - there's still plenty of length and they're making good physical contact with the commutator, but I guess it can't hurt to try. I will also clean the commutator. I'm a bit sceptical because the 0.2A current flow would imply a resistance of ~60 ohms, which is a lot and not consistent with what my meter is showing across the leads of the motor (< 2 ohms). I've ordered a new pump now (we need a resolution soon) but I will try your suggestion before I unpack it and hopefully I won't need it and can send it back. \$\endgroup\$
    – hollandlef
    Dec 7, 2020 at 8:43
  • \$\begingroup\$ It turned out to be a combination of factors - additional resistance in the motor due to brush wear and reduced output from the solar panel. Cleaning the stator and hooking it up to a generator with a 12V output worked. \$\endgroup\$
    – hollandlef
    Dec 20, 2020 at 8:10
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For most of that time I had it connected to a small timer controller to enforce a 2hr on/30mins off duty cycle in accordance with the \$\color{red}{\text{manufacturers recommendations}}\$.

And...

Since then I've had the panels connected directly to the motor via a float switch.

(thus contravening the \$\color{red}{\text{manufacturers recommendations}}\$)

And...

Connected directly to a known-good 12V battery, it behaves the same

$$\boxed{\text{Your motor is broken}}$$

something that I could credibly fix myself

Are you able to rewind it? Are you able to open it and investigate without breaking the submersible seals? Do you feel lucky?

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  • \$\begingroup\$ The pump is actually quite well constructed and easy to open so yes, I've had it completely open. What's odd about it is that the motor still turns, just with a lot less current drawn and a lot less powerfully. Does this mean some but not all of the windings are damaged? I would have thought that if some of the windings had melted there would have been some visible damage/I would have noticed some increased resistance/open circuits around the commutator \$\endgroup\$
    – hollandlef
    Dec 5, 2020 at 19:01
  • \$\begingroup\$ I have no idea. It’s bust in some respect or other so, in my book my gut says replace it. \$\endgroup\$
    – Andy aka
    Dec 5, 2020 at 20:05

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