Thread: Bell Mouth Question

Originally Posted by Chris Parks

I would do this myself except I do not have a bell mouth to hand and if I did I would have to dissasemble my duct system to do it. If a duct has a bell mouth added is the extra load enough to show a significant difference in current draw measured either at a VFD or using a clamp meter.

It depends on the flow or more specifically the pressure at the collection point. With a smaller DC at the end of a long run of ducting ie lower/pressure flow it might not be possible to pick up any difference at the 0.1A level but on my system I can see a clear 0.2A difference @50Hz. At some point I will get out my 0.01A clamp meter and get a more precise reading.

Thanks Bob

I also just tried @60Hz and the current increases by about 0.4A

So it appears that the fan performance has not fallen over at 60hz which makes sense as these are primarily designed for 60hz. It makes the case for 3 phase more compelling.

Originally Posted by Chris Parks

So it appears that the fan performance has not fallen over at 60hz which makes sense as these are primarily designed for 60hz. It makes the case for 3 phase more compelling.

Just to be clear, my impeller is not a Clearvue, Its just a generic 13" impeller originally fitted with a 3HP motor but I fitted a 4HP 3P so I could run it at 60Hz.

BP reckons he can turn a 15" impeller with a fraction over 3HP @60hz. I don't know at what inlet size but knowing his liking for 6" most probably that. I suppose impeller design must also affect the load as well so it is not apples for apples. If I get a chance in the next few days I might have a play and see what the 1800 does up to 70hz.