230 volts spark gap

[Quentief]

To deal with the saturation phenomenon, I decided to rewind the core in an other way. Indeed, I have read on this article https://en.wikipedia.org/wiki/Choke_(electronics)#Near_magnetic_field_emission_reduction that it is possible to make the magnetic fluxes to cancel each other.

To do this, the each coil has the same number of spire, but the middle coil is winded in a different direction than the two others. Moreover, the iron section of the middle coil equals the sum of the two others sections coils. So in theory, the magnetic flux of each coil will cancel each other, and the core cannot be saturated, according to what I read on the article.

Do you think it is possible ?

I calculate the inductance of the coil, I find 60 mH, but I am not sure. The details are available in attachement.

Then, I connect the coil with my setup to try to make electric arcs from the main 220 V. It does not work better as my previous attempt.

[klugesmith]

You should measure the inductance using an AC voltmeter, resistor, and low voltage transformer.
If you really have 60 mH, the impedance is 19 ohms at 50 Hz.
With the coil in series with 19 ohm resistor, both elements will have the same AC voltage (each about 71% of the applied voltage).
If voltage across coil is lower than voltage across resistor, then inductance is less than 60 mH, and you can get it from the voltage ratio and known value of resistor.

You might substitute a 230 V heating appliance for the resistor, and skip the low voltage transformer.  1 AC ampere should produce 19 AC volts across a 60 mH inductor.  Incandescent light bulbs are not good to use as reference resistors.

Connecting winding sections to oppose each other increases the saturation current.  But it reduces the inductance by the square of that factor.  You are wasting wire.

If the coil winding directions are gauche, droite, gauche when viewed from top, then they are not going to cancel. 
When middle coil is forcing flux up in middle leg of core, the outside coils are forcing flux down in the outer legs.
Heads I win, tails you lose.
Even if you want three coils to aid each other, using the outer legs can't increase the effective number of turns.  All turns have to fit through the same winding window!  Turns around an outer leg are less well coupled to the middle coil, and are coupled in opposite direction to the other outside coil.

You can change the relative winding directions without uncoiling the wire. 
Bring out two wires for center coil, and two other wires for the outside coils.
Then you can externally connect them in series, or in parallel, with any polarity.
Learn something by measuring the inductance of series connection both ways.

[Quentief]

Hi everyone,

I did not check the inductance of my homemade coil. However, I replace it by a big transformer from my homemade spot welder (which is built from a microwave oven transformer). Here is the result : https://photos.app.goo.gl/z85tXhyhWVKtNm5K7

(sorry for the bad quality of the video, I was scared to damaged my phone because of the UV)

It works pretty good but I am wondering if the 10 amps are not dangerous for the MOT, the current may be too high for the primary coil and could overheat the transformer. I am wondering if I could use an electric motor instead, like a hair dryer motor : its universal motor should be able to run on the rectified 230 volt and its rotation should cool down its coil.

I have also another question, not linked with the topic : I cannot identify what is this component (it cames from an old PC power supply). Please, could you tell me what is it ?

Many thanks for your help

[Quentief]

here is the component

[johnf]

quentief
That is an X2 rated AC capacitor ie AC275 volts used in common mode filter on mains input

[davekni]

John gave you the answer, but you can also get that information and more by a simple google search of the writing on the part.  Try searching for "MEX-X2 MPP 1.0K 275V 40/100/21".  This works for many parts.

Concerning microwave transformer current, look for the power rating of the microwave it came from (or a typical microwave if you don't know).  Use that power and the line voltage it was designed for (ie. 230V) , and calculate a current.  The actual will be a bit higher due to power factor and losses in the magnetron, but that provides a lower limit to the likely value.

[Quentief]

Thanks for the component informations.

The power of the microwave oven was about 1300W, so about 6 Amps under 220 volt. In my setup, the current is limited with a 2200W kettle, so the current is about 10 Amps. I do not know if a such current can be tolerate for long time by the transformer.

[davekni]

10A would almost certainly be too much.  Depending on the actual voltage split between the 2.2kW kettle and your circuit, the current might be low enough.  A fan blowing on the transformer would allow slightly more current.

[Quentief]

Hi everyone,

I made some tests with different inductances :

This is a motor stator from an old vacuum cleaner. I cannot calculate its inductance but by considering the cable resistance, its section and the copper electrical resistivity, I calculated the lenght of the coil wire : 35 meters. Then, I was able to estimate the number of wire turns ; about 441 for each coil.

https://photos.app.goo.gl/jwejCwZyEb1WGdBH6

I was able to make some little arcs through this setup.

https://photos.app.goo.gl/JMVTAXJ9KT9xAno96

Then I tried again with the MOT with its secondary short circuited. The arcs was longer. So Davekni seems to be right : it is question of current smoothing, and the bigger is the inductance, the better are the result.

https://photos.app.goo.gl/dogYxUDi1m6iG6oGA

Then I made a last test with the MOT but this time, the secondary was not short circuited. I had supposed the inductance should be higher and I think I was right, the electric arcs were much more longer than before.

https://photos.app.goo.gl/68FDk6ufyj1T9ny2A

However, I would like to know if it is safe for my diode bridge. I mean, at each current variation, the coil produce a voltage and if it exceeds 600V, my diode bridge could be damaged. For the same reasons, I prefer to not use my multimeter to measure the voltage of the coils used in the setup, to estimate with precision their inductance.

[davekni]

Are you wearing a welding helmet or similar eye protection?  The arc conditions are not far from normal arc welding.  Viewing without protection is likely damaging little spots in your retinas.

If I understand your circuit correctly, 220VAC line goes through a 22-ohm kettle to a 600V bridge rectifier.  The rectifier output goes through an inductor to your two nails that form the arc.  In that case, the high-voltage spike across the inductor when the arc extinguishes will show up across the nails and inductor, but not the bridge.  The bridge sees only 220V (~320V peak), plus a small bit more due to stray inductance in power wiring to the 220VAC plug.  If you want more diode protection, add TVS diodes or MOVs across the bridge (input or output), or use a surge-protection outlet strip for power (which has MOVs internally).

Your DMM should be safe measuring voltage across the kettle (to determine current) or across the diode bridge.  Measuring across the inductor or arc electrodes might require some protection circuit (resistor divider and clamping diodes) before measuring.

Be very careful not to touch the MOT secondary when not shorted!

[klugesmith]

The vacuum cleaner motor stator will have _much_ more inductance when the armature (rotor) is in place.

[Quentief]

For sure, I was wearing UV glasses to protect my eyes. My smartphone however was not wearing  any UV protection, I hope I did not damage its camera (everything seems to be normal for the moment).

Davekni you are right about my circuit, here is a diagram : https://photos.app.goo.gl/2MwyUU4DuAxzMFF79

However, I do not understand why you claim the high voltage spike produced by the coil cannot reach the diode bridge : according to the Kirchhoff's circuit laws the voltage across the diode bridge output equals the coil voltage + the arc voltage. So if the coil makes a voltage which exceeds 600V, it will damage the diode bridge, won't it ? Or have I missed something ?

To answer you Klugesmith, I knew the all motor would have a higher inductance, however I chose to remove the rotor to have a motionless inductance, safer. But do you think I could add a big iron part at the center of the stator, to improve its inductance and have something closer to the MOT ? I still have difficults to calculate the inductance of coils when they have a magnetic circuit.

[klugesmith]

You said vacuum cleaner, so probably a universal motor with commutator and windings on the armature.
If you don't have brushes connected, the armature won't try to turn when you put it inside energized stator.

A solid steel cylinder in its place would introduce large eddy current losses.

When are you going to measure the inductance of your ballasts?  We've shown you how to do that with AC voltmeter and known resistors.

[Quentief]

Yes it is a universal motor with charcoal (or graphite) contacts. If the brushes are not connected, the rotor will not be able to turn. However, I do not understand why the inductance should be higher in this way ? We will just have a coil made of copper, without current through it and without magnetic property.

Concerning the inductance, indeed I would like to know how to calculate it to understand how I can increase it. But you right, I need to do some measures.

[klugesmith]

About that motor armature (rotor) ...

Remember when you figured the inductance of coil on laminated E-I cores?  One factor was the length of magnetic flux path in the steel.  Not just the part inside the coil, but the return path that includes outside legs of E-I core.

Same formula applies to your universal motor stator, with an important difference.  The flux path includes a large air gap between the poles, where the armature belongs.

The inductance formula has a path length term, which goes into the core's magnetic reluctance (resistance to being magnetized).  When computing reluctance of core with an air gap, each millimeter of air counts as much as at least 500 millimeters of steel.  In the case of your motor stator, the reluctance from air gap is much larger than that of the steel path, which makes the inductance relatively small.

When you put the armature back in place, its copper windings don't matter.  You are putting laminated steel where there used to be air.   Remaining air gap will be less than 1 mm "long" in the direction of magnetic flux, so the stator coil inductance will be more than 10 times larger than with no armature present.  Measure it!