From: Ronald Wong (ronwong@inreach.com)
Date: Thu Feb 13 2003 - 00:29:37 PST
Message-Id: <l03102800ba6f37bc4145@[209.209.18.96]> Date: Thu, 13 Feb 2003 00:29:37 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: RE: Wimshurst Machine
Recently Eric Plett asked:
>...
>First of all I had to replace the belt on the Vandegraaff and I'm having a
>problem that I've never had before. I was testing the machine out after
>replacing the belt and I noticed that the machine was very warm at the
>motor. Does that mean that I have a short? The machine is working just not
>very well and the heat worries me!
Not having the machine in front of me makes a complete diagnosis hard to
come by but one thing that may be wrong is that the motor is not turning at
it's prescribed rpm's. This would reduce the back emf and lead to a current
flowing through the motor's windings that is larger than it was designed to
handle - thus the excess heat.
Does the motor seem to be spinning at a lower speed than before when you
were using the old belt?
1. Take the new belt off and check to see if the pulleys at each end of the
support-tube are rotating freely about their axes.
Whether or not they are free to move, lubricate them. This is as good a
time as any to do this just to ensure that they will be rotating freely in
the future.
2. You may have the wrong belt for your machine. It may be the wrong
length. If so, there may be too much of a load on the bearings of the
pulleys.
Put the belt back on and, with the dome off, try to see how easy it is to
move the belt along by running the palm of your hand across the part of the
belt that goes over the top pulley. It should take next to no effort at all
to move it along.
Check to make sure that the barbed combs at each end of the support-tube
are not brushing up against the belt. There should be a noticeable gap
between the belt and the tips of the comb.
3. If the belt and pulley pass this 2nd test, turn on the motor while the
dome is off and observe the belt's motion.
In poorly designed generators, the belt wanders around on the pulleys and
sometimes rubs against parts of the apparatus or, when the belt is too
long, against each other as they pass up the length of the
supporting-column.
Frequently, the shaft of the motor is connected to the lower pulley by a
coupling. Check to make sure the coupling isn't binding in some way or
another.
If everything seems fine then check your motor during this test to see if
it's still getting "very warm". Motors run warm but if the heat is still
excessive, you may need to consider getting a new motor (although I
seriously doubt that). Then again, it might be the bearings of the motor
that are shot (or the brushes?).
At this point, I don't know what else to suggest. The generator, in it's
basic form, is a very simple device.
>
>Second, for the Wimshurst, is the answer the same as the 'chain of monks'.
>Charge up the Wimshurst, make a chain and each end grabs one of the
>discharge balls . . . Safe? I just did it last night with my family and my
>wife thought that it hurt at the wrists.
If you have the standard Wimshurst machine that one sees in an educational
science catalog, it should be safe. The voltages generated are in the 10's
of thousands and not the 100's of thousands that the demonstration Van De
Graaff generator can produce.
The big difference is that the Wimshurst machine stores it's charge very
efficiently in the two Leyden jars that come with the machine. Although the
voltages are lower, the current can be higher than with a Van De Graaff
machine. As a result, the physical reaction to the discharge is frequently
greater.
You can remove the Leyden jars from the circuit of many Wimshurst machines
and thus reduce the current in the subsequent discharges if this is a
problem.
Lots of luck.
ron
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