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I have a water pump in a pond that is rated as using 300 watts. I'm not concerned if it shuts off at night. Overcast days are super rare. I don't plan on installing batteries, just want to connect the pump directly to a kit from the hardware store.

What size off-grid solar panel kit will I need to keep it running during?

Is that likely harmful to the pump if it is sometimes shut off or gets reduced amounts of power?

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    SMH everyone wants to do solar without batteries. They think "ooh, batteries are expensive, I can get rid of them by ??? magical thinking somehow". Commented May 11, 2022 at 20:21
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    The pump is not a critical appliance like A/C, if shut off for many hours at a time, the pond should be fine, its just pumping the waterfall for appearance, so why is the battery important?
    – Village
    Commented May 11, 2022 at 20:29
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    I would budget 450W of panels (50% overhead). you will also want a small battery to prevent short-cycling the pump on an off during borderline conditions. you will also need a modest inverter to turn the solar/battery DC into AC for the pump. Most pumps can handle a bit of sag/brownout just fine, but since it uses AC, your inverter will likely not allow partial AC voltage out; it's all or nothing, hence the battery. If you got a DC pump, you could hook that right up to the panels though a buck/boost SMPS and simply let it run when it can.
    – dandavis
    Commented May 11, 2022 at 21:39
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    I don¡t even think you'd need a converter with a DC motor. Yes, the problem is magic all right — but it's Tesla's magic, and the fact that all common products make design assumptions presuming the AC magic is externally supplied by the utility. Commented May 11, 2022 at 23:01
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    There are solar pumps (and solar pump controllers) designed to work exactly this way. You don't have one. By the time you buy enough extra parts to make the pump you happen to have work, you coulda shoulda woulda bought one. The controller manages the starting, and the energy storage is all in pumped water.
    – Ecnerwal
    Commented May 11, 2022 at 23:35

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Here's the problem. Virtually all products are built on the assumption that they will be run on utility mains AC power. That assumption permits economies in design:

  • There is no need to "commutate" the motor, because the AC magnetic field will do that for you. With single-pole AC you have the "starting from dead center" problem which requires tricks like shaded pole, but the point is AC power allows design of motors with 1 moving part and 0 sliding contacts (so, no commutator or slip rings). That is fantastic for reliability.
  • With limitless current available from the utility, there is no need to do anything to limit startup surge current. That is the "SHUB!" in the air conditioner's "SHUB! Whrrrrrr..."
  • Again with limitless current, there is no need to design the motor to "fail softly" or have good behavior in a power-limited environment.

Except solar panels don't have any of that stuff. They are not AC, and they don't have a limitless buffer of startup current (that would be a battery, which is the thing you do not want). And under less than ideal conditions, they can only deliver a fraction of intended power.

So using electronics, what can we do to "glue" this stuff together so it works?

Use DC motors and simple low-power cutout

In this scenario, you sacrifice the economies of design of an AC motor, and go with an old-school series-wound DC motor. This will have a commutator and brushes, which means a high wear point.

This motor will also have a "near short" condition demanding limitless current from the utility, but it will do a much better job "settling for what it can get". And solar panels, when driving into a dead short, a) are not damaged and b) produce their highest current. So it's not a bad fit.

As the motor spins up, its current draw will drop significantly and its desired voltage will rise. This is called "back EMF".

Still, the high current may not be enough to start the motor - and you don't want the motor sitting there stalled. So you need a basic protective circuit that cuts out the motor if the available voltage is too low or current stays too high for long. It would need to remember state, so it doesn't keep "crowbar-ing" the motor but still tries to restart it periodically.

A better circuit would do "buck mode" PWM, in which the bucking of voltage causes a boost in current (e.g. it can buck 12V/1A down to 4V/3A or 1V/12A -- which is ideal for starting motors, and makes the motor appear to have a soft-start. (what it actually has is the same "dead short" start, but with voltage adjusted down so as not to be wasted as heat). Once the motor spins up and back EMF increases, the buck would increase voltage/decrease current until a balance point was found. This motor would keep working (slower) even if solar power was reduced by clouds etc.

Use a brushless DC motor (electronic commutator)

This uses a new style of DC motor that must be driven like a stepper motor, with several field windings under the control of silicon power electronics. The DC motor has 1 moving part (the shaft with a magnet on it) and the electronics switches the magnetic fields in the stator to keep the magnet spinning.

A BLDC motor can do pretty much anything you want, since it's a stepper. So the motor is easy to soft-start, and it can adjust its speed/load to accommodate the power available to it from the solar.

A Variable Frequency Drive

This works the same as a brushless DC motor, except it's using a bog-standard off-the-shelf AC 3-phase motor as as replacement for the brushless DC motor. Instead of a magnet in the rotor, it uses induction.

The VFD can do the same variable speed and soft-start tricks as a brushless DC motor. It won't be easy to use a VFD with a stock single-phase motor, because it will have a shaded pole which is tuned to work at 60 Hz, not the much lower startup frequency the VFD would provide. It really works best with 3-phase motors.

Make it work with "made for utility" AC motors.

In this case, we bite the bullet and add a battery so that the solar system can borrow from the battery to provide the "bottomless" energy a cheap, common motor expects from the utility.

Now you have a common, off-the-shelf single-phase inverter which can borrow from the battery for the motor's startup surge. It can also power the pump in "deficit spending", borrowing from the battery during short times when the solar falls short. However doing so requires some sort of supervisory circuit to keep from draining the battery flat.

I know this is your last choice, but the advantage to this choice is it is readily available commercially off-the-shelf at sane cost. All the other options are "magical" items that exist only as industrial exotica, or that require a high level of integration on your part to make them work.

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    Can you give us the motor nameplate information such as voltage, current etc and possibly a link to the pump. This will have a big impact on the final workable solution.
    – Gil
    Commented May 12, 2022 at 5:45

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