How Long is It Safe to Let a Shop Vac Run Continuously
- #7
My gift for oversimplification is about to go on display again...
The draw in power translates almost directly into heat generated with the strain on the motor compounding that. If you can't disappate the heat quickly enough, it will burn up after a while. With all due respect to Dan, I would say if it will run continously for over an hour then it probably won't get any hotter if you run it for one more or ten more. I asked a similar question about using my router speed control on a big vac some time back and one of the folks who responded was running one for days on end at a slow speed. I am not recommending that. 
- #10
OK, a number of issues going on in this thread.
First, dust collection from small machines- for good dust collection with ANY machine you need to both move a lot of air (CFM) and have suction (static pressure). If the machine has a small port (<2.5") you are limited to mainly static pressure suction like that provided by a shopvac. A small port (low cross-section) can not pass much CFM and also has a lot of static pressure resistance. So, unless you open up the port to 4" - 6" you are pretty much stuck using a shopvac. It will have good static pressure suction but not be very good collecting much of the escaping dust- think of it this way- a shopvac can pick up a bowling ball, but hold the nozzle a few inches away from a pile of dust- it doesn't pick up anything. To do that you need to move a lot of air with a DC.
If you have a DC connected to a small port there is little reason to open another blast gate to provide more air- the motor certainly doesn't need it and it will have a negative impact on dust collection. The only reason is to improve separation in a true cyclone.
Reducing a 4" or 6" pipe from a DC to something small like 1.25" or 2.5" port severely restricts CFM and gains almost nothing in static pressure suction- a waste of time.
Dust collection through pipe in sized from 2" to about 4" is kind of a gray area to make generalizations- but, depending on the equipment, is best handled by a good shopvac.
Another point - centrifugal fans and blowers (DC, shopvac, etc.) will run faster (possible premature bearing failure) and make more noise when the intake or exhaust are severely restricted or blocked. However, contrary to what is often assumed, it does not make the motor work harder and does not cause it to draw more current- both are less.
However, opening up the intake and exhaust with no restrictions can cause the motor to move a maximum amount of air and cause it to work harder- and draw max current. A DC should never be run with no restriction or pipe on the intake side and/or resistance on the exhaust. The motor will try to work too hard, overheat, draw too much current, and likely cause the breaker to pop in just a few seconds.
- #11
" If you have a DC connected to a small port there is little reason to open another blast gate to provide more air- the motor certainly doesn't need it and it will have a negative impact on dust collection. The only reason is to improve separation in a true cyclone."
I do not know the physics or aerodynamics involved but I can say that when I connect a smaller diameter hose via a reducer to my 4" DC system the smaller hose has limited suction until I open another 4" gate in addition to the gate the smaller hose is connected to. :wsmile:
Source: https://www.ncwoodworker.net/forums/index.php?threads%2Fshop-vac-question.33881%2F