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Which of the following mathematical relationships is true of a motor's horsepower if the CFM of the blower were changed?

a. HP changes directly proportional to changes in CFM
b. HP changes inversely proportional to changes in CFM
c. HP changes by the square of the change in CFM
d. HP changes by the cube of the change in CFM

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B. inversely proportional

3. a guess

a.

4. D??

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C

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A ??

Yuma,

Tryed to use a life line but forgot where I left it.

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I really expected more participation on this question. The question may seem like useless theory but it is actually a highly practical question with very important implications to every HVAC technician.

If you work with blowers or with Variable Freq Drives you really need to know this!

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This the kind of expectation that I hate about our trade. TECHNICIANS ARE NOT ENGINEERS AND DO NOT MAKE AN ENGINEERS SALARY! STOP EXPECTING US TO PRODUCE AT THAT LEVEL!

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Originally posted by Steve Wiggins
This the kind of expectation that I hate about our trade. TECHNICIANS ARE NOT ENGINEERS AND DO NOT MAKE AN ENGINEERS SALARY! STOP EXPECTING US TO PRODUCE AT THAT LEVEL!

Steve, you are totally missing the point. So here goes!

This is something you really need to know anytime you make a pulley change or adjustment on a variable pitch pulley or work with a VFD.

An increase in the CFM of a blower causes the horsepower of the drive motor to increase by the cube of the increase in CFM. That means that even a small increase in CFM makes a significant increase in both HP and amperage. Amperage also increases by the cube of the increase in CFM.

That also means that when you have the opportunity to decrease CFM you are decreasing the motor HP and amperage by the cube of the decrease in CFM. That is why VFDs save so much money! A small decrease in CFM greatly decreases the HP which is the same as decreasing the power (watts) used.

This is a highly practical thing to know and is therefore tested for on technician certification exams. This is not just for engineers. Every technician should be know this.

Norm

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Yeah you're right, I stand corrected.

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Originally posted by Steve Wiggins
Yeah you're right, I stand corrected.

That's ok Steve, that's why we are here. To learn from each other. Lots of guys immediately reject anything approaching theory without stopping to realize that theory has practical implications. I am a highly practical guy but I study theory and then look for how the theory applies to the real world. Sometimes I don't see any application right away and then later when trying to figure out a problem I face on a job it is the theory I learned that allows me to figure out the problem and what is going on.

Norm

12. Norm you are the MAN

Steve you not only stand corrected

YOU are a standup guy for saying so!

A directly proportional

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The correct answer is "d". Read my response to Steve for the explaination as well as the practical implications of this question.

Steve has the right attitude. Those who allow their ego to control them will never be able to learn from their mistakes. That just has to be the first rule of learning.

When we are wrong we need to quickly admit it. Not spend any time concentrating on it but immediately move on to learning what is correct and concentrate on that.

When I am wrong and others want to keep reminding me that I was wrong they are wasting their time because I am already busy concentrating my efforts on what is right and how I can add the new information to my arsenal of knowledge.

Norm

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