I'm not very familiar with 220V AC circuits and motors that run on them, so let me explain my question.
I'm building a soldering fume extractor for my workbench similar to what is shown in this video. It basically uses a simple ceiling exhaust fan along with some hose pipe to suck the fumes away from the desk to the window. He uses an exhaust fan which comes with a metal box enclosure and an outlet to attach the hose.
Unfortunately, where I live, I couldn't find the same type. So I settled for a simple exhaust and used the cardboard box it came in as the enclosure. I then connected 2 hose pipes which direct the air flow out to the window.
So this works all nice and dandy, but I noticed that when I close the box so that air only flows through the hoses, the fan speed slows down. I guess that's expected when the air flow reduces.
I'm sort of worried that this may put the fan motor under some additional stress and may cause it burn if I let it run for too long. Will this turn out to be some sort of safety hazard?
EDIT: I considered putting a fan regulator to the fan to control the speed, but I read online that adding a regulator to a single speed fan may lead to it getting burnt :(. So I guess that's not an option.
EDIT2: I also guess another option is to increase the dia of the hose or add more of them, but the ones I have are about 1 inch in dia and I'm not too keen on having more than 2 of them since it'll become unwieldy.
Best Answer
Fans draw a load in relation to the amount of work they do. It sounds as if you are closing the discharge duct of the fan and the fan is working to compress the air in the discharge duct.
Set your system up so the discharge duct is left open. Close the inlet duct while monitoring the Motor load (current). You should see the Motor load go down and the fan speed remain the same or go up a little.
Restricting or blocking the inlet results in the fan not having to do as much work. It moves less air and any air it does move can be freely discharged in the exhaust duct.