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How to tell if a blower is tolerant of being restricted?


Jon Kerr

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Hi guys,

Ive just bought a new blower and its fantastic... way more powerful that my last one and I'm sure its going to solve many of my issues.

Question is- how can you tell if a blower is tolerant of being restricted/choked without any bleed off for excess air?

If I plumb it in without a T Piece, this blower is almost silent which is percfect, except its so powerful that the valve is only 20% open.

How do I know if I'm damaging the blower operating this way?

If I fit a T piece it still works great but roars like a jet engine!

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A centrifugal blower won't be at all, if you cover either the intake or outlet with your hand you can feel the pressure fall off as the flow reduces.

However the motor might depend on air flowing through the blower for cooling. If so I choke the blower intake, increasing air draw through the motor.  Don't let the increased "scream" of the blower and motor fool you, they're speeding up because they're doing less work moving less air. 

A friend had to prove it to me with a multi meter which showed a clear drop in amperage when I blocked either or both intake or outlet. Less air, less work, less amperage, less heat. It was a head slapping moment, after that I listened to motor noise differently. ;)

Frosty The Lucky.

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Thanks for the reply Frosty... unfortunately I think I'm being dim so not sure quite what you mean.

12 hours ago, Frosty said:

A centrifugal blower won't be at all

Wont be tolerant or being restricted? or won't be damaged?

 

12 hours ago, Frosty said:

However the motor might depend on air flowing through the blower for cooling. Less air, less work, less amperage, less heat.

I understand both these points but they would seem to contradict each other in terms of what to do- unless I'm misunderstanding?

 

Is overheating the only likely issue that could cause damage?

If I compare the temperature of the motor in restricted and air-bleeding setups I might be able to get an idea what effect its having?

 

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An alternate solution. 

Turn the blower on and let it run unrestricted. Feel how hot the motor gets every few minutes for the first fifteen minutes.  It should reach a stable temperature and not get hot enough to burn your hand .  

After letting the motor completely cool down,  repeat the test choked down to 20%. 

If you can still touch the motor after  fifteen minutes then  it's well within it's limitations.

 

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7 hours ago, Jon Kerr said:

Wont be tolerant or being restricted? or won't be damaged?

Boy I didn't say that very well did I? It won't be damaged by being restricted. The motor is a different component and you need to know what you have to know what if any precautions to take. Is that better?

 

DRATS, I did it again I was describing two different things in one statement. My bad. <sigh> Some motors are connected tightly to the blower housing with vents exposed to open air on one end only.  This configuration uses the blower to draw air through the motor and cool it. 

Motors with exposed vents in both ends are typically independent of the blower for cooling. There are other motors that cool themselves in ways I don't understand and not commenting on. 

The other part of that sentence was referring to the blower itself and the energy it uses to move air. It is counter intuitive but blocking a blower reduces how hard the motor needs to work because it's moving less or no air.  Less work less heat. Make more sense? 

Dickb is suggesting THE way to test motor temperature. A motor will get WAY hotter than you can touch before it starts to suffer damage. 

Frosty The Lucky.

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A lot depends on the type of motor you use. A universal motor (so called because it's designed to work on either AC or DC) can run off a variac (variable transformer), while an induction motor can't. 

Regardless of what kind of motor you have, a gate that dumps the excess air rather than simply restricting it is an option. Here's what I had on my old JABOD before I got the variac:

 

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