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

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>

MOT's for mass consumption are designed to minimise the iron and copper for 
cost reasons.  It doesn't really matter to them that the power consumption 
is high, only that it doesn't melt under maximum use under fan forced 
cooling in the MO situation. They often run hot even under no load as the 
iron is close to saturation.  IMHO, this is why a lot of MO's "buzz" rather 
than the fan when they are on.  Keep a fan on it and it should run safely 
for 15 - 99 minutes but it will be very hot.  This is why you often see 
thermal cutouts attached to the MOT.

If you look at the power consumption with input voltage you will see it 
rise very significantly by the time it gets to mains level (up to 4-5 amps).
The solution is to use a variac or an external ballast or wind an extra 
30-50 turns of wire in series with the primary for 240V mains and half that 
for 110V.  The latter is only possible if you are removing the primary though.

Running under oil helps with the heat loss but not total draw.  My coils 
use 4 MOT's under oil but I watch the current draw. My best pic had the 20A 
meter hard over.

Peter (Tesla Downunder)
http://tesladownunder.com



> 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...
>
> 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)?
> --
> Best regards,
>  Grishka                          mailto:ghome@xxxxxxxxx