Maybe I will have to try this experiment again and pay closer attention to the spark characteristics and sound. I didn't spend much time on it the first time because I couldn't perceive any noticeable differences.
One advantage of the SRSG over the ARSG that I thought of might be fewer voltage reversals. If you use surplus commercial energy discharge capacitors (Maxwell, etc.) like I do then they are rated for a certain voltage reversal such as 20% and it is advantageous to stay within that spec for maximum capacitor life. I have no experience with MMCs so I can't say if voltage reversal is as important for those. It seems to me that a properly synchronized SRSG will experience no voltage reversals due to the spark gap timing. This does not include the voltage reversals due to the ringing of the damped primary circuit oscillation that happens during each firing event. For example, consider a ARSG with 4 flying electrodes and 2 stationary electrodes (2 gaps). Imagine that the beginning of a charging event begins at 135 degrees and the firing point is at 225 degrees. For the first half of this cycle (135 degrees to 180 degrees), the capacitor is being applied with a positive voltage. For the second half of this cycle (180 degrees to 225 degrees), a negative voltage of equal peak magnitude is being applied to the capacitor. This constitutes a voltage reversal to the capacitor of perhaps close to 100%. This would occur 120 times per second. It seems to me that this is not conducive to maximum capacitor life. It may be that the number of voltage reversals due to this scenario is negligible compared to the number of voltage reversals that happen during the damped oscillation of the primary circuit over one firing event.
Steve
----- Original Message -----
From: "phil" <pip@xxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, July 11, 2018 5:17:46 AM
Subject: Re: [TCML] SRSG 200bps or 100bps choice
Steve, et al
I imagine you were well pi##ed off (not sure that works in American
English) and sadly I doubt this will help, but anyway here goes:
I think an SRSG setup only really has any worthwhile benefit on the
following break-rates: 100, 200, and lesser extent 300 (120, 240, 360 on
60Hz). Those three are the ones I've physically used, 400bps was only
ever looked at using simulation software, and I can't remember what it
showed now anyway, but I doubt there was any benefit.
Out of the three I've tried and used, one stood out as being by far the
easiest to determine the correct phase point for, and that, as you've
guessed Steve, was 200bps (240 for you).
The background for others is that if you imagine the upper positive half
of a sine wave (just the 0 to 180 degrees), then 100 bps will have one
firing occur in that 180 deg, and altering the phase of the RSG motor
merely alters the firing point position on the sine wave of the mains
voltage, resulting in a higher or lower MMC firing voltage.
200 bps will have two firing points on that 180 deg curve, so altering
the phase with 200 bps gives a higher voltage in one of the firing
points, and a lower voltage in the other one, so you aim for a phase
position that gives an equal firing voltage in each firing.
300 bps gives a confusing result where altering the phase results in one
firing voltage point increasing, but it alters the other two, so you aim
to have three firings (left to right) of medium voltage, highest
voltage, and again medium voltage, with the two 'medium' firings being
of equal voltage (ish) levels.
Now we know the voltage changes occurring consider the effect on the
sound the coil makes,
100bps gives no real change in sound, while 300 has the tone sound a
#bit# less ragged when correctly phased, (because the two medium voltage
peaks will even out with the higher single peak sandwiched between
them), but 200 bps has by far the greatest audio (& performance)
advantage, as the two equal firings create a steady tone.
(Incidently you can get a similar sounding tone with a well setup static
gap as well I've found.)
Steve, if your phase controller was altering the firing point, and it
seems from your setup it should be, I have no idea why you couldn't
detect a difference, if at least audibly if not performance wise.
As can be seen in the explanation above if the phase is shifting, the
firing point is shifting, meaning peak firing voltages must be altering
as well.
I can only suggest maybe the cap value was far too small (or big even)
or the motor was not correctly 'locked' to the mains frequency (others:
all motors have 'slippage' so a 3000 rpm rotating magnetic field in the
windings only drags the central rotor round at about 2880 ish rpm and by
milling the flats you remove this slippage, (locked) so it then revolves
@ 3000rpm)
Often if running the phase controller near the far end of its effective
range can result in slippage a bit as well. Maybe the nearly 90 degrees
may only have been [say] 70 deg, meaning the rotor must be set more
accurately (getting 90 degrees of shift often exposes the motor to
excessive voltages as well or overheating - both are bad news to long life)
Also when / if it does decide to all work as intended increasing the
main input voltage, will mean adjusting the phase a bit to compensate,
as the firing position becomes affected (spark advance). The correct
'slot' can be quite narrow as well sometimes, as little as 10 divisions
on the phase variac's scale (assuming full advance reads about 80
divisions, which is the case on mine). Amazingly an increase in RSG gap
clearance from my usual 8 thou to as little as 15 thou affects the
firing position. You don't to have an 8 thou gap, but whatever someone
does choose, a little increase can affect things.
Is it worth the hassle? Well to squeeze the utmost power then yes, as it
also gives a consistent firing voltage with no nasty high spikes and can
make a safety gap redundant (I DON'T advocate that though)
I'm altering the phase several times in the video link below to
deliberately encourage ground strikes)
Listen to the change at around 55 seconds when I put it to its correct
position, and also alter it several times at the end as well.
https://youtu.be/yRBqDZCP0jc
(Doing this though can blow the MMC voltage rating if you go too far -
as I found out two weeks ago.)
A long winded post explaining what should be happening, but maybe no
real direct help I'm afraid, but as you can see - logically, there has
to be a difference if everything works as intended and it may give some
ideas.
If anyone is still reading this, good luck
Phil T
(UK)
On 10/07/18 16:31, Steve White wrote:
When I built my pole pig powered coil I also built a SRSG designed for 240 BPS at 60 Hz. I also built one of the electronic variable phase controllers based on a large variac and capacitors which you can read about elsewhere. The phase controller allows the phase to be varied electrically from 0 to almost 90 degrees confirmed through testing. My motor has 2 flats ground on the armature for synchronous operation. Using my oscilloscope and an LED sensor, I carefully aligned the SRSG to fire at phase angles of 0, 90, 180, and 225 degrees without the use of the electronic phase controller. My intention was to then use the variable phase controller to experiment with different firing phase angles off of nominal.
I then operated the coil without the phase controller and noted the spark length and characteristic. I then used the phase controller to adjust the phase angle. To my surprise, the phase angle made no discernible difference in the spark length or characteristic. I tried offsets of 0 to almost 90 degrees. My only conclusion is that the phase angle makes little difference at 240 BPS at 60 Hz at least at the 4.8 KVA power level that I am currently using. This makes me think that the easier-to-build ARSG might be a better choice at the 240 BPS rate for high power coils.
Steve
----- Original Message -----
From: "phil" <pip@xxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Tuesday, July 10, 2018 3:57:10 AM
Subject: Re: [TCML] SRSG 200bps or 100bps choice
Do you why that would be the case? Ive never built a rotary gap coil and
would like to know more :)
Thanks!
Matt
On Sun, Jul 8, 2018, 4:22 PM <pip@xxxxxxxxxxx> wrote
Do you why that would be the case? Ive never built a rotary gap coil and
would like to know more :)
Thanks!
Matt
------------------------------------------------------------------
Matt.
My observations were based on just that - observations. We, that is
myself and another UK coiler, have ran both SRSG and ARSG coils, as well
as statics and each tends to display different streamer characteristics,
although it's mainly the rotary coils where this is noticeable.
A very good explanation on RSG and static gaps and the pros and cons is
at Richie Burnett's excellent web site, as my own brain cells have lost
their edge lately on the more technical aspects.
Richie's site is at www.richieburnett.co.uk/ and there is info on my
own site http://www.hvtesla.com. Also try the archives on here.
Basically once you've decided on either a static or rotary (RSG) gap,
then if you choose rotary you then have two flavours: SRSG or ARSG. The
latter Asynchronous rotary is quick and easy to build, while a
Synchronous rotary is more efficient at lower bps rates, but also a lot
more work to achieve, needing motor modification and careful control of
the phase using some form of phase control, and also care is needed with
the MMC size**. Probably only worth it if you want to squeeze every last
bit of performance out of a coil.
An SRSG at 100/120 bps (50 or 60Hz) can be particularly challenging as
you can then run into some very high resonant voltages if you get the
phase wrong. This is now thought to behind the damage at my other recent
posting:
https://www.pupman.com/listarchives/2018/Jul/msg00019.php ; entitled "Bad
strike to a 12 inch traditional coil (somewhat
terminal)"<https://www.pupman.com/listarchives/2018/Jul/msg00019.php>
To sum up:
Static: - Easy to build, generally suited up to 6 or 7kW (yes I know
others have ran twice that)
Rotary ARSG:- Easy to build, handles power better than a static with
theoretically no upper level for hobby coiling.
Rotary SRSG:- Lot more work involved, most efficient up to 300bps
(particularly at 200/240 bps) if pursuing 100% power, harder to run,
handles power as ARSG above.
People often try SRSG but don't get success because MMC size and phase
are critical. (See "resonant charging" on Richie's site)
I've left out 'triggered gaps' another thing that used to interest me,
but never got around to trying.
Good luck experimenting.
** Note: MMC resonance is affected by both mains frequency AND the bps
rate if using synchronous. Richie calls it resonant charging and has an
old TCML post reproduced here with the graphs he refers to (now dead links):
http://www.hvtesla.com/richie.html
Phil
On 09/07/18 15:27, Matthew Sweeney wrote:
Do you why that would be the case? Ive never built a rotary gap coil and
would like to know more :)
Thanks!
Matt
On Sun, Jul 8, 2018, 4:22 PM <pip@xxxxxxxxxxx> wrote:
If you have wondered on how each perform, especially in their streamer
behaviour, have a look here:
https://youtu.be/Sm8SoFmnlLM
There is a general feeling that 200bps will give marginally longer
strikes, while 100bps is better for ground strikes.
Phil T
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