Okay - let's say you've managed to build the thing and get it going
satisfactorily, as described in the
previous page
. Now what?
Well, that's only the beginning. There are plenty of experiments you can do,
varying from quite simple observations to some quite advanced and
thought-provoking investigations.
When my Dad and I first built this project, there was among my Dad's
possessions a small neon light bulb, of a similar size to a 15W pilot light,
which ran off the mains. There was also an extension cable with a bayonet
light fitting on the end. With this, we could move the light reasonably
close to the little machine in a darkened room and observe certain effects.
As mentioned earlier, our original flywheel was white with a red dot. Neon
lights (in a clear, colourless glass case) produce a soft orange glow. With
this, the red dot showed up very nicely against a subdued orange background.
A neon light powered by AC flickers at double the mains frequency, just as a
fluorescent tube does; so four equally-spaced reddish patches appeared on
the wheel when it was running. With the light held close to the wheel, the
effect was quite pronounced.
Unfortunately, the neon light has disappeared! I can't find it anywhere.
I've never seen anything like it anywhere else; I suspect they're a thing of
the past, and can no longer be purchased.
However, all is not lost! We now live in the age of the Light-Emitting Diode
(or LED). These little electronic devices have been developed over the last
forty-odd years to the point where it is now possible to obtain very bright
ones, in several different colours, for a reasonable price from electronics
shops.
Being diodes, they pass current in only one direction. Normally, they are
wired into DC circuits the "right way around", so that they will operate as
lights without any bother. Nobody in their right mind connects them the
"wrong way around" (unless by mistake).
However! Here we have a use for them in an AC application, in which we can
very usefully exploit the fact that they only pass current in one direction.
If we wire two LED's of different colours (red and green, say) with the
anode of each connected to the cathode of the other, put the parallel
combination in series with a suitable current-limiting resistor, and run the
whole thing from a suitable AC source, this is the result:
In one half of the cycle, the red LED comes on; and in the other half, the
green LED comes on. Only one is ever on at a time; and they are both off for
a moment twice in each cycle as the voltage changes through 0V.
A simple circuit like this is very useful for viewing the synchronous wheel.
As it spins, you will still see the four equally-spaced bright spots - just
as under the fluorescent lamp or the neon light - but this time, two of the
spots (diametrically-opposed) are red, and the other two are green. This
emphasizes the fact that the wheel rotates once every two cycles.
If only one LED is connected - say the red one - only two spots will appear,
red in this case, and diametrically-opposed.
These are stroboscopic effects. There's plenty more to be said about
them and done with them. Read on!
Having succeeded in getting the rotor spinning happily at the "normal"
speed of half the supply frequency, if you then switch off the electromagnet
but keep the lighting system switched on, as the rotor starts to slow down
you will initially see the four spots appear to rotate in the direction
opposite to that of the wheel's rotation.
Very soon, they will start to blur together. Then the blur will separate out
into five spots, apparently spinning in the same direction as the rotor.
This apparent rotation will slow down, and the spots will become less blurry
as they stop completely; then they will appear to start spinning in the
opposite direction at an increasing rate and blur together again.
Shortly thereafter, six spots will appear, apparently spinning increasingly
slowly in the same direction as the rotor; they too will appear to stop, and
then begin to rotate in the opposite direction and ultimately blur together
yet again.
This trend will continue, with the effects happening ever faster and for
shorter durations until you really can't see any details. Eventually, the
wheel will be spinning so slowly that the eye can follow the single dot as
the rotor eventually slows to a stop. (Mine takes just over a minute to
grind to a halt once the power is switched off.)
This same effect is at work when the wheel is spinning stably, when the
spots appear to be slightly out of alignment with the poles of the
electromagnet:
wheel spinning clockwise
wheel spinning anticlockwise
As mentioned in the
previous page
, frictional forces are always present in the model, tending to slow the
rotor's rotation, and the electromagnet supplies enough attractive force
every half-cycle to keep it from stalling. So the wheel is always lagging
behind the light, which is (for practical purposes) in step with the AC
supply. The greater the friction, the more the lag (measured in degrees) of
the wheel, and hence of the spots.
My own model appears to lag by about 15 degrees. (This is the figure I used
when preparing the two spinning-flywheel graphics above.)
Thus, if you want to adjust your model to run as efficiently as possible,
you can gauge how well you are doing by the size of the lag angle. The
smaller the angle (i.e. the better the spots line up with the magnetic
field), the better the model must be running.
We'll return to the idea of playing around with coloured lights shortly.
In the meantime, let's keep our main attention on the wheel itself.
A natural question to ask is: can it spin at any speed other than half the
AC supply frequency (i.e. 25 revs/sec in Australia)?
I know from experience that it can. When we first made our model, we would
sometimes be able to spin it at half that speed (or one-quarter of the
supply frequency), i.e. 12.5 revs/sec. It wasn't easy, and the rotor was only
just managing to keep going at that low speed; but we did occasionally
succeed in doing it. Under the neon lamp, it would then show eight spots,
spaced around the wheel at 45-degree intervals. (Clearly, if this were done
under a two-LED lighting system like that described above, you would see
four spots of each colour, alternately spaced.) Spend a few moments
considering how the rotor must interact with the magnetic field under these
conditions.
(Since having rebuilt the project, I haven't yet succeeded in making it spin
at 12.5 revs/sec.)
How about a speed slower than the usual speed of half the supply frequency,
but greater than one-quarter of that frequency? In particular, could the
rotor be made to spin stably at a speed equivalent to one-third of
the supply frequency?
At that speed, six bright spots would be visible under a fluorescent light,
spaced at 60 degrees. With the two-LED system, three of the spots would
appear red, alternating with three green spots.
I can't recall ever having succeeded in making this happen; but neither can
I see any reason in principle why it shouldn't be possible - although I
suspect that the model may need to be extremely well-built, with frictional
losses - and the associated lag angle - reduced to an absolute minimum, for
this to occur. (My model may not perhaps meet the required standard.) If you
build the project and achieve such a result, I'd be very interested to hear
about it!
Can the rotor be made to spin faster than half the supply frequency?
In his book, Bulman suggests that it is possible to get it spinning at twice
that speed (i.e. at one revolution per AC cycle), or at some other low
multiple of that speed. However, I've never managed to do it. About the
fastest that I can spin it is so that just three spots initially appear
briefly under a fluorescent light, indicating a rotational speed of
two-thirds the supply frequency (can you see why?), and the rotor is shaking
rather badly at that speed. It then rapidly slows down.
Presumably, if the project were made very carefully and to extremely high
standards of workmanship, it would be possible to have it spin at quite high
speeds. This comes down to how much time, effort, and possibly expense a
person is prepared to commit in order to achieve a top-quality result. At
what point does it cease to worth the bother?
While on the subject of high rotational speed:
I mentioned in the
previous page
that just spinning the rotor at high speed and hoping it would "lock in" to
a stable rotation as it slows down was a waste of time. If it happens at
all, such an incident may reasonably be called a fluke. (If you build a
model which proves me wrong on this point, I'd very much like to hear about
it!)
However, at some point I did discover a useful little trick. If I spun the
rotor fast - using some kind of strobing light system to monitor it - and
then, as it slowed down, very lightly brushed the end of my index finger
against the needle's length above the flywheel in the same direction as the
rotation just as the four spots were coming into view, the rotor could be
persuaded to "lock in". I do it now almost without having to think about it,
and it saves a lot of frustration. I recommend that you try it yourself!
So far, we've only considered effects which have to do with a power source
of a single frequency. If we run both the project and a strobing light
system ultimately from the mains, we are restricted to observing the kinds
of effects so far described. Interesting as these are, there's plenty more
that can be done with some extra equipment.
If you can gain access to a signal generator, you can use it to run a pair
of LED's at a frequency different from that of the supply used to drive the
synchronous wheel's electromagnet. Again, you will need to use an
appropriate current-limiting resistor. (The important thing with LED's is
that the current needs to be kept down to no more than 30mA, usually, to avoid
damage - and you may well find that with really bright LED's, you can use a
lot less current than that to get a good effect.)
So: we set the rotor happily spinning at its usual speed. Starting with the
signal generator set to the same frequency as that of the power supply, we
observe the flywheel. What will we see?
Assuming that the frequencies are exactly the same, we should see
four stationary spots, two of each colour as described earlier. You may well
find, however, that the apparent "lag" is quite different from that
previously observed (i.e. about 15 degrees, in the case of my model).
In the very unlikely event that the signal from the "sig-gen" is
in phase with the power supply , then of course the "lag" will be the
same as before. Otherwise, it will be different!
Let's assume for the moment that the rotor is spinning clockwise.
Now: adjust the sig-gen's frequency ever so slightly. What will happen?
If the frequency is increased, the flywheel will not quite complete a
quarter of a rotation in the time between consecutive flashes of light. Thus
the spots of light will appear to rotate slowly anticlockwise.
If the frequency is increased further, the speed of apparent rotation of the
spots will increase. Eventually they will blur together, and then reappear
as five spots. With further increases in frequency, the spots will
again blur together, and then reappear as six spots; and so on.
Here's something to think about: if the number of spots is even, then half
them will appear red and half green, alternately; but what will happen if
there are an odd number of spots?
On the other hand, if the frequency is decreased, the spots will
appear to rotate clockwise, as the flywheel completes slightly more
than a quarter of a rotation in the time between flashes.
Decreasing the frequency further will increase the apparent speed of
rotation of the spots. Again, blurring will occur; this time they will
reappear as only three spots; then two; and eventually only one.
Further decreases in frequency will produce only a blur.
Clearly, if the rotor is spinning anticlockwise instead, then the
above analysis will be correct, provided that the words "clockwise" and
"anticlockwise" are interchanged.
It's also possible to use the signal generator to control the actual speed
of rotation of the wheel. PLEASE NOTE, however, that it's probably
not a good idea to connect it to the electromagnet directly!
A signal generator is designed to deliver a signal of sufficient
strength to be used as an input to an electronic test device such as an
oscilloscope, not to provide a source of power - at least, almost
certainly not enough power to run this project.
What is needed is a suitable power amplifier - a device which can take a
signal from the sig-gen as input, and produce a much more powerful output
waveform with the same shape and frequency characteristics as the sig-gen;
this stronger signal may then be used to run the electromagnet.
You may be able to build or otherwise obtain such a amplifier. At this
stage, I do not have a suitable circuit that I can publish here. This is a
project which I may attempt at a later stage. (If you have a suitable
circuit which you are prepared to share, perhaps you could send it to me,
and I'll then place it in this page, subject to appropriate copyright
considerations.) In the meantime, what follows should be viewed simply as a
source of ideas.
Assuming, then, that you have a sig-gen and a suitable power amplifier, you
can use them to do similar experiments to those suggested above - but with
the rôles of wheel and lights interchanged.
However, you can also use such a set-up to test the performance of
your synchronous wheel. You could very gradually reduce or increase the
frequency to the point where the wheel stalls, and thus determine its
"operating range". You could read the frequency straight from the sig-gen's
dial, if it is accurate enough, or use an oscilloscope - or even a frequency
counter, if you have such available.
If you are fortunate enough to have access to two signal generators,
you can really have some fun. You can use one of them (with a suitable power
amplifier) to run the wheel, and the other to run the LED's. Your
imagination is the only limit with a neat setup such as this.
Alternatively, you could just run the wheel from the usual AC power source,
and use two sig-gens to run two pairs of LED's (red/green and blue/white,
for instance). Generate your own miniature psychedelic light show!
If you are using one or more signal generators in any of the ways suggested
above, and if you also have access to a suitable oscilloscope, you could
produce
Lissajous figures
, like those shown above, with the signals used to run the electromagnet and
the "light show" also used as inputs to the 'scope, set to X-Y operation.
This, in combination with some method of projecting the "light show" onto a
screen, could provide a powerful teaching tool in a suitable setting. (If
you are a teacher or lecturer who decides to do this, I'd like to think that
you'd provide your students with the
URL of this website
, of course. Credit where it's due...!)
Finally: some comments about photographing the synchronous wheel, with a few
examples.
Whenever any camera is used to take pictures of a moving object, there will
be some blurring effect. The only way to reduce this to the point where it
is not noticeable is to use a special high-speed camera.
Even in a movie, the individual frames contain some "blurriness". However,
if the frames are photographed - and replayed - at a high enough speed, the
human eye is fooled into thinking that smooth motion is being viewed. The
minimum rate needed for such an effect is usually given as 24 frames per
second.
With a digital camera capable of taking short movies (mpeg's), it is
possible to observe some interesting effects with the synchronous wheel. I
borrowed such a camera from a friend and used it to photograph this and
other projects within these pages.
Here is a picture of the wheel, taken with this camera in normal ambient
light, rotating at 25 revs/sec:
Interestingly, the white dot appears as a large blur going almost all the
way around the flywheel. What are we to make of that?
Well, I'm not sure what to make of it. I don't really know exactly
how the camera works, or what its capabilities are.
I took some mpeg's of this and other projects while I still had the camera,
with some surprising results. With the synchronous wheel rotating at 25
revs/sec, we saw a single, stationary, very wide white spot, just as in the
still photograph above:
Mad Teddy's synchronous wheel, mpeg 1 (5 seconds,
~119.3Kb)
Thus, apparently, the camera operates at 25 frames per second when filming
mpegs. This ties in with what I have read elsewhere.
In this and other mpegs I took of the project, you can occasionally see a
slight drift - either clockwise or anticlockwise - of the "spot". Any such
effect seems more likely to be due to deviation of the camera's frame-rate
from exactly 25 frames per second, rather than variations in the mains
frequency - but I'm not sure!
Here's another mpeg, taken immediately after the wheel had been started
spinning and while it was settling into its stable pattern. You can see the
spot swinging from side to side (with very gradually decreasing swings), and
also hear the soft, slow throbbing "whirr-whirr-whirr" effect I
mentioned in the
previous page
:
Mad Teddy's synchronous wheel, mpeg 2 (5 seconds,
~119.5Kb)
(At the time of writing, I'm doing experiments - getting the wheel spinning,
and then using my watch's stop-watch facility - to try to establish a
relationship between "whirr-rate" and supply voltage and frequency. No
theories yet...)
For the next mpeg, I asked my wife to switch off the power to the wheel
as soon as I'd started filming (you can hear me say "now"). This one goes
for a full minute - almost enough to record the entire slow-down. You can
observe some astonishing and very intriguing phenomena:
Mad Teddy's synchronous wheel, mpeg 3 (1 minute,
~1.3Mb)
Having filmed these various weird effects, I'd really like to know more
about exactly what is happening. It seems that my little motor has become an
instrument for investigating of the workings of another, more complex device
- the camera!
A PARTING SHOT
The pictures and movies on these and other pages in this website are only a
very small subset of the many taken while I had access to the camera. (I
took lots to make sure I had plenty of material to draw on.) I had the
camera for the 2005 Easter break, which gave me several days - but there was
a lot I wanted to do with it, and I seemed to be flat out. At various
stages, I got a bit frazzled with it all.
At one point while taking movies of this model, my son was helping by
switching off the power once I'd started filming. The first time, when I
said "now", he didn't realize that I meant to switch off immediately so that
almost the entire movie would feature the slow-down; my fault because I
hadn't made it sufficiently clear. (It wouldn't have been a very good movie
anyway: the lighting was too dim, and I was much too far from the action.)
The resulting mix-up was captured, and I decided to feature it here just for
a chuckle:
Mad Teddy's synchronous wheel, mpeg 4 (11
seconds, ~253.5Kb)
Note that, in all four movies, the wheel is spinning clockwise.
CONCLUSION
In his book, "Model Making for Young Physicists", early in the chapter
headed "A synchronous wheel", A.D.Bulman says:
"The motor here described is little more than a spinning top but it clearly
illustrates this principle of synchronism."
I think he's far too modest. In the early twenty-first century, when
"education" often means little more than rather narrow training for a
particular vocation, educational books like this one and others
mentioned in these pages shine like beacons, showing what education was once
about - and what hopefully it will be again in time to come: training for
the mind.
For all its many faults, the twentieth century - following on from the
exciting romantic era of the nineteenth - started out as a time of cultural
and technological development. The technological side continues apace; but I
venture to suggest that in these somewhat philistine times, the cultural is
now seen increasingly as an economically unviable encumbrance.
If these web-pages can do even a small amount to help reverse this ugly and
alarming trend, they will have been worth every bit of time and effort
put into them.
Back to Mad Teddy's synchronous wheel, page 1
My home page
Preliminaries (Copyright, Safety)
Mad Teddy's synchronous wheel, page 2
Return to Electromechanical models sub-menu