Re: Primary Resonance

Tesla List wrote:
> >From armitage-at-tiac-dot-netTue Jul 30 22:25:45 1996
> Date: Tue, 30 Jul 1996 09:12:19 -0400
> From: Graham Armitage <armitage-at-tiac-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Primary Resonance
> Hi,
> This is directed primarily to Scott Myers (as suggested by Malcolm Watts)
> but it's open for discussion.
> I believe this topic has been discussed before (prior to my joining) but it
> is really giving me a headache. I am running neon sign trannies (4, 15kV
> 30mA) through a 15A variac. I am using an air quenched spark gap and have
> taken care to build all components carefully and properly.
> When I begin to turn up the variac (at about 20%) it begins to fire,
> indicating 15kV across the secondary. Obviuosly I do not want to open my
> gaps as this would be disastrous. I know that with matching capacitance the
> system should want to resonante at 60Hz, however there is something else
> that I am observing. With capacitance almost matching there is a distinct
> beat discharge. I investigated this with a HV probe and the scope and found
> a superimposed frequnecy on the 60Hz cycle. This frequency increased (as did
> the beat - as expected) when the variac was turned up. I immediately assumed
> that the changing variac inductance was somehow influencing this. By
> calculation based on these observations I estimate the output voltage would
> be 130kV at full power (if it could have gone this high)!
> I have just fried one of my caps and was wondering how people have tackled
> this problem in the past? Malcolm has had the same frustrations and I guess
> now it's my turn. I never used a variac in the past and never had this
> problem. Any ideas would be most welcome.
> Thanks,
> Graham
> [ NOTE: Scott is currently on vacation. -- Chip ]


Since Scotty seems to be on vacation, I thought I'd give a shot at
answering your question. I'm running into a similar resonance condition.
My system has 0.02 uF of tank capacitance and two 15 KV 60 MA
transformers. It resonates at almost exactly 60 Hz, and shows a very
sudden increase in primary current (jumping to 12 Amps) at about the 20%
variac setting, and my gaps begin sporadically firing - sounds similar to 
your system. However, I'm not seeing the "beat" frequency behavior that
you are, so your system is probably resonating at a frequencly slightly
higher or lower than 60 Hz. If we assume yours is slightly higher (say 62 
Hz), you will see a lower "beat" frequency at 2 Hz (and may also see an
upper beat frequency at 64 Hz). Even though the circuit is excited by
60 Hz, you'll still see significant resonance effects since the "Q" of
the neon-tank capacitor system is fairly low (typically 5-10), puttin 60 
Hz well within the bandwidth of the tuned system.

On my system, I actually see a significant drop in primary current as
the effective "Q" of the resonant circuit declines when the gaps begin
firing. With increased primary current, you may be seeing a slight
reduction in the apparent series inductance of the transformers and
variac as you have theorized. Since you are getting a higher than normal
primary current for a given voltage input, you may also be starting to
saturate one or more of the cores in your neons. This would reduce the 
apparent inductance, and would cause an increase in the observed beat 

But will I blow up my neons or caps???
As long as you use a static gap adjusted to a break down voltage below
that which would cause damage to the transformers or tank capacitors (I 
use a maximum gap of 0.50") you will probably be OK for short-term, low 
duty cycle operation. As you approach 60 Hz series resonance, the tank 
capacitance partially cancels the current limiting effects of the 
internal magnetic shunt inside the transformer. Under these conditions 
you're supplying significantly higher current levels than the transformer 
secondaries were designed for. While this makes for EXCELLENT coil 
performance, due to more rapid recharging of the tank capacitor between 
gap firings, it's also very hard on the transformers, and can also 
overstress the capacitors. The heavier than normal secondary current 
causes significant heating of the fine secondary windings. I've found 
(the hard way..) that 15 KV 30 MA transformers seem to be significantly 
more fragile than 60 MA ones when overstressed in this manner. 

The SIMPLEST fix is to "overpower" your system by adding one more neon
transformer to the bank. This will shift the new resonant frequency
below the point where it will cause you a problem while still providing
you with excellent coil performance. The next best solution is to reduce
the size of the tank capacitance so as to raise the resonance point
significantly above 60 Hz. However, this may reduce your output voltage
and sparklength. With either fix, you'll notice that the variac setting
will now need to be significantly higher before you the gaps begin to
fire,  since you'll no longer be seeing the effects of 60 Hz resonant
rise on the secondary. Your transformers will thank you for it by not
smoking quite as quickly.... your capacitor(s) will have to fend for 

[Note: I've also played with power factor correction capacitors connected
across the primaries, and they DO reduce overall current draw. However,
they do NOT fix a 60 Hz resonance condition].

Safe coilin' to ya! :^)

-- Bert --