×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Measuring moment of inertia
3

Measuring moment of inertia

Measuring moment of inertia

(OP)
I would like to measure the moment of inertia of an axial fan.  Can anyone tell me a method for doing this?

RE: Measuring moment of inertia

Hello
You can do that by mesuring the electrical absorbed power from the fan motor which is equal P = I x V
where I : current (A)
V: Voltage (V)

Theoritacly most of this power converted to mechanical power which is P = 1/2 x I" x w^2
 Where I" : moment of inertia ( requested )
w : rotational speed ( rad \ s)

equate both equarions you can find I"

Good luck

RE: Measuring moment of inertia

If you have access to the fan blade outside of the assembly, there is an experimental way of determining the moment of Inertia.  It is often used to measure the inertia of motor rotor assemblies.  In fact the procedure is described nicely in the NEMA standard for motors.  The NEMA standard is MG-7 if you want to look it up yourself, but I'll try to describe it here.

Suspend the fan with the center shaft oriented vertically using two parallel wires.  You may have to make an attachment fixture, but if you keep it's geometry simple, it's added inertia can be calc. and subtracted out in the end.  The wires should be attached equally spaced from the centerline of the fan.  The ratio of length of the wire(L) to the distance between the wires (D) should be about 10.

Once setup, rotote the fan a small amount from equilibrium and release.  After release, measure the frequency of oscillation.  The moment of inertia can then be calc. using the following:

I = c*m*D^2/(L*f^2)

I = moment of Inertia about the rotation axis
m = fan weight
D = distance between wires
L = Length of wire
f = measured frequency in HZ
c = conversion factor depending on units used

If Inertia is in kg-m^2, with 'm' in kg and "L" & "D" in meters, the c = 6.2e-2

If Inertia is in lb-ft^2, with 'm' in lb and "L" & "D" in feet, the c = 2.04e-1

If Inertia is in lb-in-sec^2, with 'm' in lb and "L" & "D" in inches, the c = 7.61e-2

If desired, don't forget to subtract out the inertia effect of any attachemnt fixture.  The procedure works well for motor rotors.  I think if you keep the amount of rotation low so it doesn't spin to fast such that the drag forces of the fan blades is not to high, it should give a good value.

Good Luck.

RE: Measuring moment of inertia

    I don't think you can use the current (I) method suggested because most of the electrical energy is used move air. The best way will probably be to calculate it mathematicaly. The solution using the wires also sounds good.
    If you can run this fan in a vacuum and measure the current draw to get up to running rpm, then you can use the mathematical relationship between
the change delta I (amps) and delta rad/sec to obtain I (moment of inertia)

Hope this helps.

You might try contacting the fan manufacturer who might already have this  information.

Don

RE: Measuring moment of inertia

I am sorry bescause i did not mention in my first reply that mesuring should be done in aprrox. vacume conditions.

good luck,

RE: Measuring moment of inertia

This sounds like fun.

If the fan can be repositioned to the center of the axle, try this...

Orient two-horizontal edges (creating a channel) to support the ends of the shaft with the fan suspended into the channel (from the side, it kind-of looks like a table saw).

Attach and wrap string around the axle then attach a known mass to the string (preferably hung by parallel strings spanning the fan blades).

When you let the mass (weight) hang freely, the weight will rotationally and translationally accelerate the system.

Torque = I * alpha (the rotational acceleration)

Force = mass * a (linear acceleration), include the hanging weight's translation

a and alpha are related.

and solve for I



Alternately...

The mass falling a given distance and now moving horizontally with a velocity, was caused by the work of the mass being lowered.

weight * distance fallen (potential energy)= the SUM (of the rotational and translational kinetic energies)



If all else fails try attaching a tin-foil shroud over the fan blades, looking at it axially it kind-of looks like a minature bicycle wheel covered with tin-foil, then use the electrical current method previously proposed.



As a last resort...

If you can catch the next Space Shuttle, you might give the fan (freely held in your hands) an axial spin (and release it), then, have an observer measure the rotational velocity of the fan and the rotational velocity of your body.  Then you simply back-calculate the moment of inertia of the fan, through a simple relationship, incorporating your body's moment of inertia, but that my friends will have to be another topic of discussion.


Well gotta go,
pi

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper – Data Security and Know-How Protection
Our data is constantly exposed to the danger of being intercepted or stolen as it wends its way over global data networks. Data security measures and measures for protecting intellectual property should not, however, first be implemented when data is exchanged – companies must lay the foundation for these measures within their own organization. Download Now
White Paper – Collaboration in the PLM Context
The influence exerted by the Internet of Things (IoT) means that there is a steadily growing need for collaboration in industry. Partners from new industries and areas of application need to be integrated in cross-company business processes to ensure that the lifecycle of smart, connected products can be managed from end to end. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close