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Re: [TCML] Old TC restoration advice Update



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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