RE: [CAD_CAM_EDM_DRO] Re: Plasma table gear reduction

Dave,
I did do a quick run on motioneering for you concerning
servo gear ratios. Here is a quick rundown of the results:

1) direct drive with 1" diam timing pulley
600 in oz torque 0.4 oz in sec^2 rotor inertia

2) 2:1 reduction drive with 1" diam timing pulley
400 in oz torque 0.4 oz in sec^2 rotor inertia

3) 3:1 reduction drive with 1" diam timing pulley
250 in oz torque 0.1 oz in sec^2 rotor inertia

4) 5:1 reduction drive with 1" diam timing pulley
280 in oz torque 0.1 oz in sec^2 rotor inertia

5) 10:1 reduction drive with 1" diam timing pulley
400 in oz torque 0.1 oz in sec^2 rotor inertia

6) 20:1 reduction drive with 1" diam timing pulley
no motor found

The numbers are peak torque...continuous required is about a quarter
of that figure with the friction I assumed. More details are required
to get really good friction numbers though.

Se what's happening? At very low reduction the motor has to
get pretty big, although not huge.

As more reduction is used the motor torque required goes down, but
only up to about 5:1 ratio. beyond that the torque requirements rise
again, and by 20:1 no practical motor can supply the needed performance.

Why does this happen? It's caused by the square term of reflected inertia
vs gear ratio. The motor is turning faster and begins to really fight it's
own
inertia in an attempt to rapidly change speed.

So with the numbers I used you can see that there is a sweet spot at about
3:1 to 5:1 where the peak torque required is minimum and a reasonable and
typical
rotor inertia gives a good impedance match.

This is not anywhere near the point where the motor delivers best power or
is
most efficient. It is simply the point of best economy for your design.




Leslie M.Watts
L M Watts Furniture
Tiger Georgia
(706) 212-0242

Main page:
http://www.lmwatts.com
Engineering:
http://www.lmwatts.com/shop.html
Cnc surplus for sale:
http://www.lmwatts.com/forsale.html
Carved signs:
http://www.lmwatts.com/signwp.html


-----Original Message-----
From: turbulatordude [mailto:dave_mucha@...]
Sent: Tuesday, December 21, 2004 2:21 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Plasma table gear reduction

> The big problem with running servos slow is heat. For the same

output

> power if you halve the speed you double the torque required.

Current is

> proportional to torque so the current doubles. Now for the killer -

the

> heat squares! (heat = I^2 * R). To make matters worse the slower

the

> armature turns, the less effective the cooling is.
>
> Les

I don't see that as a problem. An external fan would suppliment any
low speed heating problem. But then knowing to solve a problem is
half the battle !








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