HUMIDITY
First see the article about SOLVING HUMIDITY PROBLEMS. VandeGraff
generators need low humidity in order to function. Humid air
causes insulating surfaces to become slightly damp, and they become a bit
conductive. If you build a brand new generator and it won't work the very
first time you run it, your generator might be bug-free, yet
it will make no sparks at all. Before looking for other problem,
always try the blow-dryer trick
first.
And use the balloon/armhair test to verify low humidity.
OIL
Oil and VDG machines don't mix. If the tiniest bit of oil gets on the
rollers and belt, their surfaces will no longer touch. (Oil touching oil
does not generate contact-electrification.) If you must lube your motor
bearings, use great care and definitely DON'T use a WD-40 spray can under
any circumstances. If you suspect that oil has coated your system,
remove and clean both rollers and scrub down your belt with alcohol.
Don't just wet and scrub, also use a bit of running alcohol, then rinse
under running water. Your goal is to flush off the microscopically thin
oil film, and if you don't flush with running water, you'll only stir the
oil film around without actually removing it. If you're really paranoid
you can even wear rubber gloves to eliminate contamination from skin
oils. (At least scrub your hands thoroughly before starting the cleaning
process.)
If one roller is made of felt, it might make more sense to replace the
felt or the roller, rather than trying to clean out all the soaked-in
bits of microscopic oil.)
CHOICE OF MATERIALS
What did you use for a belt? Beware, some types of black rubber are
conductive and will not work. They're full of carbon, and will fail when
installed in a VDG. They short everything out. Grey rubber or light tan
rubber should be OK. To test your rubber for conductivity, simply rub a
piece of it on your arm hair, then hold it near that hair and verify that
the rubber surface attracts the hair and makes it rise. (Of course this
test can only be performed when humidity is low!) If in doubt, avoid
using black rubber.
Is your main support column made of insulating material? New and clean
plastic water or sewer pipe is good. Wood is bad; it becomes
electrically conductive when the weather is damp. Cardboard is similar:
humidity makes it conduct. Assume that wood and cardboard are "resistor"
materials, they make poor insulators unless they are baked in an oven and
sealed to prevent water absorption. Stay with plastic instead. (If you
have no choice, then bake your cardboard or wood until it is extremely
dry, then coat it with layers of oil-based varnish.)
Roller materials are an issue. There should be no problem if one roller
is polyethelene, and if the other roller is metal, and your belt is
rubber. Some kinds of belt material and roller materials may not work.
For your very first generator project, it is wise to use a reliable
design. Once you have it working, you can experiment with other
materials. If you have built a "powered" type of generator (having a DC
hi-volt power supply inside,) then you can use metal for both rollers.
For non-powered VDG machines, the surfaces of the rollers MUST be made
from two different materials. (Think: if both roller surfaces were the
same, then how could the generator "know" which end was (+) and which
end was (-)? ) PS: if at least one roller is plastic, then you can
experiment by placing other kinds of insulator on its surface. Try
rubber sheets, teflon pipe-tape, paint or varnish, and verious kinds of
adhesive tape.
In a contact-electrified or "frictional" type of generator (w/no HV
supply inside), avoid making both rollers from metal even if you plan on
coating one with plastic. A plastic-coated metal roller will probably
form a sort of capacitor, and this would greatly reduce the roller's
voltage. In a capacitor, opposite charges are attracted from the metal
and then sit on the metal just under the dielectric surface. These
opposite charges would vastly reduce the e-fields at the tips of the
combs. So, if you intend to experiment with various roller materials,
it's better to use two plastic rollers, then coat them with desired
materials (tape, aluminum foil, etc.)
COMBS
Are your combs positioned properly? The tips of the combs must not drag
upon the moving belt. The tips of the combs should be within 1/4" of the
surface of the belt. Hint for adjusting combs: short out your generator
with a microamp meter, then set the comb positions for maximum
microamperes (typically 5uA to 20uA). It's possible to make a microampere
meter for a couple of bucks: an NE-2 neon bulb with a 0.01uF capacitor,
and maybe a 1Meg resistor in series, for safety. The faster it flashes,
the higher the microamps. (This from the old CL Stong, F. Lee
Scientific American article
on
atom smasher" generating a half-megavolt electron beam with VDG and hard
vacuum drift-tube.)
The combs must be properly connected. The upper comb must be electrically
connected to the hollow sphere. The lower comb must be electrically
connected to either the grounded case, or if you are building the
"powered" style of generator rather than the "frictional" style, to the
power-supply terminal
REDUCED VOLTAGE
Does everything in your generator look OK, yet it will only produce tiny
sparks? This problem can have several causes.
Normal electrode leakage: if you bring your fingertip near the generator,
no long sparks will jump. This is normal. Your fingertip is too "sharp",
and so it creates a corona discharge (tiny blue glowing spot seen in the
dark). A corona discharge creates conductive air, which allows the charge
to silently flow to your fingertip. To make longer sparks, bring the back
of your wrist or the side of your arm near the main generator sphere. Or
hold the bottom of a metal mixing bowl near the main sphere.
Sharp Edges: if your generator's main sphere has protrusions, then regions
of corona discharge will appear upon the protrusions. The charge will not
build up on the sphere, instead it will leak into the air. External
protrusions such as screws, joints, edges of mixing bowls, etc., must be
eliminated. Plastic protrusions are OK, it is usually only the sharp
edges of metal which cause problems. Protrusions within the sphere are
OK. The inside of your generator sphere can be a sharp-edged mess, it's
only the outer surface which is important. (Keep any sharp-edged metal
parts a couple of inches away from the belt and roller.) The outside of
your generator's sphere should be smooth, and should have a donut-shaped
area where the belt and support column enter the sphere. Quick and dirty
fix: put thick layers of silicone caulk on any metal protrusions (thicker
than 1cm.)
Tiny burrs: if a sharp metal part is exposed to strong e-fields, corona
will appear, and electric charge will leak into the air. Therefor, make
sure there are no sharp metal parts near the moving belt. If there are
metal parts near the belt, they should be large and smooth, and have no
sharp edges or tiny burrs. If in doubt, cover them with a thick coating
of silicone caulk.
Corona jets: if the main sphere has some invisible corona leakage,
sometimes you can find the trouble spot by searching for air jets. Tiny
sparks of corona discharge will emit charged wind, and this charged wind
feels ice cold. While your generator is operating, wave your hands slowly
around the sphere while keeping them a few inches away from the metal. If
you feel a jet of cold air, then you have found a source of leakage. My
lips and face are more sensitive than my hands, so I move my face all
around the sphere in order to locate the tiny sharp spots which emit cold,
charged air.
First see the article about SOLVING HUMIDITY PROBLEMS. Commercial
generators usually fail because of a combination of dirt and humidity. If
the column, belt, or rollers are clean, then the generator might work ok
even in fairly humid conditions. If they are dirty, then the generator
might fail even if it is fairly dry out.
If you are certain that humidity is not your problem, verify that the
belt and rollers move when the switch is turned on. You might hear the
electric motor running, but does it actually spin the rollers and make the
belt travel around the loop?
Verify that the "combs" are electrically conected properly. The lower
comb must be connected to the metal of the base, and thence to ground.
Or, if your generator uses a separate Hi-volt power supply within the
base, then the lower comb must be properly connected to the power supply
terminal. The upper comb must be electrically connected to the hollow
sphere. The tips of the combs must within 1/4 inch of the belt. They
should not drag on the belt; they do not need to touch it at all.
And as I mentioned above, a microamp meter is very handy for tuning up the
belt/rollers/combs assembly. Use a microamp meter to short out your
generator by connecting the meter leads to the sphere and to the metal
base (or to ground.) Run the generator and the meter should indicate a
few microamps. Do anything you can to make this value higher (tweek the
comb positions, remove and clean the belt, dry the parts with blow-dryer,
etc.) This improves the charging current but it cannot fix high-voltage
problems caused by filth on the column or dust-speck corona-points
attracted to the main sphere.
If your generator still does not work, try more of the fixes discussed at
the top of the page. In particular see the part about oil contamination.
.