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130kW Coil -- Oh No!




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From:  Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent:  Sunday, February 01, 1998 4:47 PM
To:  Tesla List
Subject:  130kW Coil -- Oh No!

Hi All,

The final development and testing efforts of the coil during the
last few weeks have brought considerable progress, and a few setbacks
as well.  Most of the major problems so far have had to do with the
dynamics of the rotary gap system.

The latest setback on the rotary gap system has to do with
the sharp pulsations of air pressure exerted on the stationary
electrode holders by the quickly moving rotating electrodes.  As the
moving electrode passes the stationary one (at 420 km/h with a 
clearance of 0.9mm), the shock wave tends to hammer the stationary
electrode supports, which are supported by an array of large porcelain 
insulators.  These shockwaves are strong enough even at half speed 
that hearing protection is required anywhere near the gap.
Although the porcelain insulators are quite massive, they appear to 
be susceptable to vibrational fatigue, and during power tests
last Friday one of these insulators failed completely, resulting in 
a rather impressive spray of porcelain shards, ejected out of the 
vault through the access door.  T. Leonard was far enough away, and
I was behind my favorite steel plate, so no one caught any action.

On a positive note -- at the time of the insulator failure, the coil 
was generating 22 ft long discharges into the air from the top sphere,
with the mains at one-fourth of full power!  Electrically speaking
at least, the coil appears to be doing quite well.  
I finally got the Fluke ScopeMeter to work with the coil energized,
and grabbed some waveforms just before the rotary exploded.
The measured waveforms on the charging reactor and HV circuitry 
have come out fairly close to the projected values from PSPICE, 
with the exception that PSPICE predicted more ringing on the diode
stacks at the end of the charging cycle than was actually observed
with the HV scope probe. (those pesky ideal components)

* The charging reactor waveforms indicate that the coil is _not_ 
in fact drawing 2.5 times the normal amount of power (as the 60Hz
CT's on the mains indicate).  The current waveform measured on the 
charging reactor is a haversine 4.8A tall by 3msec long.  PSPICE
predicted a haversine 4.6A tall by 2.4msec long.  With a HVDC of
12,700V and a rep rate of 200PPS, this gives a power usage of 32kW,
where the CT's on the mains say 120A per 400V leg!

Does anyone have experience with standard 5A FS panel meters and
300:5 CT's?  How sensitive are they to non-sinusoidal waveforms?

Also, we were able to bring the primary voltage to 55kV in the 
single-shot mode (~1PPS), at which point the top sphere issues
7' to 8' streamers, and the RF current at the base of the coil
is 60A pk.

Our plan for the electrode support insulators involves replacing 
all 8 of the porcelain posts with 3.5" x 6" x 27" solid slabs of G-10.  
The material is already in hand; however the machining efforts, 
modifications to the rotary gap frame and testing will probably 
set our schedule back another two weeks.  

I'll report on our progress again when the G-10 is installed.


-GL