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Jacob89
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Posted: 13 September 2017 at 10:43am | IP Logged Quote Jacob89

Hi all, I'm a long time lurker, first time poster, lots of great information to be found here.
My uncle and I have been experimenting with using a ~2hp DC treadmill motor to power an old piston pump directly from solar.
The pump was originally powered with a 3hp 3 phase motor, so the DC motor we have is a little undersized, however we figure if we gear it down we can the pump slower from the smaller motor and still pump a reasonable amount of water.
We did a little test, running the pump from the DC motor through a single belt, and powering it directly from 10 250w panels. We let it run for about an hour in the middle of the day and it performed quite well.
My question is whether it is necessary or worthwhile to add some kind of MPPT controller or linear current booster, for times of low sun, early/late in the day, and during cloudy weather. My uncle prefers to keep things as simple as possible, and I think he would happily accept reduced output in exchange for the simplicity of having no form of controller.
But I'm also concerned about the possibility of damaging the motor by running it direct. At times of low sun, the motor won't turn, but there will be an amount of current flowing through it. I wonder if this is likely to damage the windings or the brushes over a long period of time?


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Warpspeed
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Posted: 13 September 2017 at 11:06pm | IP Logged Quote Warpspeed

Yes definitely, and welcome to the Forum.

The problem with a dc motor is that it can very easily pull the solar panel voltage way below the maximum power point, effectively reducing the power available from the solar panel.

That becomes particularly a problem with any kind of piston pump, because it always has to start up with maximum mechanical load applied. It will just stall with hydraulic lock unless the available starting current is very high.

What you need is to arrange your panels so the maximum power voltage (mppt) is a bit higher than the required motor voltage with everything running flat out on a mid summers day.

Use a buck converter to control the current going to the motor.
Control the duty cycle of the buck converter from the voltage across the solar panels.

As the solar panel voltage rises above the mppt voltage, increase the duty cycle.
If the solar panel voltage tries to fall below the mppt voltage, reduce the duty cycle.

If its working correctly, there should be a constant voltage at the solar panels from dawn to dusk from clear blue sky to total cloud cover. And your motor will then do the best it possibly can from whatever current it gets from the buck converter.

Realize too, that a buck converter can magnify the current. So at very low duty cycles the motor current can be several times what the solar panel is producing.

The motor may then turn very slowly, but at least it should be able to start up and run with minimal available power.

The more sun, the faster it will go...

With a piston pump, consider fitting a VERY heavy flywheel to the motor. It will help a lot.


Edited by Warpspeed on 13 September 2017 at 11:16pm


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Jacob89
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Posted: 15 September 2017 at 1:37am | IP Logged Quote Jacob89

Thanks Warpspeed.
Is a buck converter in that configuration doing a similar(or the same) thing as a linear current booster?
I think something like this would be good for this application, also because the panels will be kept about 100m away from the pump in order to get good enough sun, so it would be best to keep the
panel voltage as high as possible to reduce line losses.

I'm guessing I'll have to have a go at building something myself for this, there doesn't seem to be much available for buck converters in the kind of current and voltage range I need. Its something I'd like to have a go with, I'm handy enough with a soldering iron, but my electronics knowledge is lacking, so it would be a good learning experience.

Luckily, being a treadmill motor, it has quite a heavy flywheel on it all ready.
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Warpspeed
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Posted: 15 September 2017 at 2:00am | IP Logged Quote Warpspeed

I am not familiar with the term "linear current booster" but a buck converter will increase the output current in the same ratio that it reduces the output voltage.

Higher voltage is definitely going to be more efficient. Most treadmill motors I have seen are rated for 180v, but there are some exceptions. There should be a rating sticker on the motor that will say.

A typical 24v solar panel might have a maximum power voltage in the region of 30v, so six or seven connected in series might be about right.

Something will need to be built especially for this, but it need not be too complicated.



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