From: Tedd Dillard<tedd.dillard@xxxxxxxxx>
To: Tesla Coil Mailing List<tesla@xxxxxxxxxx>
Subject: Re: [TCML] Tesla Digest, Vol 132, Issue 5
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<CABebsSmOTy+syG067FzyQN+mAkJep0PMKOT4hjT7PGU9sDby7A@xxxxxxxxxxxxxx>
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Greg,
I am missing something, please help me understand sir.
I do not understand why the rotary spark gap motor would necessarily need
to be so big as the size increases.
The diameter of the rotor would get bigger and so the weight would also
increase so the starting torque would also increase.
But as there is no mechanical friction involved the only power increase it
seems would be the increased windage from the rotating parts.
I can see maybe 5 horse power or so and if the motor is a regular induction
motor modified to be a synchronous motor there would be a loss of torque
due to the increases air gap but I must be missing something. So getting it
going could be an issue but once it was up to speed where is the large
power going?
Teddy
On Wed, Nov 21, 2018 at 5:20 PM Greg Leyh<lod@xxxxxxxxxxx> wrote:
>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. ;>