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Rotary Spark Gap

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  • Eric Rodemeyer
    How does one figure the number of breaks per second for a coil? I have built a rotary gap but am not comfortable trying it until I know a little more. I did my
    Message 1 of 6 , Jul 5, 2004
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      How does one figure the number of breaks per second for a coil? I have built a rotary gap but am not comfortable trying it until I know a little more. I did my best to make it adjustable from 1000-3000 rpm configurable from 2-8 breaks per revolution asyncronous. I am running a bank of 16 - .68 mfd caps and a pair of 7500 - 60mA Franceformers. 
    • Bert Hickman
      ... Eric, This particular forum is usually not really very useful for answering practical Tesla Coil design questions. Instead, the Pupman Tesla Coil Builders
      Message 2 of 6 , Jul 6, 2004
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        Eric Rodemeyer wrote:
        > How does one figure the number of breaks per second for a coil? I have
        > built a rotary gap but am not comfortable trying it until I know a
        > little more. I did my best to make it adjustable from 1000-3000 rpm
        > configurable from 2-8 breaks per revolution asyncronous. I am running a
        > bank of 16 - .68 mfd caps and a pair of 7500 - 60mA Franceformers.
        >
        ><SNIP>

        Eric,

        This particular forum is usually not really very useful for answering
        practical Tesla Coil design questions. Instead, the Pupman Tesla Coil
        Builders List or Tesla-2 Lists would likely get you more responses:
        http://www.pupman.com
        http://tesla-2.org/

        Asynchronous gaps are quite notorious for destroying Neon Sign
        Transformers (NST's), since gap misfires can result in the NST and
        tank cap interacting to build up excessive voltages across the NST
        secondary. Since your system uses 16 0.68 uF caps in series, the tank
        cap is Larger Than Resonant (LTR) at 60 Hz, so you should be OK.
        However, you'll still want to make sure you have a safety gap in
        parallel with the rotary gap - its cheap insurance. Also, the
        electrode spacing between the flying and static electrodes will need
        to be quite close (0.050 or less), since 7500 volts is a relatively
        low voltage for a rotary gap system. Small, low power systems usually
        operate much better with either a forced air or vacuum static gap or
        with a synchronous rotary gap.

        Because you are running an LTR system, you'll get the best results at
        low rotary break rates - between 120 - 240 BPS. If you are using 4
        breaks/revolution, this translates to 1800 - 3600 RPM, and using all
        8 rotary electrodes will drop you down to the 900-1800 RPM range. Make
        sure you have a metal or Lexan guard around the rotary to catch the
        shrapnel in case you have an accidental rotary "crash".

        The RPM is related to the desired BPS and the number of flying
        electrodes (N) as follows:

        RPM = BPS*60/N

        Good luck and play safely,

        -- Bert --
        --
        --------------------------------------------------------------------
        We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
        Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
        Technical Books. Stoneridge Engineering -- http://www.teslamania.com
        --------------------------------------------------------------------
      • Bert Hickman
        Eric, Oops... I just noticed that you said you were using a PAIR of 7500-60 NST s. My previous response assumed only one NST. With the pair, your tank cap is
        Message 3 of 6 , Jul 6, 2004
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          Eric,

          Oops... I just noticed that you said you were using a PAIR of 7500-60
          NST's. My previous response assumed only one NST. With the pair, your
          tank cap is sized to be resonant with your NST at the mains frequency.
          It's critical that you use a safety gap across your NST, and you
          should also use one directly across your rotary gap as a safety valve
          in case of rotary misfires or gross mistuning. When the tank cap is
          sized to resonate with the NST, the NST output voltage may try to
          "ring up" to a very high voltage if the main gap fails to fire for
          some reason. The voltage may try to rise to as much as 5-10 times the
          faceplate voltage value of the NST(!) - as the voltage climbs, it
          eventually destroys either the NST or the tank cap along the way. \

          Best regards,

          -- Bert --

          Bert Hickman wrote:

          > Eric Rodemeyer wrote:
          >
          >>How does one figure the number of breaks per second for a coil? I have
          >>built a rotary gap but am not comfortable trying it until I know a
          >>little more. I did my best to make it adjustable from 1000-3000 rpm
          >>configurable from 2-8 breaks per revolution asyncronous. I am running a
          >>bank of 16 - .68 mfd caps and a pair of 7500 - 60mA Franceformers.
          >>
          >><SNIP>
          >
          >
          > Eric,
          >
          > This particular forum is usually not really very useful for answering
          > practical Tesla Coil design questions. Instead, the Pupman Tesla Coil
          > Builders List or Tesla-2 Lists would likely get you more responses:
          > http://www.pupman.com
          > http://tesla-2.org/
          >
          > Asynchronous gaps are quite notorious for destroying Neon Sign
          > Transformers (NST's), since gap misfires can result in the NST and
          > tank cap interacting to build up excessive voltages across the NST
          > secondary. Since your system uses 16 0.68 uF caps in series, the tank
          > cap is Larger Than Resonant (LTR) at 60 Hz, so you should be OK.
          > However, you'll still want to make sure you have a safety gap in
          > parallel with the rotary gap - its cheap insurance. Also, the
          > electrode spacing between the flying and static electrodes will need
          > to be quite close (0.050 or less), since 7500 volts is a relatively
          > low voltage for a rotary gap system. Small, low power systems usually
          > operate much better with either a forced air or vacuum static gap or
          > with a synchronous rotary gap.
          >
          > Because you are running an LTR system, you'll get the best results at
          > low rotary break rates - between 120 - 240 BPS. If you are using 4
          > breaks/revolution, this translates to 1800 - 3600 RPM, and using all
          > 8 rotary electrodes will drop you down to the 900-1800 RPM range. Make
          > sure you have a metal or Lexan guard around the rotary to catch the
          > shrapnel in case you have an accidental rotary "crash".
          >
          > The RPM is related to the desired BPS and the number of flying
          > electrodes (N) as follows:
          >
          > RPM = BPS*60/N
          >
          > Good luck and play safely,
          >
          > -- Bert --


          --
          --------------------------------------------------------------------
          We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
          Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
          Technical Books. Stoneridge Engineering -- http://www.teslamania.com
          --------------------------------------------------------------------
        • Eric Rodemeyer
          Thanks for your help! Possibly a RSG is not the correct setup for my coil. Every static gap I have fabricated has not worked the best. I have tried the 4 PVC
          Message 4 of 6 , Jul 6, 2004
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            Thanks for your help!
             
            Possibly a RSG is not the correct setup for my coil. Every static gap I have fabricated has not worked the best. I have tried the 4" PVC method with copper pipe and the same configuration with a vac cleaner motor. My problem is that they run hot and want to melt anything I have used for insulation. I know the next comment will be duty cycle but I need a little more run time for this project.
            I will continue to search the links you posted.
             
            Thanks,
            Eric
            -----Original Message-----
            From: Bert Hickman [mailto:bert.hickman@...]
            Sent: Tuesday, July 06, 2004 8:57 AM
            To: usa-tesla@yahoogroups.com
            Subject: Re: [usa-tesla] Rotary Spark Gap

            Eric,

            Oops... I just noticed that you said you were using a PAIR of 7500-60
            NST's. My previous response assumed only one NST. With the pair, your
            tank cap is sized to be resonant with your NST at the mains frequency.
            It's critical that you use a safety gap across your NST, and you
            should also use one directly across your rotary gap as a safety valve
            in case of rotary misfires or gross mistuning. When the tank cap is
            sized to resonate with the NST, the NST output voltage may try to
            "ring up" to a very high voltage if the main gap fails to fire for
            some reason. The voltage may try to rise to as much as 5-10 times the
            faceplate voltage value of the NST(!) - as the voltage climbs, it
            eventually destroys either the NST or the tank cap along the way.   \

            Best regards,

            -- Bert --

            Bert Hickman wrote:

            > Eric Rodemeyer wrote:
            >
            >>How does one figure the number of breaks per second for a coil? I have
            >>built a rotary gap but am not comfortable trying it until I know a
            >>little more. I did my best to make it adjustable from 1000-3000 rpm
            >>configurable from 2-8 breaks per revolution asyncronous. I am running a
            >>bank of 16 - .68 mfd caps and a pair of 7500 - 60mA Franceformers.
            >>
            >><SNIP>
            >
            >
            > Eric,
            >
            > This particular forum is usually not really very useful for answering
            > practical Tesla Coil design questions. Instead, the Pupman Tesla Coil
            > Builders List or Tesla-2 Lists would likely get you more responses:
            > http://www.pupman.com
            > http://tesla-2.org/
            >
            > Asynchronous gaps are quite notorious for destroying Neon Sign
            > Transformers (NST's), since gap misfires can result in the NST and
            > tank cap interacting to build up excessive voltages across the NST
            > secondary. Since your system uses 16 0.68 uF caps in series, the tank
            > cap is Larger Than Resonant (LTR) at 60 Hz, so you should be OK.
            > However, you'll still want to make sure you have a safety gap in
            > parallel with the rotary gap - its cheap insurance. Also, the
            > electrode spacing between the flying and static electrodes will need
            > to be quite close (0.050 or less), since 7500 volts is a relatively
            > low voltage for a rotary gap system. Small, low power systems usually
            > operate much better with either a forced air or vacuum static gap or
            > with a synchronous rotary gap.
            >
            > Because you are running an LTR system, you'll get the best results at
            > low rotary break rates - between 120 - 240 BPS. If you are using 4
            > breaks/revolution, this translates to  1800 - 3600 RPM, and using all
            > 8 rotary electrodes will drop you down to the 900-1800 RPM range. Make
            > sure you have a metal or Lexan guard around the rotary to catch the
            > shrapnel in case you have an accidental rotary "crash".
            >
            > The RPM is related to the desired BPS and the number of flying
            > electrodes (N) as follows:
            >
            > RPM = BPS*60/N
            >
            > Good luck and play safely,
            >
            > -- Bert --


            --
            --------------------------------------------------------------------
            We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
            Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
            Technical Books. Stoneridge Engineering -- http://www.teslamania.com
            --------------------------------------------------------------------


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