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Was discussing Dust Collection in the shop with a customer and he asked me something I had never really considered.  Wanted to know why DC motors are direct drive and not a belt and pulley system.  Went on about less effort by the motor to turn a smaller pulley and more power from the belt driving a larger pulley.

 

Not really something I have ever seen or researched but anyone have a reason why?:ChinScratch:

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With a smaller pulley driving a larger pulley the impeller would turn slower.  The belt would be a wear item with maintenance to keep it tight.  Furnaces are have direct drive blowers now where they didn't before.  Just seems to make good sense to me.

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Motor/fan drive selection is based on the desired ratio of air flow and pressure for the specific application, including noise.  A fan intended to deliver air to an occupied/comfort duty usually stays under 1200 rpm, usually develops about 1--3" pressure.  DC wants 5--8" pressure, and the way you get pressure is fan rpm (my little Griz runs 3450 rpm); noise need not apply.  Also, the shape of the fan blade has a lot to do with the air flow/pressure mix, and DC fans tend toward simple flat (or slightly curved) surfaces so that they won't be harmed by hunks ("material handling blade configuration").  Then, comfort apps want more precision to fan rpm to hit the mark (although the mark is often poorly understood, and then measured to death), whereas DC is "nothing exceeds like excess".  So, most apps want rpms lower than the motor (which usually is 1800/3600 nominal), and for some time (centuries almost) a pulley system that allows ratios of motor speed has been the easy solution (btw, modern belt drives "eat" about 2--3% of the fan energy).  With the advent of inexpensive motor speed controls, more and more apps use electronic speed controllers to match fan/motor to app.

 

Met my first fan, up close and personal, in 1972.  Paid for 4 kids, 2 wives, 3 dogs.

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more expense in bearings and pulleys and belts.  I remember when Ford was  looking at the question of a SST exhaust  in the late 1970s.  they rejected it because to go with SST would be ab out  $120 more at retail.

 

About DC

No insurance carrier  will  pay on a policy for a house fire if they can claim that the fire  originated or was made worse by a DC system  if that system did not have metallic and grounded ducts.

And  you know they have a motive to make that claim.

the problem is that their data all comes from heavy industry.  There is no way for us to change their minds. 

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13 minutes ago, Cliff said:

No insurance carrier  will  pay on a policy for a house fire if

You know I see this kind of comment from time to time, and yet after (uh 18 minus 72, add....um) 46 years in the biz, the last decade spent in doing forensic (lawsuit) data, I've never seen a problem with a home app.  Large commercial systems have some code requirements for grounding, and UL, but the loss history is pretty faint.  I'd worry more about ignition of flour dust in your blender (try it, particularly if you don't like your eyebrows).

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10 minutes ago, PeteM said:

I'd worry more about ignition of flour dust in your blender (try it, particularly if you don't like your eyebrows).

 I did not say that  it'd start a fire,

I did not touch on the physics of dust & O2 concentrations  sufficient to create a hazard. 

I spoke to one thing  only; that an insurance carrier will be disinclined to cover a loss if they can make the claim that it started  in the Non Metallic DC.  They don't need to prove it.

 And once they make such a statement a lawsuit may be the only way to dissuade them.  And at such a litigation the fees for the testifying experts that will be required  by  the rules of Court would likely bankrupt most  regular folk.

 

 

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