Project Tesla - An Update

by
Toby Grotz

The results of researches conducted during the summer months of 1987 and 1988 have yielded some interesting experiences. A team of ten volunteers, both full and part time, contributed hundreds of man hours and thousands of dollars to the project. A Tesla coil 51 feet in diameter was erected and a 30 foot wooden tower supporting a metal antenna topped by a 36-inch diameter metal sphere was constructed. The total height of the tower and antenna was 143 feet. The experiments have been supplemented by two winters devoted to literature searches and mathematical calculations which support observed phenomena. This report pertains to the electrical theory and operation of a 51 foot diameter primary and secondary and an extra coil 8 feet in diameter and 7.5 feet high mounted 10 feet above ground.

The facility in which the experiments were conducted is approximately 500 feet long, 75 feet wide, 80 feet tall and is situated at an altitude just above 11,000 feet. The walls of the building are corrugated steel. Large I-beams, commensurate with the size of the building and a winter snowfall of 200 inches, form the support structure of the facility. Each vertical I-beam is grounded to a 500 MCM copper cable which is buried below the building at the perimeter. The buildings at the site are well grounded due to the violent nature of summer lightning storms. The floor of the building was constructed of concrete reinforced with rebar, making it strong enough to support 123-ton GVW trucks during their maintenance when the building was used in an open pit mining operation.

The size and construction of a building can be important to the operation of a large air core resonant transformer due to distributed capacitance to the floor or ground, to the walls, and to the ceiling. In particular the resonant frequency of the extra coil, also known in transmission line theory as a quarter-wave helical resonator, varied depending on its position within the building. The extra coil was wound on a hollow core made of eleven vertical 4-inch diameter sections of PVC pipe. The vertical members are supported laterally by 2-inch PVC pipe wound in a spiral manner inside and connected to the vertical supports. The dimensions of the extra coil included a diameter of 8 feet and a height of 7.5 feet. The wire size was #6 AWG consisting of 127 turns.

The resonant frequency of the extra coil was 94.3 kHz when it was placed over the lengthwise center line of the building. Using an overhead crane to reposition the coil closer to the walls resulted in a lower resonant frequency. It was possible to lower the resonant frequency 7.3 kHz to 87 kHz by shifting the position of the extra coil within the building. Raising the extra coil 10 feet above the ground raised the resonant frequency to 94.7 kHz.

These measurements may seem mundane to those who have built and tuned small Tesla coils. But consider the differences between the three facilities which have been used to operate Tesla coils of this size and power. The facility Nikola Tesla used was made of wood and had wooden floors. The facility used in Wendover, Utah, was an aircraft hanger. The third facility has been described above. The metal walls of the aircraft hanger in Wendover were one hundred feet from the extra coil. The measurements indicate that the tuning of the Tesla coil can be affected by the location within a building as well as the type of building. Another difference between the Project Tesla experiments in Utah and in Colorado was the position of the primary with respect to the floor. The experiments in Utah were conducted with the primary about a foot above the floor (1-1). We must assume that a military aircraft hanger was built to Mil Specs and has a Mil Spec floor filled with Mil Spec rebar. We do know that the floor of the Colorado facility was full of rebar but the primary was 3.5 feet above the floor. In any case, the magnetic field of the primary will tend to be shorted by close location to a conducting grid of rebar (1-2). This could tend to act as a current limiter and reduce the power requirement of the system. This could have been a factor during the coil operation at Wendover.

These factors, and others to follow, are mentioned because at no time during the recent experiments did the coil perform anywhere close to the capability and output power of the Utah experiments. This has been confirmed not only from personal observation but also from examination of photographs and video tapes of coil operation in Utah. In fact, the coil as constructed in Colorado worked poorly, if at all.

The principal investigator during the operation of Project Tesla in Utah, Robert Golka, had developed a configuration which workked successfully. This was reported to be due to the use of a specific A.C. generator and a special arrangement of high voltage power transformers (1-3). It has also been surmised that the generator/transformer match was critical due to the “negative resistance” of the spark gap.

It appears that successful operation of a powerful Tesla coil is not dependent on the source of power or the type of transformers used and that negative resistance is not a characteristic of a spark gap. The coil operated during the Colorado experiments did not perform at peak capacity. The reasons for poor performance will be shown to be due to over-loading and improper tuning. The spark gap and the rotary break will be examined first.

Spark gaps have been used as high power switches for many years. The following is directed at spark gap dynamics:

1. Switch loss. This is defined as the amount of energy deposited in the switch during the switching process and is mainly a result of gas heating due to the finite resistance of the arc channel. Arc formation can be divided into two phases: (1) Current begins to flow while the arc resistance is high. Resistance decreases to a low but measurable value as the arc develops and expands. (2) This is the conduction phase. The arc stabilizes at a low but finite value.
2. Resistive losses. This occurs during both phases but these losses can be especially significant during the formative phase. Resistance of an arc ranges from tenths of ohms to several ohms.
3. Peak spark gap current is not a function of the gas mixture in the gap.