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500kV for Tesla Coils?
Posted by Fred Berry on 11/26/2004, 3:03 pm
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Re: 500kV for Tesla Coils?
Posted by Bert Hickman on 11/26/2004, 6:48 pm, in reply to "500kV for Tesla Coils?" Hi Fred, Interesting observations! There are many reasons for these apparent discrepancies. First, despite what some Tesla Coilers may believe, the actual output voltage of Tesla Coils is significantly lower than would be "guessed" based upon streamer length. Tesla Coils create a unique combination of high frequency and high voltage within rising voltage envelopes. These complex high voltage waveforms are repetitively generated between 120 - 500 times/second for most coils. This combination can cause sparks to become quite long - a new spark can grow to a total longer length by adding to the hot trail blazed by earlier sparks. Because of these effects, a coil that generates a peak output of 300,000 volts may generate streamers that can be 6 or 7 feet long! Tesla coils that generate sparks of only 2-3 feet are actually generating considerably lower output voltage - perhaps 150,000 volts or less despite what their builders may hope... :^) Utilities specify a much longer "safe" distance for a number of reasons. First, line voltage may have load dump or other switching surges that can add to the normal peak voltage on the line. Also, a working linemen's elbows or tools might accidentally come closer than desired, so by specifying a "worst case" distance, utilities can still keep their linemen safe from potential flashovers. The fact that thousands of amperes could flow if the distance was bridged is not really a factor - only the peak voltage is important for initially breaking down the air. However, once a conductive plasma path has been formed in air, the following arcing distance can be MUCH longer. The higher the current that can be supplied, the longer the arc will become. Even a comparatively low voltage high current source can create a very dangerous, extremely hot arc. For example, a "low" voltage high current source such as a 440 VAC feeder can create an arc that can exceed 18", and the resulting heat and shock from the explosive "arc flash" can injure or kill anyone unlucky enough to be anywhere near the event. In the case of the 500 kV arc, the voltage initially appearing across the good interrupter was much greater than 500,000 volts, since it was opening up an inductive reactor. Opening inductive loads is a very demanding task for any high voltage circuit breaker. BTW, there are actually very few Tesla coils that can generate an honest 1 million volts or more. The world's largest two-coil classic Tesla Coil actually generates about 1.6 MV and 50+ foot streamers. Check out Greg Leyh's Electrum Site at: Also, check out this shot - yes, that's Greg Leyh INSIDE the top terminal while it's running... :^) Best regards, -- Bert --
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