Microwave Heating

While microwaves have many uses, we are concerned here, only with their use in the field of heating. As those who are likely to read these pages will know, the device responsible for generating microwaves of sufficient power to cook food or weld rubber etc. is the cavity magnetron. Its potential use as a means to cook food was discovered by Percy Spencer who worked for the American company, Raytheon in the 1940’s. It was this company who filed a US patent for a microwave cooking system in 1945.

Following development by other manufacturers in succeeding years, oven efficiency progressively improved to emerge as the ubiquitous heating system we see today.

How is the heating achieved?

The microwave energy developed by the magnetron is confined within a metal box that acts as a “Faraday Cage”. Articles of food placed in the chamber are cooked by dielectric heating.

Water, and many other molecules are, in effect, electric dipoles in that they have a positive charge at one end and a negative charge at the other. Thus as they are subject to microwave energy, they are continuously moved or rotated as they attempt to follow the alignment of the alternating microwave signal. During this movement, the molecules collide with those nearby, producing friction and heat.

The microwave energy that generates this heat penetrates the food to a depth of approximately 2 centimetres. Heating to a greater depth is achieved by conventional thermal conduction.

Regulation of the cooking process

Although there are microwave ovens available in which the regulation of the cooking process is continuously variable, it is fair to say that most ovens operate cooking power level via a “mark-space ratio” arrangement.

Fig. 3. mark-space ratio power control

Thus the average power for the microwave oven under the above on/off conditions is approximately 1/3 of full power.

The significance of this arrangement becomes apparent when technicians are asked to check the oven for leakage