Previous in Forum: Towers - Mechanical Parts and Functions   Next in Forum: Two-Link Inverted Pendulum: Parameter Change and Response
Close
Close
Close
14 comments
Rate Comments: Nested
Member

Join Date: Jun 2008
Posts: 9

Calculating Fan Run-Down Time

01/23/2009 6:09 AM

We need to fit a brake to a fan and the brake supplier wants to know what the run down time of the fan is. How do we calculate this?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16512
Good Answers: 670
#1

Re: Run down time of fan

01/23/2009 7:54 AM

Quicker to time it surely? Vastly more accurate too
Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Member

Join Date: Jun 2008
Posts: 9
#8
In reply to #1

Re: Run down time of fan

01/27/2009 5:29 AM

Fan is still in design stages.

Register to Reply
Guru

Join Date: Dec 2005
Posts: 1602
Good Answers: 19
#9
In reply to #8

Re: Run down time of fan

01/27/2009 9:20 AM

See post number 4. The fan supplier and the motor supplier should be able to provide moments of inertia. Then dust off your Dynamics textbook, or beg, borrow, or steal a dynamics text to find the applicable formulas. (I barely passed Dynamics when I went to college, and cannot remember and don't have time to look the formulas up!)

Or if you're lucky, the brake saleman is smart enough (or knows someone) who can take the information and come up with a reasonable extimate for the number you need.

__________________
Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#10
In reply to #9

Re: Run down time of fan

01/27/2009 2:26 PM

No brake is needed, really not, use dynamic Braking as I posted in #5. Its cheap, as fast as you wish from almost instant (under 1 second) to any length of time you wish, just change the resistor, or use none!!!

Its also relatively cheap and if done properly will not affect the life of the motor.

Look here:-

http://en.wikipedia.org/wiki/Dynamic_braking

http://www.mosebachresistors.com/

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Guru

Join Date: Dec 2005
Posts: 1602
Good Answers: 19
#11
In reply to #10

Re: Run down time of fan

01/28/2009 10:49 PM

I am aware that this has been done for decades with rail and DC applications, but not familiar with AC fan motors.

I understand the motor will stop quickly, but without a mechanical brake in addition what will keep the motor from "drifting" from position?

__________________
Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#12
In reply to #11

Re: Run down time of fan

01/30/2009 7:42 AM

Two questions:-

1) what forces are their that could cause it to "drift position"?

2) Why does this matter?

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Guru

Join Date: Dec 2005
Posts: 1602
Good Answers: 19
#13
In reply to #12

Re: Run down time of fan

01/31/2009 12:23 AM

For a fan, momentum of moving air can keep a fan wheel or propeller moving.

If you need to brake a fan for stopping, then it matters.

__________________
Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#14
In reply to #13

Re: Run down time of fan

01/31/2009 9:20 AM

The original poster has not said either way as to whether the fan must be stopped from rotating completely in spite of a possible "stiff breeze", on you mentioned it.....it could be completely unimportant!!!.

But with a short on the motor and a very strong blast of air, it will hardly move.....at worst, very, very slowly.........

Dynamic braking is a very good way to stop motors, if you have never tried it, you don't know what you are missing!!!

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply
Guru

Join Date: Mar 2007
Posts: 2550
Good Answers: 103
#2

Re: Run down time of fan

01/23/2009 8:27 AM

Does he need the existing run down time ? I feel the braker should be more interested in required time. And that is what you have to decide based on why do you want to brake it.

__________________
Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
Register to Reply Score 1 for Good Answer
Guru
India - Member - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Oct 2006
Location: India, 200 Km. North of Delhi.
Posts: 1393
Good Answers: 53
#3

Re: Calculating Fan Run-Down Time

01/23/2009 11:13 AM

When you decided to brake the fan then you should be knowing that why you want brake it and for how long?

Register to Reply
Commentator

Join Date: Sep 2008
Location: Tiruchirapalli Tamil Nadu India
Posts: 56
Good Answers: 3
#4

Re: Calculating Fan Run-Down Time

01/24/2009 2:49 AM

You need to know the moment of Inertia of the fan and speed at which is rotating when you switch off. This will tell you the energy available in fan.

You have to identify the no. of revolutions to stop. The revolutions could be in fration of one revolution too.

Based on the above data, your supplier will determine the torque required to stop the fan in the specified time and design the braking system.


If you need to know the run down time just stop the motor when fan is running at full load, time the coasting down till fan comes to complete rest. You repeat the test two three times to get consistant results.

Register to Reply Score 1 for Good Answer
Guru

Join Date: Dec 2005
Posts: 1602
Good Answers: 19
#6
In reply to #4

Re: Calculating Fan Run-Down Time

01/24/2009 10:07 PM

Moment of inertia of the fan AND motor, plus any other rotating mass.

__________________
Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
Register to Reply
Guru

Join Date: Mar 2007
Posts: 2550
Good Answers: 103
#7
In reply to #6

Re: Calculating Fan Run-Down Time

01/25/2009 2:10 AM

And the friction characteristics of bearing(s) at different speeds, seals, the lubricant, its viscosity, gear boxes and their characteristics (in case of industrial fan) and so on ...

And since it is a fan, you further need the air drag coefficients at difefrent speeds, the air characteristics, temperature ....

Is it worth mathematical modelling ?

__________________
Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
Register to Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#5

Re: Calculating Fan Run-Down Time

01/24/2009 4:59 AM

Why buy a brake?

Removing power (switching off) the fan and shorting all the leads going to the motor together will stop it VERY fast. In fact a few low value resistances may be needed to let it stop a bit little bit slower. No resistance its very very abrupt!!

Its called dynamic braking and works with every motor (AC or DC) that I personally have every seen. Even most brushless motors.....

Usually its done with a contactor (relay to you) that supplies power to the motor....

__________________
"What others say about you reveals more about them, than it does you." Anon.
Register to Reply Score 1 for Good Answer
Register to Reply 14 comments

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Copy to Clipboard

Users who posted comments:

Andy Germany (4); Kappie (1); narendra54 (1); rakesh_semwal (1); Ried (4); sb (2); user-deleted-1105 (1)

Previous in Forum: Towers - Mechanical Parts and Functions   Next in Forum: Two-Link Inverted Pendulum: Parameter Change and Response

Advertisement