Inrush current ballasting and manual winding of a MOT

[Mephitus]

So I have been trying to use a MOT (microwave oven transformer) to run a power supply I am bodging together with off the shelf modules, like the LM2596. Using a secondary transformer in series as a guide, I was able to wind the secondary for 38V. But since inrush current on a MOT is a monster, it would trip the breaker every time. I was going to try a delayed turn on relay in conjunction with an NTC, but had a wild hare and figured I could try to just see how effective it would be to just do it with some current limiting resistors. First attempt was a 27 and second was 10 . Each of them where 10W wire wound ceramic cement resistors. Both attempts resulted in a literal explosion of the resistor within moments of switching on. (feel free to laugh at my newb fail) I am just glad I had the entire setup in an isolated clear testing box.... (I'm crazy, not stupid) Is this a common failure mode for this kind of resistor? On a related note: Is there a reference I can use that details best operating parameters for selecting different types of resistors? Such as when or why you would use a wire wound vs carbon film?

After such an exciting and educational event, I was still in the mood to experiment some more (trying to "fail faster" as Adam Savage puts it) and so tried a "poor mans variac" (100W tungsten lightbulb in line to limit current). Once the MOT was powered, I could only read 1.78V RMS from the secondary. Yes, I triple checked to make sure. I used the 2nd transformer in series again to test and was able to get a reading on the secondary of the MOT that scaled as expected to the voltage applied. I will admit that I don't have any magnet wire of appropriate gauge and used an insulated multi-stranded copper cable instead. But would this really be the cause? Does coupling of inductance with a transformer change this drastically when using single-stranded vs braided/multi-strand cable?

When it comes to my planned power supply, I am beginning to think that a dedicated SMPS primary supply would simplify things. I am pretty sure I have enough scavenged parts on hand to make one, but cant find any good schematics for something around 35-40V at 5-10A. Anyone have something they can recommend? Or should I just bite the bullet and set aside funds for a pre-made one?

[Arjan Emm]

I guess you took the magnetic core apart. Are you sure that there are no bigger air gaps when you reassembled the core?
This can lead to excessive in rush currents because the induction drops really fast with tiny gaps in iron core transformers.

edit:
I would grind the core apart, cause they are typically welded. This leaves a lot of cuttings and sharp edges that could cause airgaps.

[Mephitus]

I guess you took the magnetic core apart. Are you sure that there are no bigger air gaps when you reassembled the core?
This can lead to excessive in rush currents because the induction drops really fast with tiny gaps in iron core transformers.

Actually, I took the time to carefully hack out the secondary winding only. I was even able to leave the plastic winding spindle intact. As far as I can tell, there isnt a magnetic shunt either. I dont have the matching HV cap though as the transformer was donated by a forum member as-is.

[Arjan Emm]

Clearly a red herring. I mentioned it because a total gap of .5mm or even a lot less can already cause massive inrush currents and sustained currents.

[Mephitus]

Well, I figured out both problems quite by accident. I had gotten annoyed at the strange results I was getting, so in a fit of frustration tried to dismantle the whole thing. Only to fine A: it was not actually welded together at all and B: the laminations where hiding a hell of a lot of rust between themselves. No wonder the thing couldn't saturate. So now I'm in the process of cleaning and sanding a large stack of laminations since I have more time than money right now.

Question though, what is a good lacquer/enamel I can use?

[SeanB]

The stuff you buy at the hardware store will work, or even a litre of clear polyurethane varnish. There are special transformer polyurethanes, but in many cases the cheapie will work just as well. You just need a thin coat on the laminations to insulate them.

[joseph nicholas]

If you have the capacitor that came with the MOT, try wireing it up in series with one of the primary side windings.  This should limit the inrush current on start up.  Use the variac and light bulb trick.  Play around with this until your happy with the output on the secondary side.  Measure the resistance on the variac and put in a ballast resistor of the same approximate rating.

Hope this helps.

[Mephitus]

If you have the capacitor that came with the MOT, try wireing it up in series with one of the primary side windings.  This should limit the inrush current on start up.  Use the variac and light bulb trick.  Play around with this until your happy with the output on the secondary side.  Measure the resistance on the variac and put in a ballast resistor of the same approximate rating.

Hope this helps.

Unfortunately I don't have the cap that came with it. It was generously donated to me by forum member Fat. But I have a delayed on circuit half built using an NTC and a rather beefy 40A latching relay. But at least now I know why my previous attempted ballast resistors died so dramatically. Kinda sad though now... The biggest resistors I have left are 5W. I will have to scavenge for some more. (Recently scavenged a high voltage diode. That will be fun  )

On that topic though, I haven't been able to find a proper breakdown of how or why to select different resistor types except for fringe/specialty application. Can anyone please point me towards a good educational reference about this?

[johansen]

your average microwave oven transformers with stock primaries suck up about 600 volt amps at no load. that's 5 amps at 120vac or 2.5 amps at 240vac. full load is typically 13 to 14 amps from the outlet.

your average transformer of the same weight and size, might be rated for 200-300 watts. it will have approximately 30% more turns than a microwave oven transformer for a flux density on the order of 1.5 or less. if you add an extra 20 turns to a 120v or 40 turns to a 240v microwave oven transformer that 600VA of no load reactive power will be reduced to somewhere on the order of 200. if you add 30% more turns you should be able to get it below 100volt amps.

[Gyro]

if you add an extra 20 turns to a 120v or 40 turns to a 240v microwave oven transformer that 600VA of no load reactive power will be reduced to somewhere on the order of 200. if you add 30% more turns you should be able to get it below 100volt amps.

That sounds like a sensible way of getting the flux density down from the insane levels used in MOTs, the only reservation is that the extra turns will probably have to extend outside the insulation of the existing primary bobbin. Proper insulation from the secondary winding becomes more difficult and will need special care.

Apart from specific applications like homebrew spot welders, I suspect that MOTs are a case of diminishing returns, even if they are free (don't let me put you off trying though!).