Solar powered attic fan without battery

[bcroe]

If the open circuit voltage of your panels will not damage the motor, just put more panels in parallel
(or larger panels at the same voltage), and your fan will run at max speed for a larger part of the day.
Tilting 2 panels east and west would probably give longest peak power.

Another approach is your idea of using a much larger panel, with higher Voc, and a buck regulator. I
expect with first sun, the buck (with inadequate input) will become a short through to the fan,
remaining that way until enough power is generated to reach the fan speed/voltage you have set.
Then the buck will start switching down the panel V to the V which you set for the fan, making
available more current which the fan will not use. But as Mike says, excessive V will wear or
damage the fan. Bruce Roe

I should add, if the larger panel has the same Isc as the original smaller, the fan will start through
the buck converter at the same sun as before. But the speed will rise to max setting much more
quickly. Multiple panels, keep them in parallel to keep the
Isc up. To control top speed, use an adjustable voltage buck. Bruce Roe

[jflorey2]

I can use a buck converter to bring the voltage down

These often don't work for solar systems due to their negative input impedance characteristic. They tend to "stall" the panel at the converter's lowest turn-on voltage. You need a lot of power to overcome this, which means a lot of panel. You can ameliorate this by setting a high UVLO (around 14 volts) so you don't get stuck at a low voltage. But not all DC/DC converters have adjustable UVLO's.

it looks like fan speed is controlled by current in the systems they sell.

What sets the ultimate limit of motor speed (for most motors) is voltage. If the panel isn't beefy enough to get to Vmp, then it limits current, so you will see lower currents.

However, per what bcroe said, if the fan can handle 18-20 volts (the Voc of 12V panels) then just put more in parallel until you have your desired speed.

[bcroe]

If the open circuit voltage of your panels will not damage the motor, just put more panels in parallel
(or larger panels at the same voltage), and your fan will run at max speed for a larger part of the day.
Tilting 2 panels east and west would probably give longest peak power.

Another approach is your idea of using a much larger panel, with higher Voc, and a buck regulator. I
expect with first sun, the buck (with inadequate input) will become a short through to the fan,
remaining that way until enough power is generated to reach the fan speed/voltage you have set.
Then the buck will start switching down the panel V to the V which you set for the fan, making
available more current which the fan will not use. But as Mike says, excessive V will wear or
damage the fan. Bruce Roe

[Mike90250]

Here's an idea. Wire a automotive light bulb socket in series with the panel and fan. Obtain a selection of bulbs that fit the socket and use the bulb wattage to control the speed. Perhaps a headlamp @ 55w would be right.

Or maybe the first thing to try is another 10w panel in series with the existing panel, and you will know you won't exceed the fan specs.

Remember - the fan only has so many revolutions in it before the bearings wear out. Higher speed = sooner wear out.

Some folks have had luck with about a 50w panel and a small automotive radiator fan. Those are made to work in high heat under hood applications.

[noequus]

My first post on this forum...hope its in the right place, and clear. All thoughts appreciated.

I have an attic fan that is powered by a 10W panel; it pulls out a lot of the hot attic air. However, its not quite powerful enough for this house and I'd like to increase its RPM to get better air exchange (but still not suck cool air from the house into the attic).

The vendor sells a variety of panel sizes for this fan and motor, up to 60W; with the more powerful panels for use in larger homes. All panel sizes are 12V (nominally, ~18V Voc), and the 60W panel has an Ioc of 3.6A. The vendor said it was best not to exceed that for the fan motor; I don't have full specs on the motor but the vendor told me its okay for 1-38V. Its labeled "1.7A, 38.2V".

A friend recently gave me a large panel (320W; Voc=44V, Ioc=9A), and it would be nice to use it to power the attic fan. So...I would like to adjust the power sent to the fan so it is:
1) Within spec of the fan
2) Provides the needed ventilation, but not too much
3) Ideally, easy to adjust the fan RPM (this will help me find the right ventilation level).

The fan motor has no problem starting with the 10W panel, and it only needs to run during the day; no battery needed or wanted.

I can use a buck converter to bring the voltage down, but its not clear to me how to also control the current...or, actually, its not clear what combination of V and I to use. Since the panels the vendor sells only vary in the current they can provide (all same V, but different wattages) it looks like fan speed is controlled by current in the systems they sell. So on my system (conceptually) the V could be set to match their panels (ie, nominally 12V), and then current could be varied and (conceptually) I could match the current to one of the panels they sell that is sized better for my house.

But its not clear to me if that is possible, or practical--or even if that's what I would really want to do. In the end, I just want to control the fan speed. So: perhaps you-all have some thoughts on the right way (or, rather, a good but safe way) to do that. Thanks in advance!