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



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:03 AM 7/7/2005, you 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).


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?

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...


oil is an effective cooler, especially since it can flow. However, your point about the inner windings is valid. Consider though, that there IS some air gap, which will eventually fill with oil, and the thermal conductivity of oil might be higher than that of air.

You CAN measure the temperature of the winding. Measure the DC resistance both cold and hot, and use the standard copper wire temperature coefficient to figure out how hot it got. This is used for electric motors all the time.


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)?

The wire is probably good to 80 or 100C, and that's likely to be the limiting factor.