Re: Help with making SRSG phase control work?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx 

Hello, Scott!

In a message dated 1/27/06 2:03:32 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:

> I don't believe that your Shinano Tokki motor is either a
>salient-pole or a hysteresis-synchronous motor; it is just a plain
>old synchronous AC motor.
    OK, how does a "plain old" synchronous motor work?

>There seems to be a lot of confusion as to what type of motor is
>needed to drive a SRSG (Synchronous Rotary Spark Gap) for an
>AC-charged Tesla coil.

    Like a lot of us, I know what I'd *like*, and what I've *got*. 
Question is, how well will what I *have* work in place of what I'd *like*?

>Many people assume that a "synchronous" motor
>is all that's required to obtain a "synchronous" rotary spark gap.
>This is absolutely NOT true. There are two completely different types
>of "synchronism" involved here.

    For the purpose of clarity, let's keep calling synchronous 
"synchronous", and timing "timing".

>The objective is to "time" the system so the SRSG electrodes align
>and fire at the instant of peak voltage on the tank capacitor. This
>requires some known, fixed, and repeatable relationship between the
>spark gap electrodes and the AC sine wave. Every time the motor is
>started, this electrode position/sine wave timing relationship must
>remain identical.

    My theory is to just keep power cycling the motor until it 
"cooperates". :)

>Fortunately, almost any single-phase synchronous capacitor-start AC
>motor can be modified to provide salient-pole operation by machining
>flats on the rotor. This process has been covered in great detail
>many times in the TCML archives.

    As I mentioned before, I did the salient-pole conversion to a 
3/4 HP induction motor. Like most everybody else, I fretted a lot 
about how much material to remove. But things went quite well with a 
hand grinder and file. Runs fine, and I surprisingly smooth without 
any attempts to balance yet. This was the motor I was comparing to my 
Oriental with a stroboscope.

>Repeat the power-on/power-off/measure
>residual magnetism cycle ten or twenty times and you will find
>several of the cycles leave the rotor with a stronger residual
>magnetism than the other cycles.
    Well, after many power cycles and checks, my Oriental goes to 
synchronous every time.

>The bottom line is, if you are going to use your RSG with a
>NST-powered Tesla coil,

    That's the plan...

>you put your NST at risk unless your convert
>your motor to salient-pole operation and create a true SRSG.
>Otherwise, you'll end up with an ARSG (asynchronous RSG).

    That's a good point - I suppose an *incorrectly timed* SRSG that 
had gap presentations only at the zero-crossing points of the 60Hz 
wave could allow resonant rise in the primary circuit. But that's 
what safety gaps are for.

-Phil LaBudde