Re: Rotor inertia.

Be careful with "low-inertia" motors for another reason entirely.
Oftentimes they are unsuitable for use with switching type drives.

Motor inertia is reduced by eliminating the laminated steel armature,
leaving only the windings, which are wound in such a way as to be
self-supporting.

The absence of iron also reduces motor inductance dramatically and
that is bad if the motor is used with a switching type drive. Almost
all high power drives are switching type (PWM).

Compare two motors:

One is a low-inertia with an armature inductance of 100uH. The other
is a standard motor having 5mH of inductance. Both have similar specs
otherwise and will be run at 48VDC using a 25kHz switching drive.

The 25kHz ripple current thru the standard motor will be:

I = V / (2L * f) = 48V / (2 * .005 H * 25,000 Hz) = 0.192A

The low inertia motor:

I = 48V / (2 * .0001 H * 25,000 Hz) = 9.6A!

The standard motor will stay nice and cool while the "low-inertia"
motor will probably burn down with nearly 10A of current passing thru
it continuously.

It would require large inductors in series with the motor in order to
use it.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Jason Cox" <sinergy@o...>
wrote:

> Sorry Jon, I meant the lower inertia ones were more expensive.
>
> Regards,
>
> Jason Cox.
>
>
> ----- Original Message -----
> From: Jon Elson <elson@p...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Friday, October 31, 2003 5:17 PM
> Subject: Re: [CAD_CAM_EDM_DRO] Rotor inertia.
>
>
> >
> >
> > Jason Cox wrote:
> >
> > >Hello All,
> > >
> > >Just wondering when selecting servo motors what impact does

rotor inertia

> have on the final function of a cnc machine?
> > >
> > >>From my research motors with a far lower inertia are much

cheaper, but

> does this really matter. Surely the redution in inertia of the

rotor would

> be minimal say compared to the inertia of the machine table.
> > >
> > >
> > Yes, essentially correct. Mariss Freimanis did some calculations

that I

> > seem to
> > recall showed that it is not insignificant, but that common

motors come

> > out in
> > the right range. Under some circumstances, a motor with too

much, or too

> > little inertia can make for an unstable servo system. This is

especially

> a
> > problem when there is some springy element between the motor and

the

> > high-inertia load, such as a long thin leadscrew or couplings

with too

> much
> > flex. You end up with a system comparable to two masses

connected by a

> > spring.
> >
> > Are you sure low-inertia motors are cheaper? I happen to have

some low

> > inertia Yaskawa super minertia motors that supposedly cost $5000

each.

> > They were in computer tape drives, and had a fiber composite

capstan

> > with Beryllium hub, a ceramic shaft, ironless rotor motor of

the "thimble"

> > style, etc. But, in general, a low inertia motor of conventional
> > construction
> > is just smaller, that's all. So, a smaller motor qould be

cheaper to

> make.
> >
> > Jon
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> > Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
> >
> > Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> > Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> > List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com,

wanliker@a...,

> timg@k...
> > Moderator: pentam@c... indigo_red@q... [Moderators]
> > URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
> >
> > OFF Topic POSTS: General Machining
> > If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to

reach it if

> you have trouble.
> > http://www.metalworking.com/news_servers.html
> >
> > http://groups.yahoo.com/group/jobshophomeshop I consider this

to be a

> sister site to the CCED group, as many of the same members are

there, for OT

> subjects, that are not allowed on the CCED list.
> >
> > NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY

POSTING THEM.

> DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> > bill
> > List Mom
> > List Owner
> >
> >
> >
> > Your use of Yahoo! Groups is subject to

http://docs.yahoo.com/info/terms/

> >
> >