×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*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.

# AC Motor Sizing

 Forum Search FAQs Links MVPs

## AC Motor Sizing

(OP)
I love finding deficiencies in my education.  Can someone verify that my reasoning is correct?

We currently have a mixing maching that I prefer to call a rock tumbler (just like the ones at Toys-R-Us) because we have marbles in the slury as an agitator.  It has begun to break down more and more often so we have decided to replace it.  Since it was a custom build and all the information on it has been lost, the replace has also turned into a redesign.

To size an AC motor, the input torque and speed is needed.  I can calculate all that based on gear reductions assuming I know the initial conditions.  --Well, during my education, the initial conditions were always a part of the question.--  The RPM I can determine by measuring the existing setup.  The torque I am not so sure about.

The rock tumbler has a cylindrical container with the mix in it.  The container is supported on two rollers: one idler, one driver.  I think I can determine the torque by multiplying the weight of the cylinder by the center to center distance of the driver roller and container.  Is this correct or am I missing something?  Friction?

--Scott

### RE: AC Motor Sizing

INERTIA!
Figure out how much Mass Moment of Inertia the system that is rotating has about its rotational axis.
Figure out the gear ratio from the drum to the motor.
Figure out how fast you want this system to rotate at maximum load (weight) and how fast you want to get it to steady state RPM.
Then T=I*Alpha     (Alpha=angular acceleration)
This is the torque you need, but you still need to calculate your "Alpha"

Edson Campos

### RE: AC Motor Sizing

(OP)
Thanks Edson,

Finding the inertia of the system is going to be the fun part!

Could you also check something else for me?  I think I'm missing or adding a slug.

(1)Mass Moment of Inertia units are lb*in^2
(2)Alpha units are rad/sec^2
(3)Multiplying I get (lb*in^2*rad)/sec^2
(4)A slug is (lb*sec^2)/ft (converting without carrying coefficients is (lb*sec^2)/in)
(5)Multiplying (3)*(4)= lb^2*in.

I obviously would like torque to be in ft*lb or in*lb.  Why do I get lb^2?

--Scott

### RE: AC Motor Sizing

Mass moment of inertia has units of "ft*lbf*sec^2"
Multiplying should cancel the Sec^2 and you will get Ft*Lbf torque.
Why are you multiplying the Torque by Mass?
The mass part is taken care of in the "MASS" moment of Inertia part.
Let me know what happens.

Edson Campos

### RE: AC Motor Sizing

(OP)
It wasn't a slug that got in the way, it was the graviational constant.

Here was my original thinking:
Since in my design, a cylinder is rotating; I can determine the Mass Moment of Inertia ("MMI" so as to not be confused with the pronoun "I") by computing MMI=(mr^2)/2.  Units work out to be lb*ft^2 ~~ in^2 in my previous post.  And, the previous post follows the rest of my reasoning.  What the equation didn't tell me is that m is in units of lbm, not slugs or lbf.

But, as I now determined, m should be replaced with W/g.  That yields MMI=lbf*ft*sec^2 as you mentioned above, where W is in lbf and g is in ft/sec^2.  The units then clean up nicely to a Torque in units of ft*lbf.

Thank you very much for your help.

--Scott

#### 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.

#### 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

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. 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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!