Re: Really big MOT power supplies

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "tesla by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-paradise-dot-net.nz>

Hi Chris

Some thoughts re making a bigger MOT System

1) For six MOT's you run them as two groups with the centre at gnd phased to
give 6kV or so per side. Let me call them the "A" and "B" side.

2) The inner MOT's of each side are not modified other than removing the
heater windings

3) Select the 4 Outer MOTs of the A and B side for best looking Secondary to
core clearance

4) Lift the Secondary Eth lug connection on these 4 MOT's and terminate
carefully on a standoff.

5) in 4 above I use HOT glue to physically stabilize the exit spot where the
secondary wire exits the winding and runs to the standoff.

6) I put these 4 MOT's in xfrmr oil to dramatically improve the sec to core
insulation where the oil penetrates and it helps with heat dissipation. I
Alchohol washed all MOT's before putting in oil bath and dry very throughly
in light heat.

7) I have NO heat build up for short runs (approx 1 min) in any of my
xfrmrs, for longer runs I suspect placing the inner two MOTs  in oil would
assist but I've never needed to.

8) Use some Capacitive PFC of several 100uF on each side. I  Fuse each MOT
separately with a 15 amp 3AG fuse

9) Inductive input ballasting with a single MOT will be too hard on a single
MOT as you say, you will need to make a series parallel system for a six MOT
supply. The question of ballasting is not simple. Some have No ballasting
anywhere in the system and make all primary wiring capable of handling the
power reqd. I use 11mH per side from smoothing inductors salvaged from 50v
DC power supplies that are very common in telecommunications systems. These
older linear suppplies are now being replaced by switching designs and I
suspect the linear types will be avaialble surplus in both the USA and
elsewhere. 50v supplies for telecoms gear is one common world wide standard.

10) Secondary side capacitive ballasting has been used by some. My opinion
is that it is not yet well understood and that there is a real possibility
of achieving 60Hz resonance between this large series C and O/P inductance
of the MOT system. Having said that look on the Tesla WEB ring or thru list
archive and you will find many advocates for this idea, it can certainly be
made to work.

11) I myself wire the A  and B side primaries completely separately to their
own mains plugs, this gives me some options re split mains outlet powering.
I tune up on a single variac driving the A and B sides then set up with a
split the I/P one direct to a mains outlet and the other to the variac on
another fused outlet from the main switchboard.

12) To make 11 above work I wind the variac to 0 volts with mains I/P on,
turn on the mains to the other side then ramp up the variac to reach full
power on both sides. This order in applying power is essential BTW as is the
reverse when down powering

13) Oil containers for MOT's have been made from tupperware, ammo boxes, 6"
PVC downpipe and end caps, I like the ideas of each MOT in it's own box to
keep the individual weights down and it aids me in selecting the no of MOT's
I'm working with.

14) Before I stressed my system with volts I left it for weeks to allow the
oil to penetrate, this may not have been needed but it was how I did it.

15) Reported failure rates are low BUT from experience with a 300bps SRSG if
you select the firing times incorrectly the secondary voltage will
skyrocket. I estimated I reached 50kV in my secondary due pooly selected
firing times with 300bps SRSG. Even with this huge stress it was the warning
loud bangs from the MMC as individual caps internally arced that failed
first. These 0.33uF/1600v CD caps self healed from these events.

Best of luck Ted L in NZ BTW MOT's are leathal, CARE
 >
 > Hello Everyone!
 >
 > I hope someone can answer some of my questions about buiding a MOT tesla
 > coil power supply.
 > I haven't got the transformers yet, but I know this won't be a problem (I
 > know several people with old microwaves).  I was planning on having 6 of
 > them in series.  There would be an optional ballast (made with another
 > MOT), and the whole thing would be submerged in oil inside a large
 > tupperware container.
 > This should give me around 12000 VAC at maybe 8000VA (ballast off).
 > Is there any tips on building something like this?  I also plan to use it
 > to power other high voltage projects, such as a jacobs ladder.  Is there a
 > high transformer failure rate with this many in series (and the
 > corresponding much higher voltage)?  Would I need to use paralled MOTs as
a
 > ballast instead of one because of the power the system will draw?
 > I will have a switch to allow the circuit to bypass the ballast for higher
 > power, and also to short or unshort the ballast secondaries, to give me a
 > couple power levels to play around with.
 >
 > Thanks in advance!
 >
 >
 >