AC motors Torque Vs Speed

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haythame78
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AC motors Torque Vs Speed

Post by haythame78 » Thu Feb 17, 2005 6:55 am

Hi all<p>We want to use a variable speed AC motor (with no feedback) to actuate a fly cutter disc. We want to design the fly cutter so that we can vary the speed of the cutting disc. <p>I am wondering whether there is a need to indirectly connect the motor shaft to the cutting disc (like pulley pair) so that when reducing the speed the torque is increased and not decreased. Or do you this there is no need to use the pulleys pair since that when the speed of the motor is decreased the torque is by default increased to maintain constant power output? <p>I would appreciate your feedback and suggestions in this matter <p>Cheers

terri
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Re: AC motors Torque Vs Speed

Post by terri » Thu Feb 17, 2005 8:53 am

If you're talking Induction Motors, they strain mightily to maintain the same RPM and are built to deliver that RPM for a given line frequency. If you cut the voltage, they strain less mightily, so the torque will go down, but they are prone to burn up with lower voltage than that for which they were designed. ( Some induction motors use additional stator windings which are switched around to vary the torque, and hence the RPM, for a given load. )<p>I'm sure your machinist will tell you that fly cutters are a ROYALE PAINE IN THE TUCHAS and any time I could avoid using them, I would. They require excessive setup and adjustment time to get the job done, and I would rather use an end mill with more passes than a fly cutter any day of the week. Or even a shaper, if you've got one. And the bit and the head have to be rigidly rigidly rigidly mounted, with no play in the machine's bearings, and everything has to be 90.0000 degrees +/- zero degrees from everywhere to everywhere else. I say again, rigidly.<p>"Shotgunning" it from the low house:<p>You will probably have to go to a different motor (as with brushes and field coils) for your application, and bite the bullet and use some kind of pulse-modulation on the motor, with or without feedback. One wonders what this will do to the smoothness of your cuts.<p>You might look into using a hydraulic motor to drive the cutting head.<p>I can't imagine many machining situations where a stepped pulley system can't get you close to the desired cutter speed for a given material. However, there are belt transmissions which give continuously variable ratios.<p>Or, you just might want to buy thinner material in the first place. :) <p>Again, as with so many questions posted here, it is important to know the application, and in this case, the material to be machined as well.<p>[ February 17, 2005: Message edited by: terri ]</p>
terri wd0edw

haythame78
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Re: AC motors Torque Vs Speed

Post by haythame78 » Thu Feb 17, 2005 9:48 am

Hi Terri <p>thank you for your reply.<p>the application is somehow unorthodox. the fly cutter is a part of an apparatus whish is going to be used to slice and scan Termite mounds (material is mainly silica in addition to other materials that give it a plastic behavior). This project aims to research the homeostasis that may be featured by the Macrotermes Michaelseni Termite mounds found in Namibia and South Africa. Part of this research is the design and construction of the Slice and Scan Apparatus which will perform an incremental slice and scan on the Termite mounds to be able to capture the 3D geometry of these mounds. <p>in order to slice these mounds while keeping the structural integrity, these mounds were filled and covered with a 3 inch layer of plaster. that means the material to be sliced/ground is plaster and the mound material. the presence of the two materials may result in smearing during the cutting process depending on the moisture content of that material and the speed of the cutting stip in the cutting disc. an experiment was done on mound sample that was embedded in plaster/gypsum. the embedded sample was placed on a milling machine with fly cutter. by testing different speeds with different feed rates an optimum 372 RPM with a 10cm dimaeter fly cutter so if a 50cm fly cutter is to be used then 75 RPM may be used to get the same speed at the cutting tip. the cutting disc is desired to around 60 - 70 cm and it may way up to 20 kg.<p>so we need to be able to vary the speed of that disc depending on the moisture content of those mounds. as discussed in my first message. I hope this gives enough info about the application. waiting for your feedback guys
Please have the time to look for more information about this project at www.sandkings.co.uk.

haythame78
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Re: AC motors Torque Vs Speed

Post by haythame78 » Thu Feb 17, 2005 10:02 am

Hi Terri <p>thank you for your reply.<p>the application is somehow unorthodox. the fly cutter is a part of an apparatus whish is going to be used to slice and scan Termite mounds (material is mainly silica in addition to other materials that give it a plastic behavior). This project aims to research the homeostasis that may be featured by the Macrotermes Michaelseni Termite mounds found in Namibia and South Africa. Part of this research is the design and construction of the Slice and Scan Apparatus which will perform an incremental slice and scan on the Termite mounds to be able to capture the 3D geometry of these mounds. <p>in order to slice these mounds while keeping the structural integrity, these mounds were filled and covered with a 3 inch layer of plaster. that means the material to be sliced/ground is plaster and the mound material. the presence of the two materials may result in smearing during the cutting process depending on the moisture content of that material and the speed of the cutting stip in the cutting disc. an experiment was done on mound sample that was embedded in plaster/gypsum. the embedded sample was placed on a milling machine with fly cutter. by testing different speeds with different feed rates an optimum 372 RPM with a 10cm dimaeter fly cutter so if a 50cm fly cutter is to be used then 75 RPM may be used to get the same speed at the cutting tip. the cutting disc is desired to around 60 - 70 cm and it may way up to 20 kg.<p>so we need to be able to vary the speed of that disc depending on the moisture content of those mounds. as discussed in my first message. I hope this gives enough info about the application. waiting for your feedback guys
Please have the time to look for more information about this project at www.sandkings.co.uk.

Sterling Martin
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Re: AC motors Torque Vs Speed

Post by Sterling Martin » Thu Feb 17, 2005 11:55 am

I don't understand what you're doing, but it sounds cool! :cool:
What you really need is a variable frequency drive and a gearbox. You didn't mention horsepower, so I'm not sure what kind of torque you're needing, but if you want to go that low rpm with a regular induction motor and have any torque at all left, you need a gearbox. If the horsepower isn't very great, you might even get by with a 115 Volt input VFD like the GS1. <p>GS1

terri
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Re: AC motors Torque Vs Speed

Post by terri » Thu Feb 17, 2005 10:50 pm

With this new information regarding your exact application and the materials involved, I would recommend that you move away from the idea of a rotating cutter and go to a linear cutter akin to a band saw.<p>Parameters:<p>Throat (diameter of "band saw" wheels): Greater than the maximum height or diameter of the mounds, depending on which way you are taking the sections.<p>Kerf: Suitable for thickness of section, considering handling fragility, versus number of sections per linear dimension needed.<p>Cutter: You could probably use a regular bandsaw blade charged with abrasive, such as screened sand fom the locale, or carborundum and the like. You apparently have to cut it dry. The teeth will wear out, but the actual cutting will mostly be done by the abrasive with which you charge the cutter. You might consider using, rather than a bandsaw blade, thin aircraft control cable with the ends welded together just like a bandsaw, likewise charged with abrasive. In this case, you would need to groove the bandsaw sheaves<p>Something like this will give you a nice smooth cut, and once the rotating speed of the bandsaw wheels is set for a given specimen or specimen diameter, you can leave it alone. Suitable bandsaws with variable speed drives are on the market.<p>If this sounds suitable, you can call it "Terri's Microtome."<p>I'm outta here.<p>[ February 17, 2005: Message edited by: terri ]</p>
terri wd0edw

haythame78
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Re: AC motors Torque Vs Speed

Post by haythame78 » Fri Feb 18, 2005 3:33 am

Hi again<p>Thanks to everyone who replied and thank you very much terri for the your effort it is appreciated and it was helpful <p>It is my mistake again not to give you enough detailed info<p>I had to mention that our slicing will be destructive incremental slicing with increments of 1mm and the total length to be sliced is about 3.5 meters ( the total length of the mound) we need this to get a resolution small enough to capture the diameter of the smallest channels in these mounds which is about 1 mm. <p>so the machine should slice the first layer off (destroyed) take a picture of the sliced surface then decrement 1 mm and repeat the same action again. <p>it is worth mentioning too that the diameter at the base of the mound will be around 2.5 meters so if we to use an end mill that may take ages due to the small diameter of the band saw compared to the mound diameter.<p>so if we to use a steel cutting disc with 600 mm diameter and 25 mm (1 in) thickness that will need a high horsepower motor that is why we wanted to use an AC motor that can give high horsepower possibly 1 Hp and yet cheap. <p>the band saw is a good idea but i am afraid that while cutting across this big diameter and the 1 mm thickness the 3 meter saw (for example) may buckle or bend slightly which is gonna cause a problem for us.<p>Please let me know if you think using an AC motor (obviously with transmission agent from your discussion) would be good enough for this application. and if we are to use a hydraulic motor how much would that roughly cost?<p>cheers<p> :eek: :eek:

haythame78
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Re: AC motors Torque Vs Speed

Post by haythame78 » Fri Feb 18, 2005 3:35 am

the 2 blue faces where not ment to be at the end of the message so dont be alarmed lol <p>cheers

rshayes
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Re: AC motors Torque Vs Speed

Post by rshayes » Sat Feb 19, 2005 1:55 am

The speed of AC motors tends to be controlled by the frequency of the power applied. They can be slowed down by reducing the applied voltage, but this may leave the speed sensitive to the torque load on the motor.<p>The speed of a DC motor depends mainly on the armature voltage, with the current increasing as the torque load increases. Bodine Electric (www.bodine-electric.com) makes a line of DC motors and the associated variable speed controllers. These are available up to 1/2 horsepower. Gearhead motors are also available. This will allow a better match to your speed and torque requirements.<p>A fly cutter usually has a very small cutting edge. This will make it wear rapidly, especially when cutting an abrasive material. Sand glued together with termite saliva sounds similar to glass-epoxy circuit board, which dulls high speed steel drills very quickly and carbide drills somewhat more slowly.<p>You might look at the process used to make plywood. This uses a knife to cut a thin layer off of a rotating log. These blades are at least 8 feet (2.4 meters) long, and probably have carbide edges. The layer removed is usually 1/16 inch (1.6 mm) thick and the variation along the length of the cut seems to be quite small.<p>Since you are cutting a brittle material, driving the cutting edge with ultrasonic energy might reduce cracking. Using wax rather than plaster as an embedding material might also help.<p>I would also suggest making two passes, no matter what cutting technique you use. The first pass would be a coarse cut, possibly .9 mm. The second would be a finish cut of .1 mm. This should give a smoother surface.<p>Another alternative would be a sanding drum, with the first pass using coarse abrasive and the second pass using a fine abrasive.

Wolf
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Re: AC motors Torque Vs Speed

Post by Wolf » Tue Feb 22, 2005 5:05 am

thank you for everyone participated in this<p>it was helpful

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jwax
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Re: AC motors Torque Vs Speed

Post by jwax » Thu Feb 24, 2005 5:55 am

Have you considered wire saws, similar to what is used on cutting marble, granite, etc.?

terri
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Re: AC motors Torque Vs Speed

Post by terri » Thu Feb 24, 2005 6:01 am

That's probably a better suggestion than the one I made to use aircraft control cable and abrasive.<p>This party has also posted the "hoursepower" topic with respect to the cutting disc.<p>[ February 24, 2005: Message edited by: terri ]</p>
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