Re: Plasma table gear reduction - motors

Pardon my $.02USD here.

There is always confusion between Torque and Horsepower in general
discussions, especially in automobile benchtop racing. But Remember
Torque is your friend!

DC motors, especially series wound field motors, usually have a rated
base speed. These motors are considered to be Constant Torque from 0
RPM to Base Speed and Constant Horsepower above Base Speed.

In Kinematics, Horsepower is generally expressed as energy applied for
some period of time. For example:
1 HP = 745.6 Watts
1 HP = 2546.6 BTU/HR
1 HP = 33,000 Pound-Feet/Minute

In your example:

> I do have the idea that a DC motor is rated on a somewhat linear
> curve. and that generally speaking, a 100 volt, 10hp 4,000 RPM motor
> is also a 50volt, 5hp 2,000 RPM or 25 volt, 2.5hp 1,000 RPM motor.

While the theory is sorta correct, the thing to remember is that the
available torque is considered constant. In your example, if the Horse
Power (10) multiplied by 33,000 Pound-Feet/Minute (330.000) and
divided by the Motor Speed (4000) the result is the Torque avaliable
at the Motor Shaft (82.5) in Foot-Pounds. If this is muliplied by 192
it is converted to Ounce-Inches (15,840). If the calculations are
repeated for the motor at the each reduced speed you find that the
Hosepower does fall BUT the Torque available at the Motor Shaft,
15,840 Ounce-Inches remains the same. This would probably drive your
gantry quite well. So how can the motor be sized to drive the gantry?
This is what I do on large machines with tables weighting 50,000 # and
more.

First I determine the torque required on the Ball Screw or Rack to
drive the axis. This can be calculated or measured, I find that even
if I have calculated the required torque, I still end up measuring the
torque with a torque wrench and using that value. After measuring the
torque you will find that usually you have two different values, the
breakaway torque and a running torque due to the the "Stick/Slip"
nature of the axis. For an axis that relys on friction ways or
bearings the difference may be quite high, for Linear Ways the
difference may not be that great. I will usually use 2X the running
torque as the Base Line Torque for the axis. This gives sufficent
force to move the axis and sufficent reserves to supply cutting
forces. The cutting method of course should be considered in setting
this reserve, a plasma cutter or router requires lower cutting forces
than a milling machine. For the higher break away force, the higher
peak torque available on a servo motor handles this nicely since it an
instantous load that goes away quickly.

Now that you have the torque required, look at the axis speed. You can
get the torque anyway you can find it within reason through the use of
belts, gearboxes, etc. since this is only a force. A very small motor
can move a very large object if it does not have to be fast.

To size the motor you now must decide on the axis velocity desired.
Pick the velocity of the axis and for a ball screw for example
determine the rotational velocity of the screw. Multiply this velocity
in RPM's by the desired axis torque in Ounce-Inches and convert to
Horsepower or watts

For example, assume that you measured your running torque and it was
100 Oz-In. You doubled this to get 200 Oz-In. The Ball Screw has a
lead of .2" and you want to traverse at 80 IPM which must rotate the
Ball Screw at 400 RPM. Multiplying the torque by screw velocity (200 *
400) we get 80,000 In-OZ/M power required. This doesnot mean a lot so
if we divide by 192 Oz-In per Pound-Foot and mulitply by 60 Minutes
per hour ((80,000/192) * 60) the result is 25,000 Ft-Lb/Hr. This
divided by 33,000 Ft-Lb/Hr (25,000/33,000) gives .7575 Horsepower or
multiplied by 745.6 (.7575 * 745.6) sizes the motor at 564 Watts.

Think of it this way, Torque moves the axis, without sufficent torque
the axis will not budge. Once it is moving however, Horsepower
determines how fast you can get there. While the two cannot be
seperated, given one or the other, I always favor Torque. Given
sufficent Torque I will always get to where I am going even with lower
Horsepower, it just takes longer.

My $.02 worth, invoice will follow by snail mail.

gary

aka nitewatchman
dba SynerTech, LLC


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<dave_mucha@y...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Leslie Watts"
> <leswatts@a...> wrote:
> > Dave,
> > Power is just torque times rpm. The motioneering software selected
> motors
> > and displayed a torque curve showing the operating point was in the
> > safe region.
> >
> > Although I did not list it, the optimum motors were in the range of
> > 600W or so rated cont power. This occurs at max cont torque and max
> > rpm.
> >
> > At the 3 to 5 reduction the motor will only be going a few hundred
> rpm
> > for the gantry speed I used. The power actually delivered will be
> > only perhaps a tenth of what the selected motor is capable of. The
> > voltage will similarly be much lower than rated.
> >
> > I know this may seem really sub optimal to use the motor this way,
> but
> > we are trying to optimise cost- not motor power utilization or
> efficiency.
> >
> > The only mechanical load that really consumes real power is
> friction. That
> > is
> > very low in your proposed machine. What we need is FORCE to
> accelerate the
> > moving
> > mass... and in a regenrative servo system you actually get that
> back when
> > you
> > deccelerate....it pumps up the supply cap voltage.
> >
> > So it turns out that the software selected motors capable of the
> torque
> > required
> > can generate much more power if they were run at a high rpm.
> >
> > The smaller motors at the high ratios run at higher rpm, but they
> have to
> > use
> > more torque to fight their own rotor inertia. As you can see the
> software
> > found
> > NO practical motor that would give the selected machine performance
> above
> > 20:1.
> >
> > Let's say you had a car that you wanted to be fast off the line.
> You can use
> > smaller and smaller motors with more and more gearing to get the
> same
> > torque.
> > At some point the time to wind up the smaller motors to high rpm
> would
> > actually
> > be the limiting factor rather than the car's mass.
> >
> >
> > Les
> > Leslie M.Watts
> > http://www.lmwatts.com
>
> Sorry for being so dense.
>
> But, it seems one of your earlier posts mentioned flat and large
> diameter motors are bad and long skinny ones are good. That makes
> sense as the flywheel effect from the large motor would get in the
> way of quick response. Ditto cast iron timing pullies vs. plastic
> ones.
>
> I do have the idea that a DC motor is rated on a somewhat linear
> curve. and that generally speaking, a 100 volt, 10hp 4,000 RPM motor
> is also a 50volt, 5hp 2,000 RPM or 25 volt, 2.5hp 1,000 RPM motor.
>
> so, generally speaking, I can make a prototype axis a few feet long
> and test the motors I have with different gear ratios to see the
> acceleration.
>
> Dave
>
> Dave