Motor kw to rpm? Am i mossing something?

[thesilentone]

It is worth looking at the operational efficiency of each motor.

In simple terms the 4kw could be operating at 80% efficiency, the 7.5kw at say 50%.

As has been touched on a above, life expectancy and more importantly Kwhr consumed during operation need to be compared.

However, to do this accurately you do need to know operational efficiency.

 

[Wisconsonian]

anybody know how to work out how to get maximum suck from the right size ducting regarding the fan size

I would want to look up a lot of factors to figure out a new system. I would start from scratch as you don't know if the original was working at it's most efficient, or just barely working. Everything is a balance of cost vs performance and which part will affect the performance more. A lot of this sort of thing is done by simple charts and ratings, but it's not that hard to do airflow/pressure calculations to see where a certain setup will fall on the curve, and therefore how it will perform if it's changed one way or another.

You don't need an engineer to design it, but you want a little more knowledge than "the catalog says this dust collector is good for 4-6 machines so it should work". A little more complicated than I could guess at here.