MOTs, a summary

Hi all,
I have written what is below to give a summary of using two MOTs for a
~5kV TC supply. I just wanted to check all that facts are right. Also
the hope was that if some one was to search the archives, then there
would be at least one hit that gave nearly all the info in one hit. So
those who are experts on MOTs, check to see if I've got my facts right.
I should say that the information is not based on my own experiences,
but based on the information given by list members when I asked about
this topic. So the credit goes to them. Thanks again.




Microwave oven transformers (MOTs) may be used as a high voltage power
supply for tesla coil systems instead of, say, neon sign transformers
(NSTs). MOTs step-up the mains supply up to around 2300 volts ac, but
can deliver a current as high as 500mA or more. As surplus microwave
ovens are so readily available, this makes a good choice for a tesla
coil high voltage supply.
There are several problems however, the first being the comparatively
low voltage output. This can be overcome by wiring two MOTs together to
give around 5000V mark, about 4600V.
It should be emphasized now that MICROWAVE OVEN TRANSFORMERS ARE LEATHAL
and deserve respect when working with them. No precautions skipped.

In general you simply connect the two mains primaries in parallel, but
opposite in phase, and the secondries in series. To connect the
secondries together you place them side by side with their cores
touching. Then make a good electrical connection between the two cores
with a heavy conductors, for instance, a couple of metal straps. The two
cores bolted together form your systems earth connection, and so should
be connected to the mains earth and not left floating. Of the four
secondary cables you now have, two will be connected to the cores, which
is now your earth. You can connect these two wires together and keep
them out of the way. The remaining two secondary cables are the high
voltage leads to your tesla coil system, there is 4600V between them,
and 2300V to earth on either one.
The primaries are connected in parallel and in anti-phase. For example,
say you had two identical MOTs, both of which had a red and black cable
going to the primary. You simply connect the black of one to the red of
the other, and red to black, this gives you the anti-phasing required.
To test the set-up, connect the secondary HV leads to a spark-gap and
stand well back while switch on the current. If no power arc forms, then
either your primaries are not wired anti-phase, or your domestic power
breaker has tripped. If it is neither of these, then probably your MOTs
are dead.
It is not impossible to use two different MOTs, even if they are of
differing powers, but this is not to say it will work all the time.

The total available power that can be delivered by the MOT pair is
around 2300VA, this initially looks good, but actually proves to be more
of a hindrance. The high current makes quenching of the primary circuit
spark-gap very difficult, the spark quickly deteriorating into a power
The reason for this is that MOTs have a very poor built in current
limit, resulting in a massive current being drawn by the short circuit
load presented by the tesla coil system. A neon sign transformer has a
magnetic shunt as part of it's construction which limits the current.
However MOTs do not have this sort of design, simply because it is not
required for their particular application, driving a voltage multiplier
to power a magnetron. For this reason, some tesla coil builders include
the voltage multiplier circuit in their paired MOTs to charge the tank
capacitor with a DC voltage (in the range of 10000 volts).
So, if the paired MOTs need to be current limited to reduce these
effects, as well as to stop domestic power breakers from tripping! This
can be done in one of two ways. The first is by connecting a high power
resistive load in series with the primaries of the paired MOTs. This
load can either be a 100W lamp or a heating element, of course they will
emit the excess power. The second method is to use another MOT, the
primary of which is in series with the primaries MOT pair, and the
secondary leads connected together. The latter option is the most
preferred, however, MOTs are highly inefficient when used in this