Single Resonant MOT

Huge Arcs from a Single Resonant Microwave Oven Transformer


Introduction

The Microwave Oven Transformer is a normal, large, laminated iron core transformer found in Microwave Ovens. This transformer steps up the 240 or 120VAC voltage from the mains, to about 1800 to 2200VAC for use in the magnetron (to generate Microwaves). These Microwave Oven Transformers (MOTs) are very robust and handle 600W (for small ovens) up to 2000W (for the biggest ovens). These MOTs are also cheaply and easily found, making them favourite components of High Voltage enthusiasts. Other useful components inside a Microwave Oven also include the high voltage capacitor (usually 2kVAC at 1uF) and a high voltage diode.

Here, I present the simplest way to create huge power electrical arcs using only a single MOT and some Microwave Oven Capacitors.

Warning: The experiment described here is very dangerous due to the presence of high voltage and extremely high currents. Any accidental contact with the HV output will certainly cause severe injury, and can be lethal. Everything you do is at your own responsibility and risk.


 29 Nov 2013 

Schematic and Construction

For this experiment, I used a small MOT (probably from a 700W Microwave Oven), as well as two Microwave Oven Capacitors. I also used a 2kW variac to power the primary of the MOT to allow me to control power, as well as to provide a small amount of ballasting which would otherwise trip the mains.

Principle of Operation

A single MOT produces 2kVAC, which is insufficiently high to jump much of an air cap (about 2mm). However, after a spark is formed when the outputs are briefly touched, the high current (100mA +) flowing through the spark quickly heats up the air and ionizes it, forming a plasma channel which can support even more current. This creates a huge bright-white flaming arc. When the arc is drawn too long, the power cannot keep up with the constantly cooling plasma channel. Eventually, the arc extinguishes. For a single MOT, making arcs longer than 20cm can be difficult.

However, by adding a series capacitor, we bring the system into series resonance.

MOTs are designed to have what is known as a large leakage inductance. When a heavy load is applied on the secondary of the MOT, instead of dropping to 0 inductance, the MOT core is designed in such a way for the inductance to drop to this leakage inductance. Because we are driving the MOT at mains frequency (50 ot 60Hz), we can calculate the correct capacitance to bring the system into resonance.

The result of this is that much larger currents can be drawn, as well as resonant rise in output voltage, several kV higher than we would normally expect! All this results in a arc that has much more power --> larger, longer, and also more dangerous!

Finding the Resonant Capacitor Size

In order to find the right resonant capacitor size, one needs to find out what the leakage inductance is. It is generally challenging to find the inductance without proper equipment. However, depending on the MOT, experiments have shown that a capacitance of 2 to 3uF on the output generally leads to good results. Thus, one can simply use Microwave Oven capacitors in parallel, which are perfectly suited for these task.

Schematic

The schematic is straightforward.

Note that some MOTs have only one secondary output. The other is tied to the metal body of the MOT. The components are quickly wired up.

The arc is very hot and powerful - large electrodes must be used otherwise they will melt!

Additional Work

For even bigger arcs, two MOTs can be used instead, with the secondary wired in series and a small resonant capacitor (albeit rated at a higher voltage). For example, two MOTs could have their primaries wire in parallel (with the phasing checked to be correct) and secondaries in series. This is then placed in series with a 1uF 4kV resonant capacitor on the output.


Results

The results are very impressive. A single MOT would create roughly 20cm arcs at best. With the resonant capacitor, arcs grow to in excess of half a meter (difficult to capture the exact moment of maximum length with a camera). The resulting arc is very hot and very bright, and contains significant UV. Proper eye protection is very important.

Very impressive flaming arcs which ignite the oxygen in the air forming plasma blobs even after the arc has extinguished.

At these power levels, the MOT heats up very quickly. The MOT should only be run for short periods of time.


Links

Here are some useful links to find out more about resonant MOTs.


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(c) Gao Guangyan 2013
Contact: loneoceans [at] gmail [dot] com

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