Brake Horsepower 3

[rockman7892]

I have been doing a lot of research lately regarding a 1500hp 4.16kV motor used to drive a fan. The airflow through this fan is controlled by dampers.

Recently I have come across the fan curves for this particular fan. On the second attached fan curve I see that on the curve relating to the dampers 100% open, it shows a Brake Horsepower (BHP) of 1050hp.

My understanding of BHP is that this is the total hp required by the load which in this case is the fan. This does therefore not include an losses through a gearbox or any other losses in the system (due to friction etc...)Is this an correct interpertation of BHP?

If this fan is listed as a 1050 BHP, then why would we need a 1500hp motor for this application. Would this be due to the fact that there are losses in the mechanical system that would require a larger motor to account for these losses? The motor is directly coupled to the fan shaft.

Is the larger motor typically due to a reserve capacity factor as part of the design decision?

Does the larger motor size possibly have to do with starting requirements.

I'm just trying to understand for my own knowlede what throught process goes into a motor selection process when you are given the load information that I have shown attached?

 

[ScottyUK]

A few thoughts...

Starting and accelerating torque might be a factor - a smaller motor would take longer to get up to speed, and in the process might exceed its thermal capacity. Extended slow starts are generally worse for a motor than DOL starts, provided the supply is stiff enough to allow the DOL start and the load is mechancially able to withstand the 'snatch' during a DOL start.

Air density changes too, sometimes to a denser, colder state than the standard conditions the load is declared at: more mass flow requires more horses.

Motors last longer when they have a bit of spare capacity. Many specs for the power and heavy process industries call for a minimum of 10% excess motor power above the rated shaft load.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

ScottyUK shoots... he scores! Nice response.

Load curves are usually shown by equipment manufacturers in BHP because they are making no assumptions as to how the power arrives at the shaft, i.e. how the prime mover is powered or even what the prime mover is! When the ME does the shaft HP requirements calcs, for all he knows this will be coupled to an engine, or a low speed synchronous motor with a gearbox or belt drive etc. etc.. When it is packaged with a motor, all of the stuff ScottyUK mentioned goes into the decision.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[rockman7892]

Thanks guys great information.


Jraef

I'm trying to make sure I understand some of the terminology used in your response. When you mention "how the power arrives at the shaft" I'm assuming that you are referring to the fan shaft. This being the case the fan engineer has no idea what type of power losses may occur between the motor and the fan shaft because they do not know how it is driven (gearbox, pullys, etc...)

What are you refering to as the prime mover? Is this the shaft or whatever moves the load?

"When the ME does the shaft HP requirements calcs, for all he knows this will be coupled to an engine, or a low speed synchronous motor with a gearbox or belt drive etc. etc.."

Is the shaft reference in this statement again the shaft of the fan. The ME has no idea how this shaft is being moved so he cannot say what hp is required between the shaft and motor (i.e. losses)?

 

[jraef]

Yes, that's what I meant. From a pure design standpoint, the ME designing the fan is looking at things such as blade pitch, tip speed, diameter etc. in order to attain a specific air flow through the fan assembly. When he is done, he ends up knowing, and publishing, the bhp at the shaft of the fan. At that point, he doesn't yet know whether that shaft is getting the necessary mechanical power delivered to it from an internal combustion engine, an electric induction motor, a synchronous motor, or 100,000 squirrels in a cage and if the connection involves a direct shaft coupling, gearbox, or 40,000 rubber bands on a grooved slab of wood. The input HP of that "prime mover" is determined separately with all the losses, speeds, industry standards etc. factored in.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

Are the 100,000 squirrels a part of renewable energy program ?

 

[waross]

I'm okay with the squirrels but I am intrigued by the grooves in the slab of wood! Grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

Ok. so the "Prime Mover" is what is supplying the power, weather it is an induction motor or 10,000 squierls.

 

[ScottyUK]

Yes.


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If we learn from our mistakes I'm getting a great education!

 

[quark]

Does BC mean Backward Curved? The fan curve seems to be of forward curved blade type. These are generally overloading in nature, which means the power consumption increases continuously if the system resistance is decreased (or flowrate increases). The motor selection might have been done for the end of the curve operation (this is a widely used term, though technically incorrect)

What worries me more is that your operating point seems to be much closer to the unstable zone (i.e one static pressure corresponding to two flowrates). Your fan may hunt and subsequently damaging the fan and motor. You can avoid this by shifting your operating point towards right, by reducing resistance in the system, or having a good flow control system.

If the fan is of backward curved type then overloading is not an issue.

 

[edison123]

Hi quark

Any good books on fan design and performance ?

 

[quark]

Edison,

There is excellent knowledge available on net for free. For example,

http://www.lorencook.com/pdfs/cookbook.pdf

http://www.northernblower.com/download/fanfacts.pdf

http://www.bee-http://www.bee-india.nic.in/index.php?module=tri&id=4

(many other technical things apart from fans)

http://www1.eere.energy.gov/industry/bestpractices/pdfs/fan_sourcebook.pdf

http://www.tcf.com/TCFBlower/literature.asp

and go down to Engineering Data Letters

Fans Reference Guide

If you still insist on spending, go for Fan Engineering Handbook by Buffallo Forge Company. Pretty expensive but worth having.

 

[edison123]

Thanks quark.

 

[jraef]

10,000 squirrels are definitely a renewable resource, they work for peanuts... (sorry, couldn't resist)

...

Grooved slab of wood: My mind slipped a cog and I could retrieve the word "sheave" at that moment...


The reference came from the fact that since becoming unemployed, I am helping out an old walnut farmer in my town who designed and built his own production and packaging machinery. He has one "prime mover", a 15HP Repulsion-Induction motor from around the early 1900's that his grandfather bought for a lathe. He has adapted it to a complex system of hand made wood sheaves and leather belts to run 4 or 5 machines, one at a time, when it's harvest time. As a walnut farmer, he has spent a lot of time shooting squirrels but as suburbia has encroached he can no longer do that, so he traps them. I joked with him last year that if he kept a bunch of his squirrels and fed them culled nuts, he could operate his farm off the grid!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

quark

I have ordered the fan engineering cd from buffallo forge. Cost 100 bucks but I think it is worth it. Thanks again.

jeff

Nice. I love them, crossed belts.

Sorry about your job loss. Weren't you working for Siemens or some such big oem ? Best wishes. Man of your calibre cannot stay hidden for a long time.

 

[roydm]

But how do you get all the squirrels to run in the right direction, Oh I see thats why you have the crossed belt on a couple of the drums.
Nice joint work.
Roy

 

[jraef]

Yes, I had worked for Siemens. I'm freelancing again, but only temporarily mainly because a friend needs me to help him launch a new start-up.

By the way, that squirrel cage sculpture isn't mine, I stumbled across it on a website a while ago when looking for pictures of squirrel cage motor designs. Cool website for tinkers though...

http://blog.makezine.com/archive/2008/09/distributed_squirrel_cage.html

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[quark]

Off topic

Edison,

That is relatively cheaper. As I don't like reading from a screen, I purchased hard copy and the postage charges were 40USD. BTW, where are you now? Chennai or Andaman?

Jraef,

I thought you were working on perpetual motion machines

 

[edison123]

quark

The shipping cost was the reason I opted for CD. Besides, I get to print my own copies of the book now .

I am in Chennai now.

jeff

Good luck to your friend.