Calculating Fan Run-Down Time

Quicker to time it surely? Vastly more accurate too
Del

Fan is still in design stages.

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.

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/

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?

Two questions:-

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

2) Why does this matter?

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.

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!!!

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.

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

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.

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

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 ?

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....