Application for 100W panel

[sdold]

If the panel open circuit voltage is at or below the motor maximum input voltage, there will be no problem. Bruce Roe

It wouldn't be a problem for the motor because as the voltage rises, current flows through the motor and the panel can't rise to open-circuit voltage. A solar regulator is different because there are times where the controller could open the charge circuit and let the panel rise close to Voc.

[solardreamer]

If the panel open circuit voltage is at or below the motor maximum input voltage, there will be no problem. Bruce Roe

I agree that's an easy way to check for compatibility. Another way to ensure compatibility is to add auto buck/boost converter the will allow you control the voltage and current to the DC fan. I actually use one to ensure compatibility but also to maintain stable fan speed and noise.

[bcroe]

It wouldn't be a problem for the motor because as the voltage rises, current flows through the motor and the panel can't rise to open-circuit voltage. A solar regulator is different because there are times where the controller could open the charge circuit and let the panel rise close to Voc.

If you use a 100W panel on a 10W motor, at times it is necessary. The big panel will keep the motor running more hours and
over more weather. Bruce Roe

[solardreamer]

It wouldn't be a problem for the motor because as the voltage rises, current flows through the motor and the panel can't rise to open-circuit voltage. A solar regulator is different because there are times where the controller could open the charge circuit and let the panel rise close to Voc.

The panel voltage may not be able to rise to Voc but it can still rise too high for the motor. I found out the hard way when I tried to use 50W panel Voc~20Vwith a 20W 12V fan that was supposed to handle up to 18V but because the motor does not draw enough current during particularly sunny days to pull down the panel voltage enough and burned out. Twice. I finally realized what was happening when I caught the panel voltage going above 19V under load and immediately disconnected the fan before it burned out again.

[sdold]

That's the reason for wiring the motors in series.

[bcroe]

I sometimes wonder what happens internally when DC motors are placed in series. Putting a couple small brush motors
in series for reduced speed has worked for me. But a brushless DC motor has electronics that draw big pulses, and those
are not going to line up time wise for 2 motors in series. Bruce Roe

[littleharbor]

A friend of mine ended up with a 100W panel like that and I told him to get rid of it, that it wasn't worth much at home. Well, against my advice he bought two automotive 12V radiator fans and connected them in series, I think. Lo and behold, when the sun is around mid day, the fans go like hell. He made an attic fan out of it. It was weird how the current draw worked out just about right. If it was me, I'd put in on Craigslist.

I've done exactly that with a pair of 12 volt radiator fans and a 65 watt 12 volt panel. This moves hot air out of my flat roofed garage in Baja. It has worked flawlessly for 2 1/2 years now. A single fan was screaming at a really high RPM but when I added the second fan it slowed down the fan motors to a reliable speed the more direct the sun's angle the faster they run so they kind of self regulate, depending on the time of day.

[LETitROLL]

I've done exactly that with a pair of 12 volt radiator fans and a 65 watt 12 volt panel. This moves hot air out of my flat roofed garage in Baja. It has worked flawlessly for 2 1/2 years now. A single fan was screaming at a really high RPM but when I added the second fan it slowed down the fan motors to a reliable speed the more direct the sun's angle the faster they run so they kind of self regulate, depending on the time of day.

Solar panel and fan(s) only?, i have always wondered about the "grey zone", when going direct, my guess is it could be not a one size fits all solution, You have times with good sun when everything works at some level depending on the match and balance of the components, then you have times with not enough light, so not enough power to run the fan, system appears off, but the grey zone exists in some situations where there is enough power to turn the fan very, very, very slowwwly, and doesn't that cause a problem with heat buildup in the motor windings?
Well built high quality motors would have more than enough headroom to handle this i would think since current would be low at that point, i just have always wanted to know if every motor/fan are able to handle that "stall" situation, that may go on for an hour, or if that shortens their life, etc.?

[solardreamer]

That's the reason for wiring the motors in series.

That's another way to ensure compatibility. The key point is one needs to make sure the fan motor max voltage is compatible with the panel max voltage one way or another.

[sdold]

The panel's max voltage is its open circuit voltage, but the voltage on the motor can never be that if the motor is connected and drawing current, it will be something less than that, and you don't know what it is until you try it. If you really want to be safe you could size the panels so that the Isc is about what the motor current draw at 13-14V is. Bruce, there is probably DC input filtering that is required for RFI suppression that takes care of that situation you described.

[solardreamer]

... you don't know what it is until you try it ...

This is the inconvenient part as it's dependent on the specific fan and weather condition to verify. I did this and it was tedious. So, Voc would be more convenient albeit conservative spec to verify even though the panel voltage under load will never actually get that high.

[bcroe]

Bruce, there is probably DC input filtering that is required for RFI suppression that takes care of that situation you described.

It might be a interesting experiment to see just how a couple seriesed brushless DC fans act. They might find a way to sync
themselves together so the pulses line up. These are pretty low audio frequencies, so an RFI filter won't help. Bruce Roe

[littleharbor]

I guess I just got lucky in my situation after listening to all this debate. As I said it has worked flawlessly for 2 1/2 years now. When The sun is bright and especially directly overhead it runs the fastest and moves the most hot air.

[sdold]

It might be a interesting experiment to see just how a couple seriesed brushless DC fans act. They might find a way to sync
themselves together so the pulses line up. These are pretty low audio frequencies, so an RFI filter won't help. Bruce Roe

Maybe, but if the panel Isc is less than what one motor draws in normal operation, I think you'd be OK even if such a thing occurred.

[SunEagle]

I guess I just got lucky in my situation after listening to all this debate. As I said it has worked flawlessly for 2 1/2 years now. When The sun is bright and especially directly overhead it runs the fastest and moves the most hot air.

I am glad you have a working system.

Moving the fastest when the sun it overhead make sense. The problem comes if the fan speed exceeds it's mechanical design. That is all I want to convey to anyone that try's this out.