Sizing a motor

[jonruk99]

Test setup: I need to have a disk, diameter = 1ft, spinning at 955rpm. A normal load of 2866lbs via an aplicator whose diameter is 0.25ft would be applied to the disk surface at 0.25ft from disk center. Ultimately I know I have to calculate the inertia of the system, torque or stalling torque, and the power (hp), however, I do not know generate my formulas for the particular application. How do I size the motor correctly. Any input would be greatly appreciated. Should you need more info, please feel free to contact me. Thanks

 

[RichLeimbach]

Is the motor and flywheel supposed to keep a constant speed while this 716.5 ft-lb (2866 lb* .25ft) torque load is applied? With the 955 RPM speed, this gives about 13.3hp necessary just to keep the motor turning at this speed under this load. You'd probably want to pick a motor 20hp or above just to be safe.

On the other hand, if you want to accelerate the 1ft dia disk up to 955 RPM quickly, you'll have to calculate the necessary torque at all points during the ramp-up for the curve that matches your motor type.

If you give some more info on how your application operates, it might be helpful to see which of the two cases are most important.

 

[jonruk99]

Yes the motor and flywheel are supposed to keep a constant speed. The load will then be applied to stall the motor. Thanks for your inputs RichLeimbach

 

[RichLeimbach]

If you expect to stall the motor with this load, then you don't need a 20 hp motor. This size was to keep the motor running at this speed even after the load was applied.
If it is OK to slow or stall the motor after the load is applied, then you really only need to be concerned with how fast you want to bring the inertial load up to speed. If you've got some time, then a fractional hp motor can do the job. If you need it really quick, then you might need even more than the 20 hp (depending on the type of motor and the inertia of your disk).
What type of motor are you planning to use (DC, AC, etc.)? If you can describe the application a little more, you can usually get a lot of feedback in these forums (sometimes more than you want).

 

[jonruk99]

RichLeimbach, could you kindly forward me your email address so I can send you a sketch of the test setup. Thanks

 

[RichLeimbach]

Not sure how to get you my email without posting it on the website (which I don't really want to do). Any ideas?

 

[TD2K]

Couple suggestions on email addresses.

1. Set up a free address with Hotmail or Yahoo and then just never use it again.

2. Jonruk99 posts his email address here and Rich can then contact him.

Anything to get these two kids together

 

[jonruk99]

Hey RichLeimbach, I will be using a DC motor. By the way, which formula did you use to calculate your 13.3hp. I did my calculations using 716.5 lb-ft and I'm getting 130hp with no factor of safety.

 

[RichLeimbach]

You're right. I did the calcualtions in metric form and used kgf when I should have used N. Sorry for the mistake.

 

[jonruk99]

Hey RichLeimbach and all, sorry it's been a long time, I've been on travel. Here is a default yahoo account I made: mastafu77@yahoo.com. Could you email me so I can forward you a copy of my test setup. All suggestions are welcomed. Thanks guys.

 

[Metalguy]

If you expect your setup to stall an electric motor, you had better start thinking about starting torque, etc. instead of HP.

 

[rhodie]

What mat'l are you gonna make the flywheel out of?

Can it sustain 16.5 rps (with it being acted on by 716.5 lb-ft) without coming apart?

 

[chicopee]

If the applicator load is normal to the disk surface, develop an estimate of the frictional force from the applicator using equation Frictional Force= mu(static and dynamic)*2866. The External Torque from this friction is then equal to Frictional Force*.25 . Horsepower is then equal to External Torque * 955/ 2544???. That 2544 may or may not be correct as I dont remember it however this value can easily be checked out. Tack on an additional 20% to overcome inertias and friction from bearings

 

[jonruk99]

Please correct me if I'm wrong chicopee, did you mean to type: Frictional Force=ma(static and dynamic)? If not, what do the mu(static and dynamic) mean? I'm guessing m=mass and u=displacement??? How do I do this? Forgive me if I should know these things, I've just become rusty over the years and my books are out of reach.

 

[That1Guy]

Mu should be the coefficient of friction (static and dynamic), spelled as it sounds, like the greek nu or alpha, it's mu (myoo). The basic equation is F=mu*N or friction force equals Mu times the normal force. I beleive this is what you're asking about.