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Hi Steve,
Turns out that IGBTs become easier to deal with than rotary spark gaps
as the coil scales in size. Here's a few reasons:
a) The freq drops with increasing size, and in the lower 10's of kHz
range the bigger traction IGBTs start to work quite well.
b) The motor size on a rotary gap starts getting rather unwieldy at
higher power levels. Electrum needed a 30HP motor. This 40ft coil
would need almost 100HP... not a very portable rig!
c) IGBTs can work at much lower impedance, typically a small fraction of
an ohm, compared to many ohms for spark gaps. That allows this coil to
work with primary voltages <1kV. Electrum was 44kV, and this coil would
have to be 60-80kV for a rotary gap system.
Since I had no funding, this scale prototype design had to pursue
cutting costs as far as possible. IGBTs provided the best value
overall. However, I totally agree that a rotary gap system for 240kW
service would look much cooler than a box of IGBTs. ;>
On 11/21/2018 9:10 AM, tesla-request@xxxxxxxxxx wrote:
Message: 1
Date: Wed, 21 Nov 2018 00:22:15 -0500 (EST)
From: Steve White<steve.white1@xxxxxxxxx>
To: Tesla Coil Mailing List<tesla@xxxxxxxxxx>
Subject: Re: [TCML] Where is everyone?
Message-ID:
<261609294.29019341.1542777735527.JavaMail.zimbra@xxxxxxxxx>
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Hello Greg,
Very impressive coil. The biggest that I have seen since that one in Oklahoma on rails a number of years ago. I don't have much experience with solid state coils as this one appears to be. Wouldn't it be easier and more reliable to use spark gap technology for a coil of this size and power level? My question is prompted by your picture of a blown IGBT which I am sure costs hundreds of dollars. But then again maybe you have a special application in mind that requires a variable firing rate or something else that I don't know about. Or maybe it is just the challenge.
Steve White
Cedar Rapids, Iowa