Operation of Microwave Oven - Essay Example

The operation of a MICROWAVE OVEN. The microwave oven did not come about as a result of someone trying to find a better, faster way to cook. During World War II, two scientists invented the magnetron, a tube that produces microwaves. Installing magnetrons in Britain's radar system, the microwaves were able to spot Nazi warplanes on their way to bomb the British Isles. By accident, several years later, it was discovered that microwaves also cook food. Called the Radar Range, the first microwave oven to go on the market was roughly as large and heavy as a refrigerator. The idea of using microwave energy to cook food was accidentally discovered by Percy Le Baron Spencer of the Raytheon Company when he found that radar waves had melted a candy bar in his pocket. Experiments showed that microwave heating could raise the internal temperature of many foods far more rapidly than a conventional oven1. The first Raytheon commercial microwave oven was the 1161 Radarange, which was marketed in 1954. Rated at 1600 watts, it was so large and expensive that it was practical only for restaurant and institutional use. In 1967, Amana, a division of Raytheon, introduced its domestic Radarange microwave oven, marking the beginning of the use of microwave ovens in home kitchens. Although sales were slow during the first few years, partially due to the oven's relatively expensive price tag, the concept of quick microwave cooking had arrived. In succeeding years, Litton and a number of other companies joined the countertop microwave oven market. By the end of 1971, the price of countertop units began to decrease and their capabilities were expanded2. All electromagnetic energy can be characterized as waves with a specific wavelength and frequency distributed over a continuous range known as the electromagnetic spectrum. For example, some radio waves have a wavelength of 6 feet (2 meters) and a frequency of 50 million hertz (Hz-cycles per second). Visible light waves have a wavelength of 400 to 700 millimicrons, and typical X-rays have a length of 0.01 millimicrons and a frequency of 30 x 1012 millions. Microwaves (short waves or high frequency radio waves) are the shortest of radio waves, with a length of 0.1 millimeter and a frequency of 3 x 101 Hz. They are found in the non-ionizing portion of the energy spectrum, between radio waves and visible light. "Non-ionizing" means that microwaves do not detach charged particles and produce atoms with an unbalanced plus or minus charge. Microwaves can therefore safely produce heat and not cause food to become radioactive. Microwaves are reflected from most metals but they produce inductive resonance's in the atoms of many other substances. It was the discovery of their reaction to metals that led to the invention of radar. It was their ability to produce resonant coupling that led to the invention of the microwave oven3. Microwave ovens use various combinations of electrical circuits and mechanical devices to produce and control an output of microwave energy for heating and cooking. Generally speaking the systems of a microwave oven can be divided into two fundamental sections, the control section and the high-voltage section . The control section consists of a timer (electronic or electromechanical), a system to control or govern the power output, and various interlock and protection devices. The components in the high-voltage section serve to step up the house voltage to high voltage. The high voltage is then converted microwave energy. Basically, here is how it works: As shown in Figure 1, electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits. These circuits include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs4. If all systems are normal, the electricity passes through to the interlock and timer circuits. When then oven door is closed, an electrical path is also established through a series of safety interlock switches. The purpose of the interlock system is to interrupt the production of microwave energy when the oven door is opened, and similarly, to prevent any microwave output until the door is firmly and safely closed. Generally speaking, the normal sequence of switch operation when the door is opened is as follows. First the primary switch opens its contacts. Second, (yes) the secondary switch opens. Finally, the interlock monitor switch closes its contacts. The fail-safe system works like this: if any of the switches and/or relays included in the monitor loop (or circuit) fail to open their contacts properly when the door is opened, a short circuit is created when the monitor switch closes its contacts. The closed contacts of the monitor switch and the faultily-closed contacts of the defective switch combine to cause an immediate short circuit, which, in one way or another (depending on the model), blows the line fuse, or otherwise disables the oven. All this happens before the door can be opened far enough to allow any dangerous levels of microwave radiation to escape5. Generally, the control system includes either an electromechanical relay or an electronic switch called a triac as shown in Figure 2. Sensing that all systems are "go", the control circuit generates a signal that causes the relay or triac to activate, thereby producing a voltage path to the high-voltage transformer6. It might be said that the high-voltage transformer is the "muscle" of the microwave oven. With an input of 120 VAC (or 240 VAC in many commercial models) applied to the primary winding, the high-voltage transformer (also referred to as power or plate transformer) steps up that primary voltage to a very high voltage. This high voltage is then boosted even higher by the voltage-doubling action of the capacitor and diode. The resulting voltage, about 3000 - 5000 volts DC (depending on the model), is available at the high voltage (output) tap7. By adjusting the on-off ratio of this activation signal, the control system can govern the application of voltage to the high-voltage transformer, thereby controlling the on-off ratio of the magnetron tube and therefore the output power of the microwave oven. Some models use a fast-acting power-control relay in the high-voltage circuit to control the output power. Most models of microwave ovens control the output power by governing the on-off time of the magnetron tube. This is most commonly done by cycling on and off the line voltage that is applied to the primary side of the high-voltage transformer. Models that use a high voltage relay accomplish this power control by actually switching the high voltage on and off. This is quite a feat for this little reed relay, so it relies on very carefully timed signals from the control panel. In the high-voltage section (Figure 3), the high-voltage transformer along with a special diode and capacitor arrangement serve to increase the typical household voltage, of about 115 volts, to the shockingly high amount of approximately 3000 volts! While this powerful voltage would be quite unhealthy - even deadly - for humans, it is just what the magnetron tube needs to do its job - that is, to dynamically convert the high voltage in to undulating waves of electromagnetic cooking energy8. The magnetron is a diode-type electron tube which is used to produce the required 2450 MHz of microwave energy. It is classed as a diode because it has no grid as does an ordinary electron tube. A magnetic field imposed on the space between the anode (plate) and the cathode serves as the grid. While the external configurations of different magnetrons will vary, the basic internal structures are the same. These include the anode, the filament/cathode, the antenna, and the magnets. The anode (or plate) is a hollow cylinder of iron from which an even number of anode vanes extend inward. The open trapezoidal shaped areas between each of the vanes are resonant cavities that serve as tuned circuits and determine the output frequency of the tube. The anode operates in such a way that alternate segments must be connected, or strapped, so that each segment is opposite in polarity to the segment on either side. In effect, the cavities are connected in parallel with regard to the output. This will become easier to understand as the description of operation is considered. The filament (also called heater), which also serves as the cathode of the tube, is located in the center of the magnetron, and is supported by the large and rigid filament leads, which are carefully sealed into the tube and shielded. The antenna is a probe or loop that is connected to the anode and extends into one of the tuned cavities. The antenna is coupled to the waveguide, a hollow metal enclosure, into which the antenna transmits the RF energy. The magnetic field is provided by strong permanent magnets, which are mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode9. So, the microwave energy is transmitted into a waveguide, which feeds the energy into the cooking area where it encounters the slowly revolving metal blades of the stirrer blade. Some models use a type of rotating antenna while others rotate the food through the waves of energy on a revolving carousel. In any case, the effect is to evenly disperse the microwave energy throughout all areas of the cooking compartment. Some waves go directly toward the food, others bounce off the metal walls and flooring; and, thanks to special metal screen, microwaves also reflect off the door. So, the microwave energy reaches all surfaces of the food from every direction. All microwave energy remains inside the cooking cavity. When the door is opened, or the timer reaches zero, the microwave energy stops--just as turning off a light switch stops the glow of the lamp10. Appendix. Works cited. 1. Cowan, Ruth Schwartz, A Social History of American Technology. Oxford University Press, December 1996. 2.Gallawa,J.Carlton,HowAMicrowaveOvenWorks.. 3. Gallawa, J.Carlton, The Complete Microwave Oven Service Handbook 2005. New York: W.W. Norton, 2004. 4.Gallawa,J.Carlton, The Magnetron Tube Used In Microwave Ovens . 5. Gallawa, J.Carlton, The Purpose and Function of Interlock Switches Used In Microwave Ovens. . 6. Gallawa, J.Carlton, The Purpose of the High Voltage Transformer in Microwave Ovens. . 7. Schulman, Peter, Everyday Devices. Boston, 2002. Read More