I'm not sure what you mean about the tuning being within 1/2 turn accuracy?
Generally speaking as you apply a higher voltage using the variac, you have
to retune the primary tank frequency lower to compensate for the longer
sparks which have more capacitive loading. This would tend to explain
why your sparks are not increasing when you go from 120 volts to140 volts.
This is to be expected, unless you retune. I've always seen this effect in
my work. I don't think MOT saturation has anything to do with the problem.
I've never seen MOT saturation limit my spark length in any MOT that I've
used. Yes the MOT will saturate, but you'll still get longer sparks.
If you retune, then I think you should obtain 23" sparks.
John
-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil List <tesla@xxxxxxxxxx>
Sent: Mon, Feb 5, 2018 9:30 pm
Subject: [TCML] VTTC MOT question
Having recently completed my 833A VTTC based on Steve Ward's schematic, I am getting 19" sparks with staccato which is satisfactory but I think there should be a little more. I have a very large MOT now installed. I see very little difference in spark length when going from 120 volts to 140 volts on the variac. The tuning is within 1/2 turn accuracy on the primary so I know that is not the problem. The resonant frequency is 361 Khz. Connections are short and thick. What I suspect is that my MOT is saturating above about 120 volts.
The MOT that I am currently using is quite large and is probably from a 1100 watt microwave oven. I have another MOT which has 50% more iron in its core that shouldn't saturate as easily. It also appears to have more windings on the primary and secondary. It is from a very old Amana Radarange circa 1975. I would like to try this MOT out which leads me to my questions. Does anyone have any information on this MOT? Here is what I know about it:
* Dimensions are 145 x 108 x 72 mm
* The are a total of 7 terminals: 2 are for the filament, 2 are for 120 volt power, and 3 are on the HV side
* There appear to be no shunts
* The core is not connected to the HV secondary
* It is made by Advance, the model is AC-180, the catalog number is 09-123-18
My question concerns how to connect the 3 HV terminals. 2 of the HV terminals are spade lugs only 1/2 inch apart. The 3rd terminal is a wire coming right out of the secondary. When I measure the resistance between the 2 spade lugs, I get about 3 ohms. The resistance between the wire coming coming from the secondary and either of the 2 spade lugs is about 67 ohms which seems correct for a HV secondary. I thought that the 3 terminals must be for a center-tapped secondary but the ohmmeter readings say no because of the very low 3 ohm resistance between the 2 spade lugs. It almost seems as if the 2 spade lugs are connected but I would think the resistance between them would be essentially zero and not 3 ohms. I think the 2 HV leads to use are the wire coming from the secondary and one of the spade lugs. I don't know what the other spade lug is for. I assume that since the core isn't connected to the secondary that I must ground the core and one end of the HV output myself.
So, in summary, here are my questions:
* How do I use the 3 terminals connected to the HV secondary?
* Has anyone ever seen a MOT without shunts? Maybe they did things differently back in 1975 when cost wasn't as much of an issue.
* Does anyone have any data on this MOT?
I have searched extensively on the internet for info about this MOT but I can't find anything. Any information about this mystery MOT would be appreciated.
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