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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Have you actually tried just connecting the MOTs in parallel? I'm pretty sure you wouldn't notice any difference in heating.
As I said, the 100 volt difference is gonna produce around 50mA of current. Standard MOT has 100 ohm secondary resistance, power wasted=IIR=0.25W. That's a lot of power
I'd be more worried about the no-load primary current, and I would focus on reducing it, either by adding a saturable inductor in series with mains, or by removing the shunts and filament winding and wind additional turns here. Note that if you do this, you can wind less turns on the MOT with lower voltage output to make the outputs ballanced (with the shunts removed, the ballancing current would be higher so it would be desirable to match the output voltages).
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
The shunts seems to be very well glued to the core. I was just too afraid to not damage my MOTs (it is hard to get them, sometime I am waiting mounths for a single xfrm).
I am sure that with an appropriate core I can reduce the output difference as you suggest. I wound in 10 turns increment and measure the difference (the inductor was in serie with the primary from the higher output MOT). For 150 turns the max reduction was 62 V, but adding more turns the difference begin to increase again, so I supposed that it was all I can get from that core. I didnt have another one, but I had all the parts to build the monstruosity seen in my pics.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
vasil wrote ...
The shunts seems to be very well glued to the core. I was just too afraid to not damage my MOTs (it is hard to get them, sometime I am waiting mounths for a single xfrm).
I am sure that with an appropriate core I can reduce the output difference as you suggest. I wound in 10 turns increment and measure the difference (the inductor was in serie with the primary from the higher output MOT). For 150 turns the max reduction was 62 V, but adding more turns the difference begin to increase again, so I supposed that it was all I can get from that core. I didnt have another one, but I had all the parts to build the monstruosity seen in my pics.
Well, if you used the centertapped autotransformer as I suggested, it will not limit much the no-load difference with "open" secondaries. Only after you connect the HV leads together, the voltage on the transformer starts being induced and the secondary ballancing current will be very small. Try to measure it.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Well...I have a guess, that my MOTs are weaker than I thought, so I measured the core section of each transformer. They are identical 3.5 cm x 6.7 cm, so a cross section of the core = 23.45 square centimeters, thats mean 549.9 watts each, so just a merely 1099 watts output with the two paralleled transformers.
So, I upgraded it with two new MOTs (they are completely different kinds): - one with 3.5 cm x 6 cm core (21 square cm core section ) P = 441 watts - another with 3.3 cm x 7 cm (23.1 square cm core section) P = 533.61 watts
Adding the two transformer in my setup would rise the max output power to a 2074 watts power capability. Had to add a new stage on the source.
The final result:
Schema is here:
The source with the GU81 vttc together:
The diodes are 22 kV @ 3 A each, doubler cap = 5 microF @ 2.5 kV Filter cap = 10 nF @ 20 kV - not figured in the schema ( I made it 10 times more than the primary cap as Dr Spark suggested in his posts, now I am runing a primary cap = 714 pF)
I stop here (no more MOTs avalaible), have to work on the new vttc.
I run the same voltage on the doubler, but with higher current capability. The little movie shows very well how important is the power input for the vttc to get out a good spark. So not only the tube is important but the transformer you are using.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well this does not apply to MOTs, as they use nearly 2x the magnetic flux in the core so they can put out more power with the same copper heating, what's problem however is the core heating.
I have a standard-size MOT which was used at 1250W even when the core cross-section would suggest ~500W. Of course they must be cooled with a fan to allow continuous operation at 1250W.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Seems that I have no idea about the power I run through it. If it would be twice the calculated power I would have almost 4 kW....thats would be wonderful for me. Good news Dr Kilovolt!
Another thing is that my 2 kW variac heats a lot, so maibe I have more power in the circuit. Anyway I am using it only for short run times.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
vasil wrote ...
Seems that I have no idea about the power I run through it. If it would be twice the calculated power I would have almost 4 kW....thats would be wonderful for me. Good news Dr Kilovolt!
Another thing is that my 2 kW variac heats a lot, so maibe I have more power in the circuit. Anyway I am using it only for short run times.
If you want them to run continuously @1kVA, consider adding a largish fan such as the one used in microwave oven. (at least one fan for two MOTs)
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High voltage high current power source
