Re: [TCML] Old TC restoration advice Update

[David Speck <Dave@xxxxxxxxxxxxxxxx>]

Replying to my own query, I thought I'd let the list know how the 
vintage coil rehab worked out.

First of all, thanks very much to John F, Dave R, and Stephan for their 
input.

I ended up using neither of the options I had proposed.  I recalled that 
I had an unballasted 9 kV, 150 mA, 1350 VA Hammond Industries plate 
transformer that came with yet another vintage coil.  Primary is 120 VAC 
at 11.25 amps, max faceplate rating.  It was wired, encased, and ready 
to plug in, and weighs a ton.

I dug out one of the saturable reactors from their hiding place in the 
garage and threw together a diode bridge, heatsink, and line cord to 
power the control winding.  Using a calculator based on the core 
dimensions previously published on the TCML, the reactor was rated 
conservatively for about 850 VA, or more likely 1 KVA, if you push it a 
little.

I ended up controlling the coil with three Variacs -- one for the 
transformer primary input voltage, one for the saturable reactor for 
current limiting, and one to control the ASRG speed.  I set the ASRG 
speed by ear, from my memory of the sound of previous year's operation.

Initially, I had a lot of trouble with the internal safety gap firing.  
Paradoxically, raising the primary voltage a little while reducing the 
max current a bit stopped the firing of the safety gap nicely.

Eventually, I just left the primary voltage Variac at one sweet spot, 
and controlled the coil  operation entirely with the sat reactor control 
current.  It took very little control current to enable the coil 
operation.  The control winding drew less than 5 amps at a maximum of 
120 VAC impressed on the bridge, but its current draw was decidedly 
non-linear.

I didn't have a lot of instrumentation on the setup, but the reactor 
control Variac rarely went above 20 volts, and the operating control 
current didn't register at all on my clamp-on ammeter (200 amps AC full 
scale).  I'm not sure what the primary voltage was on the plate 
transformer, as that Variac lacked a graduated dial.

After I got everything adjusted, the operation of the coil was a smooth 
as it had ever been, and it may have even been delivering sparks ~10% 
longer than previously.  It would have been easier to assess if it 
hadn't been so windy on Halloween night.  The coil was still a big hit 
among the 391 trick or treaters who visited.

The most interesting feature of the modified coil operation was that 
even when I switched off the power to the saturable reactor control 
winding, it took something like 3/4 to 1 1/2 seconds for the TC output 
to cease.  (This is not a DC coil, so there was no stored energy in the 
system, other than the primary tank cap.)  The TC discharge just shrank 
back slowly as if I was backing off on the primary voltage Variac at a 
leisurely pace.  It must take several hundred AC cycles to re-scramble 
the magnetic fields in the reactor core to raise its impedance back to 
its quiescent level.

I'll definitely consider using the saturable reactor for a bigger coil 
next year.

Thanks again for your suggestions.

Dave


On 10/30/2011 9:25 AM, David Speck wrote:
> Esteemed list,
>
> Halloween is rapidly approaching, and I find myself with no working TC 
> to demo to the Trick or Treaters.  Looks like the weather will be good 
> this year, so I can't use rain and snow as an excuse for not having a 
> working coil.
/snip/==================
> So my question is:  On short notice, would I be better off finishing 
> the Terry filter, and trying one or two parallel 9 kV NSTs and 
> retuning the primary,
>
> Or,
>
> Should I throw together an 8.8 kV four MOT stack and use an external 
> ballast?
>
> Any suggestions will be appreciated.
>
> Thanks,
>
> Dave
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