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Re: Rotary Spark gap
---------- Forwarded message ----------
Date: Tue, 30 Sep 1997 08:35:42 -0400 (EDT)
From: FutureT-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: Re: Rotary Spark gap
In a message dated 97-09-30 00:31:30 EDT, you write:
<< First of all I've never seen a "sync" motor and I'm not sure what
> they do. ie. Why do I want to use a sync motor in a rotary spark gap?
Kevin,
Picture a 60Hz sine wave to represent the voltage input to the Tesla
coil. Using a regular non-sync rotary, the electrodes can line up and
permit the rotary to fire at any point on the sine wave depending on
the relationship between the 60Hz and the motor speed, number of
electrodes, etc. Motor speed, number of electrodes, will determine
the break-rate. In most cases, the break-rate will not be an even
multiple of 60Hz, so the operation will not be synchronous...the
points on ;the sine wave where the gap firing occurs will drift and
change over time.
In contrast, using a sync-motor the break-rate is an exact multiple
of 60Hz. The result of this is that the gap will fire at the SAME
exact points on the AC half cycle every time the gap fires. It is
most common to use an arrangement that causes the gap to fire
only once at the peak of each half cycle. Since there are two
half cycles in each cycle, the break rate has to be 120 breaks
per second to fire on each half cycle.
Then there's the issue of phase, or timing. This adjustment is
what determines WHERE on each half cycle the gap fires. And
of course we want the gap to fire at the peak of the sine wave.
Provision for this phase adjustment can be made in two ways;
mount the motor in a cradle so it can be rotated a little at a time
to find the proper position, OR mount the motor steady, and move
the stationary electrodes a little at a time concentrically to the motor
which accomplishes the same thing. You'll know when the adjustment
is correct because you'll get the best spark output.
The speed of a sync-motor is an exact multiple of 60Hz. So there
are 1800 rpm motors, 3600 rpm ones, etc. (At least the sync
motors we use for rotary gaps are like this, some other types of
sync motors for other applications may have differnent arrangements.)
A sync motor actually "locks in" with the frequency of the 60Hz AC,
unlike other motor types. In appearance, a sync motor can look
exactly like an induction motor...the clue is in the speed rating that
is indicated on the motor...if it says 1800 rpm, or 3600 rpm, then it's
probably synchronous. Of course if you create your own motor by
modifying a regular induction motor, then you'll start with a 1725, or
1750 rpm, or 3450 rpm or something similar, and after the mod, it
will lock up with the 60Hz ac and run at 1800, or 3600 rpm , etc.
Now for your final question, "why use such a sync-beast"? Well
there are a number of reasons:
1. In a neon tranny powered TC, the use of a non-sync gap may
permit the voltage to rise too high and destroy the transformer
due to resonant rise effects in the transformer-capacitor circuit.
The sync gap fires when the tranny has charged the capacitor
to a proper and suitable voltage, and actually limits the voltage
by firing before the voltage rises too high.
2. Every time a spark gap fires, it tends to short out the power
supply tranny. The more breaks per second, the more energy
may be wasted during these gap firing times, so a sync system
may be more efficient.
3. If resonant charging is used, there will be a phase shift which
will cause the capacitor to discharge (fully charged) when the
incoming AC is at it's zero crossing point. This is good
because then the tranny doesn't waste any power when it
gets shorted out when the gap fires.
4. (I forgot to mention. In these sync-gap scenarios, I'm assuming
that a break-rate of 120 BPS is being used. A higher break-rate
can be used such as 240 BPS, 360 BPS , etc,but much of the
advantages of a sync gap will be lost). If resonant charging is
maximized, the capacitor will be charged to a higher voltage
also increasing the efficiency of the TC. (when I refer to the
capacitor in this posting, I mean the main TC cap, the main
tank circuit capacitor.).
5. There may be other advantages which don't come to mind at
the moment. Non-sync gaps have their advantages also.
Regarding the look of the flat areas, here's a picture:
................
. .
. .
. O .
. .
................
In this view, you're holding the motor armature so that the "O" in
the picture is the motor shaft, and it's pointing right at you. You can
see the two flats on the top and the bottom where some of the
"roundness" of the armature has been filed or milled away. In my
previous posting, I gave the proportions of cut and uncut areas,
which you can relate to this drawing.
> What is a good target range for BPS? Around 200-400?
Best BPS is 120 breaks per second as explained above.
> OK last question (which requires most detial) If I wanted to modify a
> motor to run snyc, I understand that flats need to be filed on the
> armature, is there somewhere I can see pictures that detail this sort of
> work. Its very hard to "visualize" what to do from a short text
> paragraph telling me to file flats 1/8 the size of the armature etc.
See drawing above.
Regards,
John Freau
> Thanks so much for the reems of usefull info on the list,
> hopefully my understanding of TC's will grow to the point where I may
> have info to contribute. But for now I'm just trying to learn from all
> of you. Thanks for the help.
> Radtke
>>