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MICROWAVE OVEN REPAIR DIAGNOTIC

Microwave oven not heating
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Microwave oven turns on by itself
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microwave oven repair

Commercial microwave oven

Microwave Oven

A microwave oven is an electronic appliance, used in the process of heating or even cooking food. This device functions by the generation of electromagnetic waves in the microwave frequency, which is around 2.45 GHz.

Index

  1. How an electronic  works.
  2. History
  3. Precautions

How an electronic microwave oven works

A microwave is a domestic appliance designed to heat and cook food, by heating the water they contain or the liquids added. It works by the generation of high frequency radio waves. Water, fat tissue and other substances present in food absorb energy produced by microwaves in a process known as dielectric heating (also known as electronic heating, RF (radio frequency) heating, high frequency heating or diathermy). Certain molecules have a structure that creates electric dipoles; an example of those is water, this means that they have a positive partial charge on one side of their structure, and a negative partial charge on the other side. This causes molecules to oscillate in an attempt to align themselves with the oscillating electric field of microwaves. As they rotate, friction and interactions between molecules elevate temperature. Microwave oven function in this manner: a device called magnetron converts electric energy into microwave energy, and it is in that form that energy heats food. Electromagnetic waves agitate bipolar molecules present in food, such as water, and this agitation elevates temperature. This agitation is a physic phenomenon, a simple motion of molecules at the rhythm of microwave frequency; therefore, no alteration of chemical composition occurs, other than those produced by the increase on temperature.

Microwave heating is more efficient in liquid water than it is, in frozen water. This occurs because motion in frozen state water is much more limited. It is also less efficient in fat and sugar because they have a smaller molecular bipolar momentum.

Sometimes microwave heating is explained as resonance in the water molecules, but this is a mistake, since set resonance only occurs in steam and at much higher frequencies (around 20 GHz). On the other side, big industrial microwave ovens that usually work at a frequency of 915 MHz (wave length of 328 millimeters) also heat up water and food effectively.

 

Both sugars and triglycerides (fats and oils) are able to absorb microwaves because of the existence of bipolar moments in their hydroxyl and ester groups. However; a lower specific heat value in fats and oils as well as a higher vaporization temperature, can cause them to attain a much higher temperature inside microwave ovens. This may cause in oil or food with a high concentration of fat tissue, such as bacon, temperatures way above the boiling temperature of water. This way a microwave can toast in a similar manner as the one done in a conventional grill or in a deep fryer. Food with a high concentration of water and very little oil or fat tissue can rarely exceed the boiling temperature of water.

 

Microwave heating may cause an excess of heating in some materials with a low thermal conductivity, as they also may have dielectric constants that increase with temperature. One example of this is glass, as thermal runaways may take place inside the microwave oven and may even melt. Microwaves can even melt certain types of rock producing small amounts of synthetic lava.  Some ceramics can also melt and they can even lose coloration as they get cold. Thermal runaways are more common in liquids with a high conductivity such as salty water.

 

It is a common mistake to consider microwaves cook from the inside out, meaning, from the center of the mass towards the exterior. This idea comes from what happens when we have a lower layer of mass that is moist and an outer layer that is dry. Since heating from a microwave comes mainly of the presence of liquid molecules; usually missing from the exterior of a mass. In most cases in uniformly structured foods, or reasonably homogeneous in their physical composition, microwaves are absorbed by the outer layers in the same way as heat is absorbed by other methods. Depending on the water content, the depth of the initial heat penetration can be of several centimeters or more in microwave ovens, in other cooking methods such as grilling, or a convention oven, heat only enters through the outer layer. The depth of the penetration with microwaves depends on the composition of the food and the frequency the microwave works at; lower microwave frequencies (longer wave lengths) are more penetrating. Microwaves can only penetrate an estimate of about 2-4 centimeters into the mass, so the center of bigger portions will not be cooked by the direct action of the microwaves, instead it will cook by the heat produced in the oven and the heat transferred by areas of the mass that do receive microwaves.

 

History MICROWAVE OVEN

Like many other inventions the microwave oven is a secondary and accidental application of a technology developed for other reasons. In 1946 in the process of an investigation related to radars; Doctor Percy spencer, an engineer of Raytheon Corporation, was testing a vacuum tube called magnetron when he discovered that the chocolate bar he had in his pocket had melted. Suspecting that it was caused by the waves emitted by the magnetron, Dr. Spencer placed some popcorn seeds near the tube as an experiment. Corn cooked and popped; Spencer repeated the experiment using a chicken’s egg. Because of the rapid increase of temperature internal pressure made the egg exploded. These results encouraged him to continue experimenting with other elements.

 

Dr. Spencer designed a metallic box with an opening to allow radiation from the magnetron to enter. Metallic walls contained the radiation within, as energy of the electromagnetic field rises. When food is introduced inside of the box, its temperature would raise. Engineers of the Raytheon Corporation set out to improve the prototype of Doctor Spencer and by the end of 1946, Raytheon applied for a patent to employ a microwave in the preparation of food. The first experimental oven, heating prepared food by the use of a microwave was installed in a restaurant in Boston. In 1947, the first commercial microwave oven when out to market. However this first units were really big and expensive, they were about 1.6 meters high and about 80 kg of weight. The magnetron was water cooled so an installation for the cooling system was required. Also, as you can imagine, price was elevated, since they were sold at the time at about 5000$ dollars; and that is around 55000$ of current currency, so they didn’t had much acceptation.

The invention of an air cooled magnetron; eliminated the need of a complex and elaborate cooling system, and allowed for the design of less expensive and smaller ovens. Fast food dinners were the first ones to discover the many benefits of owning one of these new inventions.

As the food industry discovered the potential and versatility of the new invention, applications were found in the most various areas. Microwaves were used to dehydrate vegetables, toasting coffee beans or dry fruits, unfreeze and cooking meats, opening oysters… the applications were endless. But this success was not limited to the food industry, other industries also found use for microwaves; the capacity of microwaves to interact with water and liquids barely affecting other materials found application in the drying process of materials such as cork, ceramics, paper, leather, tobacco, textiles, pencils, flowers, damp books and marches. Microwave oven were also used in the curating process of synthetic materials such as nylon, rubber, and urethane.

Despite this success, and because of mistrust to the new “electronic radar ovens”; this device was not of domestic use until the 70’s. Since there were no cases of radiation poisoning, nor were there reports of people getting blind or sterile due to the use of microwave ovens; fears and myths gave away and acceptance to the microwave ovens significantly increased.

In 1975 sales of microwave ovens were higher than gas ovens for the first time. The following year 17% of all Japanese homes were using a microwave, in comparison, only 4% of homes in the US had one. In the 1971 only 1% of homes in the US own a microwave but that number had increased to 13% by 1978 and by 1986 one out of every four homes in the United States owned a microwave. In present day, microwave ovens have timers, baking sensors, electrical resistances to aid with gratin and give dishes a golden exterior if required, since conventional microwave ovens are unable to achieve this.

 

Precautions in use microwave oven

 

Most nations defend the use of microwaves as a home appliance that is perfectly safe for health. However, some security measures are mandatory.

Never introduce metallic containers, or utensils as the may generate a spark and create a fire; although some utensils and containers are safe and designed specifically for microwave oven use.

Do not cook eggs whole with their shell, the buildup of steam pressure inside can make them explode.

Precautions must be taken when boiling water or other liquids inside the microwave; since accumulated heat can cause splatter burns, even when liquid does not appear to be boiling as you reach for the container. This phenomenon occurs because of overheating, that is, an accumulation of temperature higher than the ebullition temperature. Basically temperature rises so fast that it takes a few seconds before ebullition process begins.

As you attempt repairs safety measures must be respected, as there is a high risk of electrocution; even when the appliance is disconnected of the grid, this happens because of the presence of a high capacity condenser.

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