Microwave Oven

About 95 percent of American households own a microwave oven.

A microwave oven is an appliance that cooks or warms up food using microwaves produced by an electronic vacuum tube called a magnetron. The magnetron converts electricity to electromagnetic radiation (the microwaves), which is made up of waves of electric and magnetic energy. Once generated by the magnetron, the microwaves travel through a metal enclosure called a waveguide to a stirrer fan, which distributes the microwaves into the cooking cavity. Inside the cooking area, the microwaves are absorbed by the food, which is cooked or heated in a few minutes or seconds.

Efficient cooking with microwaves

Microwaves can be used in cooking because of certain characteristics. They pass through many types of glass, plastic, ceramic, and paper. They are reflected by metal, and they are absorbed by food. In microwave cooking, the microwaves act on the water molecules in the food, causing them to vibrate. The vibration causes friction, which generates heat, thereby cooking the food. Microwaves cook food more efficiently than conventional ovens because they act only on the food, unlike regular ovens which heat the oven walls and the air around the food. And since microwaves change to heat once they are absorbed by the food, the food does not get contaminated with the electromagnetic radiation.

First microwaved foods

The ability of microwave energy to cook food was discovered in 1945 by an engineer in the Raytheon Company. Percy L. Spencer (1894–1979) was inspecting a magnetron at work. (During World War II [1939–45], the magnetron tube had been used in radar systems.) Spencer discovered that a chocolate bar in his pocket had melted even though he had not felt any heat from the magnetron. Next, Spencer exposed some popcorn kernels to the magnetron, and, sure enough, the kernels popped.

Spencer discovered that, when confined to a metal enclosure, microwaves produced by the magnetron tube excite certain molecules, such as water found in food, causing the food to increase in temperature and eventually get cooked. The first microwave oven, called Radarange, developed by Spencer and the Raytheon Company, was about the size of a refrigerator. It was used in commercial food preparations, and cost about $3,000. In 1955, slightly smaller versions were sold as home appliances, but they were still expensive, and people were not sure they wanted an appliance that they thought emitted "radar waves."

Fast food, fast bucks

In 1967, Amana, a Raytheon division, introduced the first countertop models at a cost of $495. By the mid-1970s, microwave ovens were outselling gas ranges. Fears about the potential dangers of microwaves had dissipated, and different companies were manufacturing microwave ovens for home use. Since then, microwave ovens have become a popular kitchen fixture. About 95 percent of American households own a microwave oven. Related industries, such as microwavable foods and utensils designed to be used especially for microwave cooking, have also grown rapidly.

Design

A microwave oven has a front panel that allows the user to program the oven. The panel shows the cooking time, the power level, the time of day, and other information. The door has a window with a perforated (having small holes) metal shield that enables the user to view the food while it is cooking. Microwaves cannot go through the holes to cause any harm because the holes are smaller than the wavelengths of the microwaves. As a result, microwaves simply bounce off the shield to act on the food. An oven light is also included.

The various electronic motors, relays, and control circuits (the routes through which electricity flows) are situated on the outer casing, to which the oven cavity is bolted, or fastened. The cooking cavity may contain a round, glass turntable on which the food dish is placed. The turntable rotates the food dish for even cooking. Instead of a turntable, the cooking cavity may just have a stationary (nonmoving) glass dish, which fits the bottom of the cavity.

Near the top of the cooking cavity is the magnetron tube, which produces the microwaves. The microwaves are funneled through a metal tube called a waveguide and into a stirrer fan. The fan distributes the microwaves evenly within the oven. Manufacturers use different methods to circulate the microwaves to achieve even cooking. Some use dual stirrer fans located on opposite walls. Others, in addition to the stirrer fan on top of the cavity, use entry ports (openings) at the bottom of the cavity so that microwaves enter through both the top and bottom.

Raw Materials

The outer covering of a microwave oven is usually a one-piece sheet metal enclosure. The door and the panels of the cooking cavity are made of stainless steel or galvanized steel (steel that has been coated with a thin layer of zinc to protect against rusting). A coating of acrylic enamel is also applied to the panels. The cooking surface is generally made of ceramic or glass. The cooking cavity is usually painted with a light color so that it is easy to see from outside the oven.

Between the cooking cavity and the oven walls, electromechanical components and controls consist of timer motors, switches, and relays. Also inside these walls are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the different components are made up of metal and plastic parts, including gears, pulleys, belts, nuts, screws, washers, and cables.

The Manufacturing Process

Most parts of the modern microwave oven are mounted in the area behind the front panel, between the sidewall of the cooking cavity and the chassis sidewall. Manufacturers aim to make ovens that are not too bulky. Therefore, they make sure the parts are efficiently situated within the available space, allowing enough room for the great amount of heat generated by the magnetron.

MAKING WAVES

How does a magnetron tube make microwaves? When heat is applied to the cathode (also called the filament), which is at the center of the magnetron, the cathode produces electrons, or negative electricity. The electrons are attracted to the anode (positive electrode) cylinder that surrounds the cathode. The electrons start traveling straight toward the walls of the anode cylinder. However, two magnets on top and below the cathode-anode structure cause the electrons to spin, thereby generating powerful microwaves. These microwaves go through an antenna above the anode, pass through a metal enclosure called a waveguide, and are distributed into the cooking cavity by a stirrer fan.

The cooking cavity and door

1 The manufacture of a microwave oven starts with the cooking cavity and the door. The cavity panels are formed using automatic metal-forming presses that can make twelve to fifteen parts per minute. In metal-forming, pressure is applied by the press on metal, changing its shape to form the cavity parts. The panels are rinsed in alkaline cleaner to remove dirt or oil. Then, they are rinsed with water to remove the alkaline solution.

2 The cavity panels and door are treated with zinc phosphate to prepare them for electro-deposition, during which the pieces are immersed in a paint tank and electric current is used to apply the paint. Then, the pieces are moved through a paint-bake operation to set the paint coating at 300 degrees Fahrenheit (149 degrees Celsius) for twenty minutes.

3 After the door is painted, a perforated metal shield is attached to the window opening. The shield reflects microwaves but allows light to enter the cavity. The door will not be attached to the cavity until after the chassis, or main framework, is assembled.

The magnetron tube subassembly

4 The magnetron tube consists of a filament heater, a metal anode, two magnets, and an antenna. The filament, which functions as the cathode, is enclosed in a ring-shaped metal anode. Then the cathode and the anode are sandwiched between two magnets. These components make up the assembly that will generate the microwaves. A metal strap holds the complete assembly together. Metal cooling fins, which are welded to the anode cylinder, help get rid of the heat produced by the magnetron. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.

5 An antenna enclosed in a glass tube is connected to the anode cylinder. Then, the air within the magnetron tube is pumped out to create a vacuum. The antenna is connected to the waveguide, the hollow metal enclosure through which microwaves are transported from the magnetron tube to the cooking cavity. A blower motor is attached to the magnetron, and then a plastic fan is attached to the blower motor to provide cool air to the magnetron to prevent overheating.

Chassis assembly

6 The chassis is the major framework onto which the various oven components will be attached. The chassis assembly work is done on a pallet, a work-holding device used with other tools. The chassis is placed on the pallet, and the cooking cavity is screwed on to the chassis. The door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and chassis.

7 The electric circuit of the oven consists of the transformer (which generates the high-voltage, or powerful electricity, required to operate the magnetron tube), an oil-based capacitor (which receives and stores electric current), and a rectifier (which changes the alternating current from the transformer into the direct current needed by the magnetron). All of these are mounted directly on the chassis, close to the magnetron tube.

Stirrer fan

8 The stirrer fan that circulates the microwaves is mounted on top of the cooking cavity. Some manufacturers use a pulley to operate the fan from the magnetron blower motor. Others use a separate stirrer motor attached directly to the fan. After the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce sparks and damage the magnetron tube.

Control switches, relays, and motors

9 The cook switch provides power to the transformer by energizing a cook relay and a timer. The cook relay, also called a power relay, allows the control board to turn the microwave source off and on. The relay is mounted close to the power transformer, while the timer is mounted on the control board. Also mounted on the control board are a light switch for viewing the cooking cavity and a timer bell that rings when the cooking cycle is complete.

SUPERHEATING HAZARD

People have reported hot liquids exploding inside microwave ovens or being burned when they first introduced a teaspoon, sugar, or teabag to a hot liquid right out of the oven. These accidents are caused by superheating. If a liquid does not boil (steam bubbles do not form during heating), it tends to heat past the boiling point of 212 degrees Fahrenheit (100 degrees Celsius). Normally, when liquid is heated over the stovetop, small bubbles form on the bottom of the heating container even before the boiling point is reached. The escaping bubbles ensure that the liquid boils. With microwave cooking, the liquid typically gets heated uniformly and no bubbles form. Objects introduced to the superheated liquid initiate the boiling process, causing it to explode.

A number of interlocking switches, also called safety switches, are mounted near the top and bottom of the door area. These switches prevent the production of microwaves when the door accidentally opens during cooking.

Front panel

10 Also attached to the chassis is the front panel, which allows the user to select the various settings and features available for cooking. Mounted behind the front panel is the control board, which is an electronic board that controls the various programmed operations when the switches are pushed on the front panel. This board is connected to the front panel and other components by means of plug-in sockets and cables.

Testing and packaging the oven

12 Most manufacturers run the oven from fifty to one hundred hours continuously as part of the testing process. After testing is complete, a robot palletizer records the model and serial data of the oven for inventory purposes. A robot palletizer, which is fitted with a gripper unit, is also used in many industries for loading finished products onto pallets, or platforms. Finally, the microwave oven is sent to the packaging area.

Quality Control

Strict quality control throughout the manufacturing process is very important. Radiation that is emitted by the microwave oven can burn anyone exposed at high levels for prolonged periods. A computer-controlled scanner is used to measure radiation leaks around the door, window, and back of the oven. Other scanners check antenna radiation, as well as the seating of the magnetron tube. Each scanner operation passes on information to the next operation so that problems can be corrected.

The Code of Federal Regulations, revised as of April 1, 2001, limits the amount of radiation that can leak from a microwave oven to "1 milliwatt per square centimeter at any point 5 centimeters or more from the external surface of the oven, measured prior to acquisition by a purchaser, and, thereafter, 5 milliwatts per square centimeter at any such point." The regulation also requires all ovens to have a minimum of two safety interlocks to prevent the production of microwaves if the door accidentally opens during cooking.

The Future

Manufacturers continue to develop various models and new applications to appeal to new users and repeat consumers. Like other industries that are taking advantage of the popularity of the Internet, the microwave industry has introduced the so-called Internet microwave oven. The oven can be connected to the Internet to access recipes and other cooking information for automatic food preparation. Several companies plan to market their products in the near future.

Other new developments include a microwave oven with special brackets that allow installation anywhere in the kitchen, including inside wall cabinets and over islands (the freestanding areas usually found in the middle of the kitchen). Some models have the control panel located behind the oven door, thus giving the oven an overall sleek appearance. A trendy model that appeals to many of today's consumers is patterned after a popular computer monitor that comes in colors of red, orange, green, and blue.

DID YOU KNOW?

• Contrary to a popular myth, a microwave oven does not cook from the inside out. If the food is small, the microwaves penetrate it, cooking it all at once. If the food is large, the outside gets cooked by microwaves, while the inside is cooked by the heat transferred from the hot exterior portion of the food.

• Manufacturers caution users not to put metal utensils or aluminum foil inside the microwave oven. These objects, like the metal walls of the oven, will reflect microwaves. When the powerful microwaves act on the walls and the metal objects, the air between them becomes charged with electricity, thereby causing sparks. These sparks can damage the cooking cavity.

In 2000 two Massachusetts inventors received a patent for a microwave oven that can be installed in a car's glove compartment. The oven would be powered by the car's battery and the controls would be mounted on the dashboard. The oven comes with a bin that could store things when the oven is not being used. The oven would only work when the bin is not in place.

alternating current:: An electric current that flows in one direction and then the other direction regularly.
chassis:: A major framework to which other parts are attached.
cooling fins:: Thin metal strips attached to the anode cylinder to help get rid of the heat generated by the magnetron.
direct current:: An electric current that flows in one direction only.
electromagnetic radiation:: A wave of electric and magnetic energy moving together in space. Microwaves are a form of electromagnetic radiation.
electron:: A small particle within an atom that carries a negative charge, which is the basic charge of electricity.
galvanized steel:: Steel that is coated with a thin layer of zinc to protect against corrosion, such as rusting.
interlocking switch:: A safety switch that prevents the production of microwaves when the microwave oven door is open.
magnetron:: A vacuum tube in which electrons traveling from the cathode (negative pole) to the anode (positive pole) are controlled by electric and magnetic fields to produce microwaves.
stirrer:: The fan that evenly distributes the microwaves into the cooking cavity.
waveguide:: A metal enclosure on top of the cooking cavity through which microwaves produced by the magnetron travel to the cavity.
wavelength:: The distance between the peaks of two successive waves of a microwave.

Web Sites

Bloomfield, Louis A. "Microwave Ovens." How Things Work. http://howthingswork.virginia.edu/microwave_ovens.html (accessed on July 22, 2002).

"Explosive Cooking." Discover. http://www.discover.com/oct_issue/breakcooking.html (accessed on July 22, 2002).

"Microwave Cooking for Today's Families." U.S. Department of Agriculture. http://www.foodsafety.gov/~fsg/fs-mwave.html (accessed on July 22, 2002).

"Microwave Ovens." University of Colorado. http://www.colorado.edu/physics/2000/microwaves (accessed on July 22, 2002).