100 volt primary in Japanese MOT?

[klugesmith]

Could not resist picking up yet another scrapped MWO,  while thinking about taking half my weight in old transformers to the metal scrap yard.

First thing that attracted attention is the 2-prong power plug, a thing I'd never before seen on a MWO.
On closer inspection, the markings are all in Japanese (?) and the nominal power voltage is 100 volts.


Don't yet know if it works. 
Anybody think it could be damaged by running on 120 volts, here in 60 Hz land when the appliance is rated for 50 Hz? 
I expect core saturation would not hurt it before over-voltage or over-power in the voltage doubler circuit, magnetron, controller, or light bulb.

[davekni]

Probably work for a while, especially with some reasonable load (water or ...) inside to keep voltage from going too high.  Magnetron filament won't last as long.  Transformer saturation is about ideal for 120V 60Hz compared to 100V 50Hz.  If they used thicker laminations, eddy current and heating will be worse than at 50Hz.

[Mads Barnkob]

As MOTs are already designed to work on the edge of self-destruction, you should all some more turns to the primary coil, in order to run it on 120VAC.

[davekni]

"As MOTs are already designed to work on the edge of self-destruction, you should all some more turns to the primary coil, in order to run it on 120VAC."

Absolutely agree with the first part, but confused by the second.  60Hz at 120V generates exactly the same magnetic field strength as 50Hz 100V.  Why are more turns needed?

[Mads Barnkob]

"As MOTs are already designed to work on the edge of self-destruction, you should add some more turns to the primary coil, in order to run it on 120VAC."

Absolutely agree with the first part, but confused by the second.  60Hz at 120V generates exactly the same magnetic field strength as 50Hz 100V.  Why are more turns needed?

The Volt/turn in a MOT primary winding is roughly 1:1. The inductance/resulting impedance is so low that the no-load current draw is pretty high. So adding more turns is to bring the no-load current down and was really not associated to the original question, you are right about that. It has more to do with re-purposing MOTs into other applications

[rikkitikkitavi]

"As MOTs are already designed to work on the edge of self-destruction, you should all some more turns to the primary coil, in order to run it on 120VAC."

Absolutely agree with the first part, but confused by the second.  60Hz at 120V generates exactly the same magnetic field strength as 50Hz 100V.  Why are more turns needed?

They are designed to operate so far into saturation that the magnetising current in idle is quite high, several amps, and with thin primary wire the resistive loss and magnetic losses makes the transformer very warm in a short time, but generally they can do full load for a while before they overheat.

But at my office where about most of about 50-100 persons on each floor (6 floors) bring food to heat in microwave ovens from 11am to 1pm aboutish , they operate more or less continuously the pungent smell of hot transformer insulation spreads quickly in the kitchenette every lunch, and the microvave ovens fail repeatedly. Sometimes with the associated evacuation of the building to safe collect location at the outside  when the reception desperatly trying to prevent the fire brigade from storming the place due to the smoke activates the automatic fire alarm...

But mostly the thermosafe in the winding prevents this.

[klugesmith]

Rikki, are you referring to a thermal fuse built into MOT?  That I have not yet seen, though I have now seen a 2-prong plug.

My interest in the Japanese specimen is not to repurpose the transformer.  I have more MOTs' than variacs, and might have as many variacs as Rikki hints about in his signature string.

The hoard of MOT's is still here for two projects, both unfinished for decades. 

1. Measure the weights, dimensions, and key electrical parameters. Display in some kind of 2-dimensional chart to illustrate the range of variation found in commercial designs.  Save the out-lying specimens, which might include the 100V unit.  Bet it has a uniquely high S/P turns ratio.   Filament winding turns count is too small to adjust in 20% steps; I bet that's one reason MWO magnetrons are made with different filament voltages.

2.  Save one pair of MOT's that are well matched in turns ratio and magnetizing current at 1/2 voltage.   That's for running with primaries and secondaries in series.  Not for extra high voltage, but to run at a comfortable temperature while delivering "ordinary" MOT high voltage.  To light series strings of 20 or 25 120-V lamps, as a holiday decoration novelty.  Could use active ground fault protection in secondary circuit.

[rikkitikkitavi]

I assume that by following CE regulations for home appliances some type of thermofuse-before-turning-into-an-inferno-of-shorted-windings-and-burning-varnish . Could be an electronic one, or perhaps just somehting bolted onto the core etc. Maybe not buried into the primary winding but one would expect that to be the warmest part.

It was some time ago that I dismembered a MOT.

2-prong, ie no protective earth? huuurrgh. 

Interesting project documenting the various MOTs, I believe the scrapyard TCoiler community would be forever grateful.
I have donated a few variacs, 6 pieces totaling 20kVA have to be enough for a lifetime
Still looking for a complementary third 25A unit though. I had one in Stockholm but no time to go get it.

I am not sure about your idea with an insulated secondary high voltage series lamp string...

Apart from the safety issue with 4,6kV secondary voltage (albeit floating) what happens when one filament burns out?

Wont it start arcing immediately due to the high potential differential of the break and then the lower voltage drop of the arc compared to the filament would lead to higher current, ie overvoltaging the remainder lamps and shorten their lifespan. Or even the arcing would melt the lamp itself.
That wouldnt be detectable woth an ground fault protection.

The last lamps would go rather quickly. Could be spectacular fun though with proper safety precautions and some fusing

[klugesmith]

Here are some experiences I've had with ordinary incandescent lamps in long series-connected strings.

First exercise was in about 2001.  I wired 20 medium screw-base lamp sockets in series, mounted on a large square of plastic pipe with 5 lights on each side.   Installed 40-watt 120-V lamps and energized it with a plain old MOT.    Had some worries, such as thermal runaway of the lamp that heated fastest, but everything seemed to work fine.  Even used the original MO controller in reduced-power mode to make the lights flash on and off.  Natural current limiting due to MOT core shunts might have been helping.

A few years later I repeated the exercise (with the square in horizontal plane just below office ceiling), using 25-watt lamps in red, green, yellow, and blue colors from the factory.   Single MOT still tended to overheat, of course, without noisy fan to cool it.   

That's why I want to do the exercise with two MOT's in series, each working at 1/2 voltage, to greatly reduce the power loss in transformers.  Natural current limit would also be halved, but I expect to get 200 or 250 mA without much voltage loss.

Fast forward to 2018, when I was working on "277 volt power at home", and set up to measure secondary voltage and current in some 60-mA NST's.   I had a string of 25 Christmas lights, converted from parallel to series by cutting one wire in each section, to use as a handy high-voltage load.  Here energized at less than full brightness with the 60 mA NST. Floating digital milliammeter is partly shrouded for outdoor daytime readability.


Aside from the risks of electric shock, or arcing from accidental open circuits, the burnout issue is important.

An old friend told me that in early 20th century, 600 volts was popular in electric streetcars.   For cabin lights, it was convenient to wire five sockets in series and use 120-V lamps.  Of course a burned-out lamp could carry an internal arc, overheat, and damage the socket or even start a fire.  The remedy was to use vacuum filled bulbs instead of the more efficient, longer lasting argon filled bulbs.  Then any lamp failure would simply make whole string go dark.  There might have been something about special paper-based "fuses" for protection.

Along those lines, I think the little screw-base C5 and C7 lamps are typically vacuum filled. Enough vacuum to not arc between the ends of a broken filament, or to arc first between external wires.  Have saved a couple of burned-out specimens but never did the test.   There have been reports of people detecting x-rays when putting very high voltage on miniature almost-vacuum lamps.

[klugesmith]

Drifting off topic, here's an abbreviated story about 2018 work which I reported on 4hv forum.

I got a good deal on some 15 kV 60 mA NST's because their nominal primary voltage was 277.
Brought 240 V to workspace at home, and a dual variac that lets me go to 280 V with midpoint neutral.
The on-off switch is presently a two-pole 10-amp circuit breaker.

First 277-volt application was some low pressure sodium luminaires, that lit parking area at work before they were replaced with LED units (that are white but have fewer lumens per watt).

Next was the NST's, with a dedicated AC/DC kilovoltmeter and floating milliammeter.   Both have inexpensive LED digital panel meters and bridge rectifiers, and are calibrated to indicate DC voltage and current correctly.  I charted the NST output voltage-current curve with resistive loads; max power output is about 450 watts at 2/3 of nameplate voltage and 2/3 of nameplate current.

To fit the ongoing discussion right here, I have a picture of 20 fluorescent lamps in series, being powered by a NST whose primary voltage is set to nominal.

[davekni]

Did you parallel a bunch of NSTs to drive your LPSV lights?  The one LPSV bulb I have (bought new) is designed for ~0.58 amps.  Gather that they wear out quickly when under-driven, as not enough sodium evaporates and the resulting higher voltage drops near the cathodes causes more ion erosion.  (Same reason that power-cycles wear them out much more than steady operation.)

For my LPSV bulb, I bought a dual-tube electronic fluorescent ballast and checked that the outputs worked in parallel.  The two outputs paralleled is slightly low, ~0.53A, but close enough.  It's a really fun addition to my color & light science demonstrations.  In an otherwise-unlit room, everyone experiences what color-blind is like.  Amazing how one's mind slowly adapts to the yellow until faces and hands and such look more like shades of gray.  (Looks a bit spooky.  I like setting it up for Halloween.)  The only nuisance is the occasional fluorescent red object that is excited by yellow photons.

[klugesmith]

The LPS runs used the native ballasts at nominal voltage.  They came with primary taps for 208 and 277. Pictured running in summer sunlight.

What is the V in your LPSV acronym? [edit] Vapor / Vapour. [\edit]

When the sodium luminaires were replaced with LED's, our facility manager had the old units stacked next to the dumpster.  Pile gradually dwindled to nothing, not before I scrounged 2 complete units and an extra ballast to play with.  In another thread, perhaps under lighting, I can post some voltage and current measurements logged during a warm-up cycle.  How come the inert gas fill is neon, which is why the lamps are red when starting?

[davekni]

That makes sense.  I'd mis-associated your NST and LPSV projects.  Yes, I'm used to the abbreviation being for Low Pressure Sodium Vapor.  I'm guessing my lamp uses argon for starting, although I haven't studied the starting spectrum to see for sure.  Could be a mixture, but definitely not mostly neon.

What numbers do you have for efficacy?  I've read that LSPV run around 130 lumens/watt, and were the worlds most efficient light source until newer LEDs.  Some time ago I read of white LEDs hitting 160 L/W, but typical line-powered fixtures seem to be around 100 L/W.