# What about motor rotor size?

1. Apr 17, 2014

### SpaceLight

Specifically, this is all to do with rotor size of brushless DC motors and how it plays a part in calculating angular velocity / volts / amps / torque.

There is no single equation to be solved here.
Any relating information or equations..., any and all, please jump in.

I've been looking at all kinds of equations -- angular velocity, moment of inertia, torque, angular momentum, so on, and all interrelating factors --, but I'm not 100% on how rotor size effects the results of these equations, in more of a collective manner, so to speak. Radius is a variable in finding linear speed, but for finding angular velocity, radius variable seems to disappear. To some frail extent I realize that DISTANCE is transferred to ANGLE, but it's one big grey area for me.

In motors, one might be inclined to think that a smaller rotor would result in higher speed. Simply enough: The circumference is less, so less time to travel per revolution. You have stator electromagnets interacting with rotor magnets; call it a force pulse, if you will, created between them, and given that pulse of force the rotor proceeds to spin around until the next pulse initiated by commutation. The smaller the rotor, the more rpm; hence, more pulses initiated, more CEMF, so forth, so on, all due to increased speed by the rotor being small.

I've gone a fair distance so far, in physics and in motor design, I understand quite a bit, but I'm intermediate at best, I digress, I can't nail it down when it comes to the relationship between rotor size, force (as stated), angular velocity.

It's very important to what I'm doing; I'll take any input on the matter.
Thank you very extremely much indeed!

2. Apr 17, 2014

### Simon Bridge

I'm sorry - was there a question in there?
Electric motors, even simple ones, can get quite complicated - as well as the bulk of the rotor, you'd also have to consider the interaction with the magnetic fields.
What calculations you need depend on what you are trying to achieve.

3. Apr 18, 2014

### SpaceLight

oy

lol

4. Apr 18, 2014

### Mordred

https://www.physicsforums.com/attachment.php?attachmentid=60534&d=1374879190

Drives Engineering handbook
https://www.physicsforums.com/attachment.php?attachmentid=54974&d=1359050037

these might help, check rotor inertia in the second article, the articles are a bit big to directly attach to PF but they are from the Rockwell knowledge base they have a comprehensive list of DC Brushless motor literature there

here is the link to their free Literature

http://literature.rockwellautomation.com/idc/groups/public/documents/webassets/browse_category.hcst

also look at their product manuals as different types of DC brushless motors have different rotor characteristics IE low inertia vs high inertia rotors. So you have to be careful what series of DC brushless motor to inertia and torque specifications you need.

you will have to create and account for their knowledge base literature but its often worth it.

Your best bet is to call them and describe your specifications, they will give you a list of recommendations

Last edited: Apr 18, 2014
5. Apr 18, 2014

### SpaceLight

Nice engineering handbook.
Refreshing to have equations compiled as such. Thank you.
Have bookmarked Rockwell as well.

Just have to wade through the fog here a bit....
Presently looking through inertia & power....

Cheers, Mordred

6. Apr 18, 2014