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Frequency effect in the operation of a MOT.

Posted: Fri Oct 12, 2018 2:51 pm
by Cai Arcos

I recently became the happy owner of an old MOT. I have been perusing the FAQs and found this post (viewtopic.php?f=29&t=4843) where it is stated that fusor demo operation is normally located in the 3-10 kV range. Considering that the output of such a transformer is around 2,2kV with plenty of current (0,5 amps, according to the Internet), it seemed like the construction of a power supply consisting of the MOT and a Cockroft multiplier is a viable option.

However, I pretend to control the voltage being supplied to the chamber, which is difficult taking into account that I have no Variac, and buying one is completely out of question (money problems). I started thinking about different options of electronic control, but then I decided to search for "Microwave Oven Transformers" in this forums and found several interesting threats, such as this ones:


Apparently, not only could the voltage be increased by increasing the frequency, but also control it with a VFO. However, I was not able to find a definite range of frequencies on which the transformer could be operated. I decided to do a little experiment. I hooked up a function generator creating a 6,2 V RMS sinewave to the primary and recorded the voltage depending on the frequency. The results are presented above.

It seems like the maximun output voltage is reached at 4,6 KHz, which seems to be way up of the suggested in the previous posts. I pretend to do different tests and post the results, but right now I am intrigued to why does this peak appear. ¿Is it the resonant frequency of the transformer? ¿Can this results be extrapolated to higher voltages?
I would be extremely grateful if someone could help me with this.

Re: Frequency effect in the operation of a MOT.

Posted: Fri Oct 12, 2018 8:11 pm
by Rich Feldman
Welcome, Cai. Nice work there, measuring the transformer with different resistive loads. Then reporting with a well-labeled chart (and excellent written English). Did you make sure the primary voltage was constant during your frequency sweeps?

Have you tried making a circuit model of the transformer, including some parasitic capacitance, that would give a similar frequency-dependent effect?

A general problem with MOT's is that they are designed with exceptionally high flux densities, to save weight and cost. That means the cores are close to saturation at normal primary voltage. They depend on forced air cooling, even at normal primary voltage, even with no load.

Suppose you want to get more voltage out, by operating at 4500 Hz. It won't be easy or inexpensive to make an inverter that delivers regular mains voltage at 4500 Hz. Then the transformer will overheat even faster, or require more active cooling, because of eddy current loss in the laminated steel core. Of course it's worth a try. :-)

Re: Frequency effect in the operation of a MOT.

Posted: Fri Oct 12, 2018 8:50 pm
by Cai Arcos

First of all, thanks for the well-explained and thoughtful comment. It was my first time posting in the forum and I was afraid I was doing something wrong. With regards to the input voltage, I hooked up an oscilloscope to the primary and monitored the amplitude of the waveform during all the testing. I was surprised how stable the function generator is, taking into account it was an old unit from school.

I must say, creating a circuit model is a very good idea! After finishing this comment I will go to sleep, but in the morning I will investigate more in this area (starting, of course, with a Wikipedia page: ... ransformer.) Luckily, in the front of the MOT there is a sticker with what appears to be a serial number. I don't know how I didn't Google it before, but I expect to find a datasheet with all the important parameters in there.

Finally, I have to admit I didn't consider the effects of high flux density. I have to refresh my knwoledge on real transformers. Thanks for pointing it out!
My plan is to first build a model unit powered from 12 V with a variable frequency drive and then scale it up. It seems more prudent to test everything first at Low Voltage.

Re: Frequency effect in the operation of a MOT.

Posted: Sat Oct 13, 2018 7:25 pm
by Cai Arcos

I am making this post to present a conclusion in the brief empirical study that I tried to conduct regarding the effects that different frequencies had in the operation of a MOT, hoping that it could be useful to any demo fusioneers making the necessary Power Supply.

The conclusion reached is the following: altering the frequency will lead to WORSE results than using the transformers at the specified 50/60 Hz. Now, you might be wondering about the graph I posted before. In it, an appreciable voltage peak could be seen at frequencies around 4700 Hz. At the time, I teorised about it being a resonance effect coming into play.

Well, the answer lies in perfoming a very simple experiment: no model or mathematical relation needed. The experiment in question is simply to gradually increase the load: the voltage at 60-100 Hz will remain fairly constant, but as you increase the resistance, you will see that as frequency rises voltage actually goes down! At first I thought that the function generator simply could not supply enough current, but there are two factors that made this difficult to believe:

- As I have said (and will demonstrate later) the voltage at the lower (transformer designed) frequencies remained pretty stable over all ranges.
- Monitoring the input current reveales that it stays pretty much constant over the whole range of frequencies.

This table is the last test (with a 120 kilo-ohm load):

Input Frequency Output Voltage
60 Hz 60 V
1000 Hz 60 V
1500 Hz 59 V
2000 Hz 58 V
2500 Hz 57 V
3000 Hz 55 V

I suppose this is the flyback effect, the coil storing the energy and then releasing it in bursts, similar to what is present in TV transformers. Useful for obtaining high voltages easily, but useless when you want some current (like a demo fusor).
Being honest, I am a bit disappointed: I was hoping the desing of a high frequency inverter could allow the MOT to be used more effectively at PSs. At least, I'm glad this should serve those that had the same idea as me to not make the same mistake.

Re: Frequency effect in the operation of a MOT.

Posted: Sun Oct 14, 2018 4:30 am
by Richard Hull
I have not responded yet to allow the experiment to be done as this is the best teacher of all. Something you do to learn. In general, an iron core laminated transformer designed for 60 hz cannot be used at much higher frequencies due to the mass of iron needed to make it work at its rated 60 hz frequency. Core loses go through the roof and power into the transformer at much higher frequencies goes into heating the core.

Richard Hull