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# [TCML] Inductive Ballast for MOT Power Supply

```Hi Everyone,

I seem to keep burning out NST's with my tesla coil and that is getting expensive. Over the last 9 months or so I have been experimenting with replacing my 15,000V 60mA NST with a 4 MOT power supply. I have a link to a youtube video of my first demonstration of the 4 MOT power supply circuit.
https://www.youtube.com/watch?v=2rhz52K9qkU

In order to cut back on trying to explain how I wired the four 7.2 amp MOT's together, I have added a link below that will take you to the circuit diagram that am utilizing for the project.
http://www.twotowers.com/tesla/images_tesla/mot_tesla_coil_power_supply.jpg

The 220 volts coming into the MOT power supply circuit is regulated by a 60 pound variac that I purchased from Ed Sonderman (off of Pupman) in Spring of 2014.

I am attempting to use two MOT's as inductive ballast, one for each 120V leg that feeds the circuit.

Each ballast MOT, is limiting current to one half of the Power supply (2 MOT's). Each ballast MOT has its primary coil wired in series with one 120V leg feeding the circuit and each ballast MOT has its secondary coil output short circuited to its metal case. I have tried not short circuiting the HV secondary output and leaving it open. This does not work as excessive inductance in the primary coil limits current to such a degree that almost no current is flowing at all. Question: Perhaps someone can explain this in layman's terms, why shorting the MOT's secondary coil to the case allows more current to flow through it's primary coil? With the secondary coils short circuited, The ballast MOT's seem to limit current quite effectively; perhaps still a little too much.

The problem I'm experiencing with the ballast MOT's, is that the short circuited secondary coils get seriously hot after just a minute or so of operation. Cool down time can be up to an hour. Amongst all the individuals on the web that recommend using MOT's as inductive ballast for MOT power supplies, I have not come across anyone that makes mention of this phenomenon. I find this very odd. Because of this over heating issue, at least for me, using MOT's as inductive ballast does not seen to be a viable ballasting solution.
Comments anyone???

This brings me to my next line of thought. I thought about going to the local iron works and picking up a couple 10 inch lengths of 1.5 inch square stock and using those to wind my own inductors with 14 gage vinyl insulated wire.
Question: Will this work?
Question: I do not have a fully equipped machine shop and I'm trying to do this on a budget. Can anyone provide me with any direction as to how to best accomplish this?
Question: Does such an inductor, besides limiting current, also limit voltage? If yes, then limiting the input voltage to the MOT power supply will also reduce the output voltage. Question: is this correct? Optimally speaking, since 8000V is pretty much all I can expect to get from my four MOT's, it would seem that I want to maximize the voltage coming into the system as much as possible while at the same time reducing the amount of current coming in.

Question: Should I forget about trying to create inductive current limiters and just use my monster variac for that purpose. I thought that variacs are more about limiting voltage. But obviously when the variac is at its lowest setting little or no electricity is flowing, so it would make sense that variacs limit voltage and current. Question: Is this correct?

Any other thoughts?

As always, I'm extremely grateful for the quality of feedback that I receive from the incredibly smart tesla geeks that participate here at puman.com!

Thanks, Steffan Heydon
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