Re: Maximum MOT`s power for long, reliable operation and life - how much is it?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Grishka,

On 7 Jul 2005, at 11:03, Tesla list wrote:

> Original poster: Grishka <ghome@xxxxxxxxx>
>
> There`re lots of articles in Internet, when people use MOTs as TC
> power supply. In all cases they draw more than 1 kW from one MOT, they
> say, operational time is short (some minutes)... I then decided to
> find out, how many watts can give us one MOT in a continuos condition
> of operation. I`ve recently made rather simple experiment - I`ve
> connected in series 10 incandenscent lamps (60 watt one piece), MOT
> was working 45 minutes, it`s temperature  became 76 degrees! But - my
> mains voltage was only 206 volts instead of 220 - and let`s imagine,
> what could happen with this transformer at full 220 volts?! A friend
> of mine (father dest) has tested such transformer with NO LOAD - after
> 20 minutes of operation transformer`s core was so hot, that couldn`t
> be touched by fingers for more than 5 sec! (mains voltage was 226
> volts, air temperature in the room - 28 degrees).

That is no surprise. It is instructive to look at the magnetizing
current waveforms with the rated voltage applied to the primary and
the secondary unloaded. You'll see straight away why it gets so hot.

> According to these facts I have some questions:
>
> 1 - does anybody know, how good is the wire insulation in MOT
> (varnish)? That is - what temperature does this varnish can face
> without considerable decreasing its life time?

In my experience the varnish is less of a concern than the type of
insulation on the wire. The only insulation between adjacent wires is
that of the wire itself and the quality (read melting point) and
hardness directly affects its integrity. Wire with red insulation on
it is low quality. I once had it on good authority from a transformer
winder that he used it preferentially in power transformers "because
it is easy to solder". After hearing that revelation I began to wind
my own and to this day have wound hundreds for myself and others. I
would not guarantee the use of self-fluxing (red) wire in any
application where it could/would be subject to significant heating
and/or friction (e.g. magnetostriction). Most MOTs I've seen use this
wire. The *only* wire I use in transformers I build is polyesterimide
coated. The supplier I use won't stock self-fluxing wire on principle.
Possibly there is little demand for it as they sell mainly to motor
rewinders.

> 2 - does MOT worth to be put into the oil? Can we consider oil as "a
> good cooler"? Think about inner layers of HV winding - we can`t
> measure temperature right there...

If it is hot on the outside it will be hotter internally. How much
hotter depends to an extent on the amount of varnish, enamel etc.
present. Copper conducts heat well. Oil is routinely used as a
coolant in distribution transformers.

> 3 - In future I want to run my TC during 5 minutes continuous
> (minimum! - it`s better 10 or 15 minutes) with pauses not more than 10
> minutes (with total runtime about 1 hour). What power could we draw
> per MOT in such condition without overheating? What is overhreating
> for you (see question #1)?

For me it depends on the wire insulation. Too hot to hold a hand on
for 1 second = too hot for self-fluxing wire in my opinion.

Malcolm