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 [EMAIL PROTECTED], "turbulatordude" <[EMAIL PROTECTED]> wrote: > > --- In [EMAIL PROTECTED], "Leslie Watts" > <[EMAIL PROTECTED]> 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 Addresses: FAQ: http://www.ktmarketing.com/faq.html FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/ Post Messages: [EMAIL PROTECTED] Subscribe: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] List owner: [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED] Moderator: [EMAIL PROTECTED] [EMAIL PROTECTED] [EMAIL PROTECTED] [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 Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
