How To Build A Vandegraff electrostatic Generator
Cutting The Bowl Centre Tube Hole
Marking & Cutting
Forming The Flange Curved Shape
Fixing The Curved Flange To The Bottom Bowl
Constructing Simple Rollers.
Constructing Rollers With Sealed Bearings.
Positive or Negative Potential's (Checking The Polarity)
Rubber Belt Splice
Trimming The Rubber
Splicing The Belt Ends Together
Wooden Frame Assembly
Mark And Drill The Panel Screw Locations.
Fixing The Battens To The Panels
Assembling The Panels
Fitting The Uprights & Flange
Fine Tuning An Adjustments
2 @ 80mm dia(o/d) PVC flanges
2 @ 270mm x 75 mm stainless steel bowls
1 @ 600mm x 80mm dia(o/d) PVC pipe tube
1 @ 2m natural rubber chair webbing
3 @ 300mm x 300mm x 20mm compressed wood or ply timber
2 @ 25mm x wooden dowel
1 @ PVA wood Glue
30 @ 2.5 mm x 12mm screws
1 @ tube contact cement (Glue)
4 @ pop rivets
2 @ sealed roller bearings
1 @ nylon66 material (50mm dia x 60mm)
1 @ nylon food cutting board.
2 @ 30mm x 70mm metal tin strips
1 @ 25mm x 25 mm shim steel
1 @ 30mm x 120 mm tin strip
1 @ 100mm x 6mm booker rod
1 @ 4 or 6mm steel shaft cut to length
4 @ 6mm nuts & washers
2 x stainless steel bowls
1 x 80mm flange
|The sphere used on the top on the vandegraff can be constructed from two stainless steel food bowls. The bowls I used are 270mm dia x 75min deep from the outer most rim. (see figure y1) The bowls do not need to be fixed together with any screws or fixing glue. The best method is to stickytape the rims together with a complete circle of insulation tape around the entire rim. You will need to remove the top bowl at periodic intervals during construction, so don't use up all the insulation tape at this stage of the construction. Use temporary strips of 50mm long insulation tape on 4 locations around the rim, which will hold the top bowl into position while doing tests an adjustments..||
|Do not scratch or dent the bowls in any way. The more shiny and clean the bowls are the better they are at holding the high voltage charge on the surface. We want to hold as much charge as we can on the surface. Any sharp points or scratches will give a jump off point for the HV (high voltage).|
|The bowls used in the construction of this vandegraff unit are not the preferred items to use. The rims of the bowls provide a narrow edge from where the high voltage can jump off. The voltage will begin to breakdown (jump off) when the voltage starts to exceeds above 50kV. If you can obtain a perfectly round sphere that would be your best option. Most people are unable to obtain a perfect sphere so two stainless steel bowls will work well enough. You may like to have a search for a metal constructed world globe or atlas. The same world globes you use to study with at school. Currently most of the spheres|
|used in these world globes are now made from plastic, so they cannot be used in our VG project. If you happen to chance on an old globe made of metal you might be lucky enough in that the sphere may have a thread that joins the two hemispheres together. This would be an ideal purchase. Look in old antique or 2nd hand stores an markets for these items. If you happen to obtain a metal fabricated world globe you will need to remove the print or paint from the outside. You need to polish the globe to a shiny smooth finish to get maximum storage of HV potential from the Vandegraff unit. There are companies around that manufacture the perfect threaded spheres for vandegraff machines. You may be lucky to find a company that doesn't charge an arm or leg for such a purchase. For your first test bench prototype two stainless steel bowls will work perfectly. I am sure you will have loads of fun with this two bowl VG design in any case...|
You will need to cut a hole in the bottom of one of the stainless bowls. This hole is where the 80mm(o/d) PVC main centre tube an one of the flanges will be fixed. Use a steel scriber an the outer most 80mm(o/d) circumference of one of the PVC flanges as a template. Centre the flange on the bottom of the bowl an scribe around the outside of the circumference of the flange. The final hole size should be around 85 mm dia.(see figure 6) This will leave 2.5mm either side of the 80mm PVC pipe to be inserted. You will not see the extra gap as the top 80mm flange will be formed into shape to fit the both the main centre PVC pipe and the hole. You can use any method to cut out the hole. I would use a metal shop lathe or a metal hole saw. You may use which ever method you feel will work. Make sure you file a clean smooth burr free edge on the final hole cut. This will keep any HV loss to a minimum. Be sure that the outer 85mm(o/d) straight section of the PVC flange can be inserted into the hole with ease. Do not be concerned about the final alignment or the rest position of the top flange at this stage. You will use this hole size when heating and forming the flange into the curved shape. See below.
|Because the bottom of the stainless steel bowls have a curved dome bottom you will need to form only one of the PVC flanges into the curved shape of the bowl. (See top flange in figure 6.) You will need to use a heat gun or flame to heat the PVC flange until the plastic is soft an pliable. Only heat the outer most 135mm rim of the PVC as we don't|
|want to push the 85mm pipe diameter out of shape.(see figure 7) Use caution that you do not over heat the material to where it starts to burn the outer PVC layer. When you feel the flange 135mm rim is soft enough to form, quickly place the heated flange into the bottom bowl with the hole cut out. Using the second stainless steel bowl; place the top bowl inside the bottom bowl then press an hold the two bowls together firmly for around a minute. This will form the heated flange into the perfect curved shape of the bowl. Be sure you placed the heated flange from the top down into the bottom bowl and not from the bottom up direction, else you can not sandwich it between the bowls or be able to form the heated flange to shape. You must insert the heated flange (85mm section first) into the bottom bowl with the hole previously cut out else this will not work. So cut the hole in the bottom bowl first. Hint : Use an offcut waste piece of 80mm PVC pipe to hold the flange while heating the 135mm rim. This will also prevent the 85mm section from deforming during the heating process.|
Once the flange has cooled, fix the flange to the bottom of the bowl with silicon/glue and 4 pop rivets. Be sure the flange is centre most around the hole.Use the same 80mm PVC pipe waste offcut over the flange to get the flange centred over the hole in the bowl. The flange is placed on the bottom curve of the bowl from the bottom up.(see figure 3) Do not place the flange down through the hole as you did when forming the curved shape. The PVC flange must cover over any sharp metal edges that where left when cutting the hole in the bottom bowl. The flange will insulate any bad metal edges so that HV loss is reduced. After you have drilled and pop riveted the flanged onto the bottom bowl you can cover the pop rivets with a thin seal of silicon or insulation tape. This will help limit high voltage jetting out from the pop rivets.
Remember: avoid all sharp edges and points on the sphere construction. Smooth an shinny is the objective.
1 @ 600mm x 80 mm PVC tube
1 @ 80mm PVC flange
1 @ tube PVC glue
4 @ pop rivets
1 @ tube of silicon sealant.
2 @ 25mm x 25 mm shim steel
1 @ 30mm x 120 mm tin strip
|You may purchase the PVC pipe in 80mm or 90mm dia tubing. I used 80mm dia PVC for the construction of this VG unit. You may prefer to use the 90mm pipe. If you choose to use 90mm PVC pipe then be sure that you get the 90mm flanges an other material to suite the larger diameter pipe. Cut a piece of the 80mm PVC tube to a length of 600mm. This will be the main centre tube of the vandegraff unit. The belt will run up the centre of this tube to the top roller. Cutting and forming the top of the centre tube is as per figure 8 and 9.||
|You can use a handsaw, jigsaw or small grinder to cut and shape the top of the main tube. You may like to make the 35mm cut of figure 8 down to a length of 50mm if you wish. This 35mm cut allows you to adjust the belt to the correct tension. The more adjustment you can give the belt the better. You may find that your belt may need this extra adjustment. The other 30 x 50mm cut down the tube can be done in only one side of the tube if you prefer. It is not necessary to cut out this 50mm section from both sides of the tube. The 30 mm|
|wide x 50mm deep cut is where the top static comb/electrode will come through to interact with the top roller.(see figure 9,10) You can drill an pop rivet the static comb onto the bottom bowl when the bottom bowl and curved flange are glued into position on the centre tube. The curved flange of the bottom bowl should have already been glued and pop riveted to the bottom bowl. If you have not done this then do it before you glue the flange and bowl into position on the centre tube.The bowl/curved flange assembly will be glued 75mm down from the top of the tube.(see figure 9) You will need to leave the glue||
|dry on bowl/curved flange/centre tube assembly before you do any more with it. Once you have this assembly in place you will have an assembly that looks something like figure 10.The bottom flange and centre tube need to be moved up or down to find the best position. The bottom flange should not be glued into position until you have completed the motor mounting section. The final position of the centre tube and|
|bottom flange depends on your motor size and mounting location. You will need to have cut the hole in the top of the wood frame before you can mount the bottom flange. Once you have mounted the motor and the bottom roller is aligned centre most under the centre tube you will be able to slide the centre tube up an apply the PVC glue then move the tube back down to the final resting place. So only temporarily place the bottom flange onto the centre tube at this stage.|
|Once you have glued the bottom bowl/flange assembly onto the centre tube you can then pop rivet the top electrode comb into position. The comb can be made from any thin or sharp pointed metal. You can use a single needle or multiple points as in figure 12. I use thin (0.1) high tensile shim steel. You can buy shim steel in packs of various thickness from your local bearing supplies. Use the sharpest edge shim steel you can get. Cut a piece of shim steel 25mm wide x 25mm long. Cut one end of the shim steel strip into multiple sharp points with a pair of tin snips.|
|The more points you can cut into the shim steel the better. (See figure 12) You can then pop rivet the pointed shim steel onto the end of a piece of tin 30mm x 120mm long. Bend the tin strip into an 'S' shape so you can pop rivet the end of the tin down onto the bowl. You will have to manually bend the tin in an out as you adjust the electrode points optimum distance away from the belt. Important: Try to get the top comb/electrode points as close to the belt as you possibly can. This will give you the best voltage potential possible. Be watchful that you do not allow the points of the comb/electrode to touch the belt at any time. This will blunt the points and cut into the belt. The belt will flex outwards from the rollers as the motor runs to full speed. So watch that the points of comb/electrode do not touch the belt when the motor is running full speed. You will have to do adjustments to the comb/electrode to make allowances for full motor speed.|
1 @ Nylon food cutting board
1 @ 50mm dia Nylon66 round formed
2 @ 18mm x 6mm sealed bearings
1 @ 100mm x 6mm booker rod
1 @ 4 or 6mm steel shaft cut to length
4 @ 6mm nuts
You have a choice as to the roller fixing method you wish to use:
|If you chose the simple direct shaft rollers of figure 13 and 14 then you can make the rollers in a variety of ways. One way is to drill/lath a hole down the length of the nylon roller and push a longer length of steel rod firmly into the hole. Alternatively you could push a sized outer sleeve into the roller and fit a smaller threaded rod to have a more secure fixing method. (see figure 17 ) The roller choices are numerous. The advantages of making simple test rollers this way is|
|that you can test which nylon is working the way you would like. Once you have stable test results you could then opt for the better method of adding ball bearings into the rollers to ensure a long continuous life to your machine. (see figure 15 and 16) There are many various, tried and tested methods with roller materials and their construction.There is no absolute right or wrong method of construction. Feel free to add your own tried an tested methods in place of this.||
|This is my own preferred method of constructing the top roller(s). You will need access to a shop lath or machine to do the lathe work. If you have no access to these machines then you can use a hand drill machine as the nylon material is very soft and easy to machine.The outer diameter of the rollers can be any diameter you wish. You should not make the bottom roller a small diameter, else you will see the high voltage jump off the roller an short out onto the shaft of the motor. The bigger the roller diameter the less likely the HV will short out.|
|As a preference I like to keep my rollers to a diameter of around 40-50mm.(see figure 17 & 18) If you are just starting out you might like to do the same until you have mastered all the variables. I prefer not to have the top and bottom roller the same diameter. I believe this helps the voltage potential's. If the top roller is smaller than the bottom roller diameter then the top roller will be travelling at a greater rotation. I believe this greater rotational speed produces better friction on the belt which produces greater HV potential on the sphere condenser.||
|Important: You will need to use two different substance of nylon materials on the top an bottom rollers. The top roller needs to be made of a different density of nylon to the bottom roller. If you use the same material/substance on the top roller and bottom roller, then you will find problems with low or no HV potential's. If you must use the same nylon substance on the top an bottom rollers then you will need to sleeve over the bottom roller with a rubber or nylon sleeve of a different substance. It is the difference of these two nylon or rubber||
|substances which allows the VG to develop greater differential potential's. Only through your own experimentation can you prove which are the best combination of materials. As a hint, try to avoid black coloured rubber, plastics or nylons. The black colour could be carbon or iron. These substances conduct electricity. This conduction property is very undesirable and can cause untold frustration. --Avoid black materials in your search for better results. You can use a metal sleeve over a roller if the centre of the roller is insulated|
|away from any conduction. I have had moderate success using a hardwood lower roller with a metal sleeve around the circumference of the wood. You may play with all the variables to find the best results. I have been informed that teflon can also produce some good results. You may also like to try skate board wheels and/or sleeve over them.|
Personal choices for roller materials :
The above nylon choices will produce a negative potential on the vandegraff condenser sphere. Swap the materials top to bottom if you wish to produce a positive potential on the sphere condenser.
If you find that you would like a specific polarity potential from your vandegraff generator then you will have to experiment with the top and bottom roller materials. Firstly you will need to know how to check the voltage polarity potential of the vandegraff unit. Using a multi-meter from a safe distance, place positive(+) probe of the multimeter directly on to the static comb or motor mount earth of the vandegraff unit. From a distance, say 500mm away, point the negative(-) probe in the air towards the VG sphere condenser. DO NOT touch the negative multi-meter probe directly on to the metal sphere while the VG machine is running. The voltage developed on the VG may be in excess of 400kV. Sparks can jump through the air some 300mm-400mm in distance from the VG sphere condenser. The vandegraff generator only utilises micro amperage of current, the 400kV potential sparks will not harm you to any great degree, but there will be enough potential to probably destroy your multi-meter. If you still have no reading of the polarity then slowly move the negative lead in the air toward the VG sphere. If you start to get a stable reading and the meter reads as a normal reading, then you have confirmed that you have a negative potential on the VG sphere. If your multi-meter reading is showing a negative reading then you have a positive potential on the VG sphere. Be watchful that you have the flying leads of the multi-meter in the correct locations of the multi-meter. The reading you get from the multi-meter may be in milli-volts. You will never be able to read the true voltage reading from the vandegraff generator unless you own a special EHT meter or EHT probe. You can make an EHT probe by connecting 10 or more 10mega-ohm resistors in series. This will give you a slightly more accurate reading but will not be truly accurate. A rule-of-thumb for high voltage transmission through dry air is around 20,000 volts per 25mm(1") air gap.
Example rule-of-thumb: Hold one end of a plastic ruler in the air while touching the other end directly onto vandegraff sphere. Move your finger down the ruler until a spark jumps to your finger. If the sparks are jumping over a 250mm air gap, then you can roughly calculate you have a high voltage potential of 200kV. This depends on the moisture content in the air. The drier the day the better the build up of charge on the VG sphere.
Once you know how to check the polarity potential on the vandegraff; all you need do is swap the top & bottom rollers over. That is move the top nylon roller down to the bottom roller, and move the bottom nylon roller up to the top. You will now have reversed the polarity potential of your vandegraff. If the VG was first a negative potential then it will be a positive potential (or visa versa) when you swap the rollers from top to bottom. I prefer a negative potential vandegraff as there are some small health benefits from using negative ions, plus other interesting conditions when you treat plastics and nylons with HV potential's. These interesting conditions are principles based on how to make diodes and other semi conductor devices. I will leave this semi conductor topic for another web page article.... Yes your vandegraff generator can become more than an interesting toy. It has the potential to be a useful work tool or valued test bench device.
|The vandegraff belt is manufactured from a 1400mm piece of non-stretch type rubber upholstery chair webbing. Note : the rubber must be the natural beige colour an of the correct non-stretch type. Do not use the elastic stretch type or black coloured webbing as this may not build a static charge. Or if the belt stretches to much it may not stay fix to the rollers during operation.The non-stretch webbing used in|
|this design comes in large length rolls by 50mm wide. Ask at the shop where you purchased the rubber webbing to cut at least 2 meters off the larger roll to ensure you have enough length. Remember, a belt 650mm long will need about 1.4 meters of webbing. This 1.4 meter length includes the two 50mm splice joins at either end of the belt. (see figure 19) Do not be afraid to order too much. The webbing is 50mm wide, once the rubber is trimmed down there will be enough webbing to make two complete vandegraff belts each at 20mm wide.|
We will not be using the entire width of the belt so it needs to be trimmed down its entire length.Using a long metal straight edge, clamp the rubber webbing an straight edge to a firm flat bench top or length for wood. Using a sharp knife or blade, cut a 20mm wide strip down the entire length of the webbing. This will leave you with a belt 20mm wide by 2 meters long. Before you cut the belt to its final 1.4 meter length, it is a good idea to put the belt into the correct position on the vandegraff frame and roller mounts to see that you have the correct length to cover the distance around both the upper and lower rollers. If you are confident the belt is long enough then cut the belt to its final length. Do not forget to allow extra length on the belt for the 50mm splice joins. The rubber webbing is made 2 plies thick, separate 50mm of the ply from each end of the belt (see figure 19 ). It is important that you trim the two unused splice tabs from the correct side of the 50mm splice. Be sure the belt is layed out correctly and that there is no twists in the belt. You must remove one 50mm splice tab from the top of one end of the belt, an the other 50mm splice tab from the bottom of the opposite end of the belt (see figure 19 ). If you get these cut off tabs wrong the belt will not glue together correctly. Be sure to leave the string fibre layer inside the original ply on at least one of the 50mm tabs. If you don't the belt will stretch at the join and may break. Now using an 80 grit sand paper an a wood sanding block, sandpaper a taper at the end 50mm location of the splice tab cut and the main part of the belt. (see figure 19 ) This taper will allow a stronger lump free join in the belt. Its important to get no lump in the join so the belt will run around the rollers with out vibration to the vandegraff unit. The belt can run at a good speed so you want to get this splice correct.
You should be ready to glue the ends of the belt together. Add liberal amounts of contact cement to both ends of the 50mm splice tabs. Wait for a period where the contact cement feels tacky an almost dry to touch on both tabs. Slightly stretch the 50mm tabs of the belt and place the ends together to contact each other at the glue.When placing the ends together allow about 2mm of overlap from each end of the tabs. You will sand this 2mm lump off 24 hours after the glue dries. Make sure you have no twists in the belt when you place the joins of the belt together. Before the glue dries clamp the glued join between two lumps of wood firmly in a vice or clamp. Important : do not over tighten the vice or clamp else the pressure will squash the glue and rubber permanently out of position. Allow the glued join of the belt at least 24 hours to dry between the clamp before you attempt to do any thing further with it. After the glue has dried, sand paper the 2mm over lap lump off each end of the join until no lump is felt on either side. If your join has moved sideways in the clamp simply trim off any excess rubber sticking out from either side of the belt. Your belt should now be ready for use on your Vandegraff generator. Take particular note of which way the join splice end overlap travels when you place the belt on the generator. You do not want the sand papered join of the belt hitting the rollers foremost before the rest of the join. You must have the natural section of the belt travel over the rollers first before the sand papered join contacts the rollers. This will prevent the very ends of the join from coming loose at high speed. Be sure of the belts correct orientation and direction of travel around the rollers.
2 @ 300mm x 300mm wood panel board
1 @ 260mm x 300mm wood panel board
2 @ 25mm x 40mm x 300mm wood battens
2 @ 25mm dia x 260mm wooden dowel
1 @ 80mm PVC flange
1 @ tube PVA wood glue
12 @ 30mm x 3mm screws
4 @ 7mm x 30mm coach screws
|Assembly of the wood frame for the vandegraff generator is as per figures 20 ,21 and 22. The main dimension of the wooden frame is configured on a 300mm cube shape. If you have purchased the wood ply or panel board in one large sheet you will need to cut 3 individual 300mm x 300mm square panels from the main board. 1 of these board will have a further 40mm removed from one end. This panel will then become the back panel where you will mount the motor. To save on time and money I decided to build the wood frame with only three sides. As you can see from figure 21. The frame has a top panel a bottom||
figure 20 (top view)
|panel an a back panel. There are two 25mm dia round dowel uprights that support the very front of the wooden frame. Building the frame this way allows for different motor sizes and choices. It also allows for maximum access around the motor and bottom roller assembly where adjustments and changes can be made. You may also enclose this wooden frame further with acrylic or perspex plastic for a more cosmetic look and feel|
figure 21 (right view)
|Cutting the hole in the top panel is much the same as cutting the hole in the bottom stainless bowl. Using a pencil, mark a cross from diagonal to diagonal across the top panel board, this will give you the absolute centre of the board. Using the bottom 80mm(i/d) PVC flange, align the flange over the centre mark of the top panel, scribe a pencil mark around the outside circumference of the flange. This circular mark should be about 150mm from any edge. You can cut this hole with a hand jigsaw or large holesaw in a drill machine.The final hole should be 85mm in dia. Be sure the flange fits neatly into the hole. Do not be too|
|worried about the hole being ragged or
over size. The 135mm rim of the flange will cover over
any mistakes made. You do not need to fix the 80mm
PVC flange into position at this stage. Leave this
flange until last where we will also glue it to the main
centre tube. This will then fix the centre tube into its
Mark and Drill The Panel Screw Locations.
Remove the above PVC flange and secure the top and bottom wood panels together with a clamp or vice. Make sure all edges are flush to each other. Mark out the fixing screw location from the top view of figure 20. Use a 2mm drill bit, drill right through both the top and bottom panels at the same time. This will allow a more accurate alignment of the 25mm dowel uprights and the 40mm x 25mm wood strengthening battens to be fixed to the back panel. Once drilling is complete you can remove the clamp from panels and separate them.
Fixing The Battens To The Panels.
Cut the 2 @ 40mm x 25mm x 300mm battens to length.Do not confuse the battens with the round dowel uprights. Align the two battens to the top and bottom panels inside surface face, flush with the edge where you drilled 3 screws holes. Apply PVA wood glue to the 40mm underside of the aligned batten. Hold the batten secure with a clamp while fixing the batten from the top down through the 3 drilled holes. Fix the batten to the underside of the top panel with 3 fixing screws. Fix the batten to the topside of the bottom panel with 3 fixing screws.(see figure 21) Be sure you have the panels and drilled holes orientated an aligned the correct way before you glue and fix the two battens.
Assembling The Panels
|If you have not cut the back panel to size then, resize the back wood panel to 300mm x 260mm. by removing 40mm from one edge. Leave the back panel lying flat on the bench top. Be sure you have the 300mm edge align to the 300mm edge of the top and bottom panel. Stand the top and bottom panels up at right angles to the bench. Be sure that the fixed battens on the top and bottom panels are sitting down on the bench. Lift the back panel up and slip the top an bottom panel 25mm batten edge under the back panel. The back panel is now siting up off the bench and on the 25mm x 40mm wide battens. Using the||
figure 22 (front view)
|top panel apply glue to the 25mm edge of
the batten. Hold the top panel-batten and the back panel
into position with two clamps. Drill 3 holes @ 2mm dia
along the inside top edge of the back and top panel into
the batten. Be careful not to drill into the original
screws coming from the other side of the top
panel-batten. Fit 3 fixing screws into the drilled holes.
This will fix the back panel and top panel together.
Repeat the steps for the back and bottom panel. Ie. Glue,
clamp, drill and fix the back panel to the bottom panel
and batten using the same procedure as the top panel. (see figure 21)
Fitting The Uprights & Flange
|The final part of the wooden frame is to fit is the 2 @ 260mm x 25mm dia wooden dowel uprights. Cut both of the dowels to length. You should have already drilled the two holes to fit the dowel uprights into the panels.(see figure 20) These two holes are 30mm in from the front corners of the frame. Apply PVA wood glue to the very ends of each of the dowel uprights. Using a fixing screw down from the top panel and up from the bottom panel fix the uprights||
|permanently into their final position. Flip the box into the upright position with the flange hole toward the top. You are now ready to fix the bottom 80mm PVC flange into the hole of the top panel. Place the PVC flange down into the hole and fix the flange into position with 4 fixing screws.(see figure 24). Once you have complete the last piece of the wooden frame you should have an assembly that looks something like figure 23. I chose to place the batten supports to the||
|outside of the frame in my design. This allowed better freedom and movement when mounting the electrical motor. The battens could pose an obstacle to the motor mount when fitting or adjusting the motor position. You may chose to place the battens towards the inside of the frame, that is quite okay as it shouldn't make a lot of difference.|
1 @ electrical motor 12 - 240 volt.
|Depending on the motor you chose you can have from a 12V ac/dc all the way up to mains powered 240Vac motor to run on your vandegraff generator. The motor you choose is your choice and won't make a lot of difference to the operations of the machine. I used a variable 240Vac motor from an old vacuum cleaner. The motor chosen was a series wound motor, an is far too fast for the VG machine. This motor will blow the belt and rollers apart if|
|it where left to run out to full speed. If you get a series would motor be sure to limit the maximum speed of the motor. Caution: if you are not sure about wiring a mains powered motor then have some one with electrical experience help you wire the motor from the mains. There is no set mounting instructions or key dimensions for mounting the motor. Just try to centre the bottom roller over the mid point of the main centre tube. View down through the top of the tube to be certain you have the bottom roller centre most under the tube. You may have to build a special mounting frame or add packing under the motor to get the required distance from the back panel to the roller centre position. You may also move the actual wooden back panel location in further toward the centre to better mount the motor. I have mounted my motor from on the back panel because this allows for more adjustment to the belt. If I break the belt I can join it again and move the motor up to accommodate the loss in belt length. You may decide to mount the motor down on the bottom panel. That is quite ok, but you will have to add packing under the motor to get the belt tension correct. The top roller will only adjust to a set length. So you will need to move the motor to get any additional adjustment to the belt tension. Once you have your motor mounted, your vandegraff unit may look something like figure 25.|
|The bottom comb/electrode construction is very similar to the top electrode. Once again construct the comb/electrode from 25mm x 25mm x 0.1 shim steel. Cut sharp points into the shim steel. Pop rivet the shim steel electrode onto a 30mm x 60mm piece of tin. You can then pop rivet this piece of tin an electrode onto the main motor or motor mount. (see figure 26) You can then manually bend the tin in an out to allow for close proximity adjustments and fine tuning to the belt electrode distance. Only trial and error can get this adjustment to the best position. Remember the closer you get the comb/electrode to the belt, the||
|better the possible potential available to the sphere condenser. I chose to mount the comb/electrode directly onto the motor. By doing this you eliminate the need to supply an earth or ground wire to the electrode. If you choose to mount the electrode from the bottom of the wooden frame then you will need to add a wire to supply the ground potential to the electrode. You can terminate this ground wire back onto the motor if you so choose.|
1 @ 1.4m x 20mm Belt
1 @ Tin PVC glue
|Once you have the motor mounted you are ready to fit the belt, and glue the centre tube to the bottom flange.This is the final stage. Place the belt onto the Top roller and secure the roller into position. Place the sphere-centre tube assembly down into the bottom flange of the wooden frame. Now fit the belt around the bottom roller of the motor. Be sure get this tube positioned square with the belt an the rollers. If you turn the tube to far one way or the other the belt will fall off the rollers. Manually hold the tube up with both hands so the|
|tension is on the belt and both rollers.View down from the top of the centre tube to see you have the belt running straight an square on the rollers without twists or bends. Have some one turn the motor by hand so you can turn the tube to get the belt running square with the top and bottom roller. Do not worry about the top roller to much, as this can be adjusted by raising or lowering either side of the roller. Just allow yourself plenty of adjustment on the top roller. When you have the belt running square with the rollers you can mark a pencil mark around the flange and the centre tube. At the intersection of the flange and centre tube place a pencil mark around the circumference of the tube for the correct vertical position(tube height). Place a pencil mark vertical on the tube and flange for the correct horizontal position. Place a pencil mark at the bottom side of the flange and tube so you know how much glue to apply to the tube. Remove the belt off the bottom roller and slide the tube upwards to where you can see the bottom pencil mark. Apply PVC glue to the centre tube between the upper and lower pencil marks. Slide the centre tube back down into position and refit the belt onto the bottom roller. Align both the horizontal and vertical pencils marks. Hold the centre tube in this positions for about a minute or until the glue becomes firm. Be sure you get the tube into the original position aligned with your pencil marks. PVC glue dries very quickly so be sure about your settings. The final position of your centre tube may be similar to the dimensions of figure 27.|
Once you have the vandegraff generator assembled you will need to make running adjustments and fine tuning. You may need to raise and lower either side of the top roller so the belt positions itself centre most on the roller. You may need to raise or lower either side of the motor slightly to get the belt to position itself centre on the bottom roller. You may need to adjust the top and bottom comb/electrodes to find the optimum distance away from the belt while the vandegraff is running. These adjustments are straight forward but might require a little perseverance. Get the belt running correctly first before you adjust the top and bottom comb/electrodes.
You may like to rub the inside surface of the belt with methylated spirits and a light sand paper to remove any contaminates that where put there during assembly. Turn the VG machine on and carefully rub a slightly damp methylated rag on the inside of the belt and rollers. Touch the inside of the belt with light sand paper to rough the surface and cause rubber dust on the belt to produce friction on the rollers. Do not touch the rollers with sand paper. This should only need to be done once in the life of the vandegraff generator. Keep dirty oily fingers off the VG belt. Do not wipe the rubber dust off the inside of belt, leave it there while the belt is new.
Clean the sphere/condenser with methylated spirits and a clean rag. Tape the bowls together with a complete strip of insulation tape around the circumference of the two bowls. One and a half circles of insulation tape will be suffice.
You can do many thing with your vandegraff unit...
Fun for kids of all ages.
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Hello again. I see that you are seeking even higher voltages from your Van DeGraaf machine. I have another suggestion that will help with the normal leakage that will limit your upper voltage.
The PVC pipe absorbs moisture, so if you bake it (in a cardboard/metal box with small light bulbs for a week or so, about 150 deg F) then coat it right away with polyurethane.
Be careful of the fire hazard of the light in the box, I put the light in a tin can and not touching anything. Be sure to coat the tube inside and out.
Another way to compensate for the leakage that will occur is to increase the charge transfered per rotation of the belt.
Instead of generating the charge with the belt/roller contact, inject the charge onto the belt. Use a high voltage DC source like a air ionizer or static dust precipator or make your own.
The higher the voltage the better. 5000 VDC will work, but you will start to get a strong ionic wind above 15KV.
Connect the hivoltage supply to a comb mesh that is positioned with the points of the mess very close to the lower roller.
The charge will be injected onto the belt at a much higher rate than could ever be generated by a small roller at normal speeds. Clark Boyd
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