Microwaves are actually a segment of the electromagnetic wave spectrum,
which comprises forms of energy that move through space, generated by the
interaction of electric and magnetic fields. The spectrum is commonly
broken into subgroups determined by the different wavelengths (or
frequencies) and emission, transmission, and absorption behaviors of
various types of waves. From longest to shortest wavelengths, the spectrum
includes electric and radio waves, microwaves, infrared (heat) radiation,
visible light, ultraviolet radiation, X-rays, gamma rays, and
electromagnetic cosmic rays. Microwaves have frequencies between
approximately .11 and 1.2 inches (0.3 and 30 centimeters).
Microwaves themselves are used in many different applications such as
telecommunication products, radar detectors, wood curing and drying, and
medical treatment of certain diseases. However, certain of their
properties render them ideal for cooking, by far the most common use of
microwave energy. Microwaves can pass through plastic, glass, and paper
materials; metal surfaces reflect them, and foods (especially liquids)
absorb them. A meal placed in a conventional oven is heated from the
outside in, as it slowly absorbs the surrounding air that the oven has
warmed. Microwaves, on the other hand, heat food much more quickly because
they penetrate all layers simultaneously. Inside a piece of food or a
container filled with liquid, the microwaves agitate molecules, thereby
heating the substance.
The ability of microwave energy to cook food was discovered in the 1940s
by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for
the military during World War II. Spencer’s experiments revealed
that, when confined to a metal enclosure, high-frequency radio waves
penetrate and excite certain type of molecules, such as those found in
food. Just powerful enough to cook the food, the microwaves are not strong
enough to alter its molecular or genetic structure or to make it
Raytheon, the company for which Dr. Spencer was conducting this research,
patented the technology and soon developed microwave ovens capable of
cooking large quantities of food. Because manufacturing costs rendered
them too expensive for most consumers, these early ovens were used
primarily by hospitals and hotels that could more easily afford the $3,000
investment they represented. By the late 1970s, however, many companies
had developed microwave ovens for home use, and the cost had begun to come
down. Today, microwaves are a standard household appliance, available in a
broad range of designs and with a host of convenient features: rotating
plates for more consistent cooking; digital timers; autoprogramming
capabilities; and adjustable levels of cooking power that enable
defrosting, browning, and warming, among other functions.
The basic design of a microwave oven is simple, and most operate in
essentially the same manner. The oven’s various electronic motors,
relays, and control circuits are located on the exterior casing, to which
the oven cavity is bolted. A front panel allows the user to program the
microwave, and the
The oven cavity and door are made using metal-forming techniques and
then painted using electro-deposition, in which electric current is
used to apply the paint.
The magnetron tube subassembly includes several important parts. A
powerful magnet is placed around the anode to provide the magnetic
field in which the microwaves will be generated, while a thermal
protector is mounted directly on the magnetron to prevent damage to
the tube from overheating. An antenna enclosed in a glass tube is
mounted on top of the anode, and the air within the tube is pumped out
to create a vacuum. Also, a blower motor used to cool the metal fins
of the magnetron is attached directly to the tube.
door frame has a small window to enable the cook to view the food while
it is cooking.
Near the top of the steel oven cavity is a magnetron—an electronic
tube that produces high-frequency microwave oscillations—which
generates the microwaves. The microwaves are funneled through a metal
waveguide and into a stirrer fan, also positioned near the top of the
cavity. The fan distributes the microwaves evenly within the oven.
Manufacturers vary the means by which they disburse microwaves to achieve
uniform cooking patterns: some use dual stirrer fans located on opposite
walls to direct microwaves to the cavity, while others use entry ports at
the bottom of the cavity, allowing microwaves to enter from both the top
and bottom. In addition, many ovens rotate food on a turntable.
The cover or outer case of the microwave oven is usually a one-piece,
wrap-around metal enclosure. The oven’s inside panels and doors are
made of galvanized or
and are given a coating of acrylic enamel, usually light in color to
offer good visibility. The cooking surface is generally made of ceramic or
glass. Inside the oven, electromechanical components and controls consist
of timer motors, switches, and relays. Also inside the oven are the
magnetron tube, the waveguide, and the stirrer fan, all made of metal. The
hardware that links the various components consists of a variety of metal
and plastic parts such as gears, pulleys, belts, nuts, screws, washers,
Oven cavity and door manufacture
1 The process of manufacturing a microwave oven starts with the cavity
and the door. First, the frame is formed using automatic metal-forming
presses that make about 12 to 15 parts per minute. The frame is then
rinsed in alkaline cleaner to get rid of any dirt or oil and further
rinsed with water to get rid of the alkaline solution.
2 Next, each part is treated with zinc phosphate, which prepares it for
electro-deposition. Electro-deposition consists of immersing the parts
tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5
mils thick. The parts are then moved through a paint bake operation
where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius)
for 20 minutes.
The chassis or frame is mounted in a pallet for the main assembly
operation. A pallet is a vise-like device used in conjunction with
3 After the door has been painted, a perforated metal plate is attached
to its window aperture. The plate reflects microwaves but allows light
to enter the cavity (the door will not be attached to the cavity until
later, when the chassis is assembled).
The magnetron tube subassembly
4 The magnetron tube assembly consists of a cathode cylinder, a filament
heater, a metal anode, and an antenna. The filament is attached to the
cathode, and the cathode is enclosed in the anode cylinder; this cell
will provide the electricity that will help to generate the microwaves.
Metal cooling fins are welded to the anode cylinder, and a powerful
magnet is placed around the anode to provide the magnetic field in which
the microwaves will be generated. A metal strap holds the complete
assembly together. 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 mounted on top of the anode,
and the air within the tube is pumped out to create a vacuum. The
waveguide is connected to the magnetron on top of the protruding
antenna, while a blower motor used to cool the metal fins of the
magnetron is attached directly to the tube. Finally, a plastic fan is
attached to the motor, where it will draw air from outside the oven and
direct it towards the vanes. This completes the magnetron subassembly.
Main chassis assembly
6 The chassis assembly work is performed on a pallet—a
work-holding device used in conjunction with other tools—located
at the station. First, the main chassis is placed on the pallet, and the
cavity is screwed on to the chassis. Next, 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 the main chassis.
In a completed microwave oven, the magnetron tube creates the
microwaves, and the waveguide directs them to the stirrer fan. In
turn, this fan points the waves into the oven cavity where they heat
the food inside.
7 The circuit that produces the voltage required to operate the
magnetron tube consists of a large transformer, an oil-based capacitor,
and a high voltage rectifier. All of these components are mounted
directly on the chassis, close to the magnetron tube.
8 The stirrer fan used to circulate the microwaves is mounted on top of
the cavity. Some manufacturers use a pulley to drive the fan from the
magnetron blower motor; others use a separate stirrer motor attached
directly to the fan. Once 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 arcing and
damage the magnetron.
Control switches, relays, and motors
9 The cook switch provides power to the transformer by energizing a
relay and a timer. The relay is mounted close to the power transformer,
while the timer is mounted on the control board. The defrost switch
works like the cook switch, activating a motor and timer to operate the
defrost cycle. Also mounted on the control board are a timer bell that
rings when the cooking cycle is complete and a light switch that allows
viewing of the cavity. A number of interlocking switches are mounted
near the top and bottom of the door area. The interlocking switches are
sometimes grouped together with a safety switch that monitors the other
switches and provides protection if the door accidently opens during
10 A front panel that allows the operator to select the various settings
and features available for cooking is attached to the chassis. Behind
the front panel, the control circuit board is attached. The board, which
controls the various programmed operations in their proper sequence when
the switches are pushed on the front panel, is connected to the various
components and the front panel by means of plug-in sockets and cables.
Making and assembling the case
11 The outer case of the microwave is made of metal and is assembled on
a roll former. The case is slipped onto the preassembled microwave oven
and bolted to the main chassis.
Testing and packaging the oven
12 The power cords and dial knobs are now attached to the oven, and it
is sent for automatic testing. Most manufacturers run the oven from
50-100 hours continuously as part of the testing process. After testing
is complete, a palletizer robot records the model and serial data of the
oven for inventory purposes, and the oven is sent for packaging. This
completes the manufacturing process.
Extensive quality control during the manufacture of microwave ovens is
essential, because microwave ovens emit radiation that can burn anyone
exposed at high levels for prolonged periods. Federal regulations, applied
to all ovens made after October 1971, limit the amount of radiation that
can leak from an oven to 5 milliwatts of radiation per square centimeter
at approximately 2 inches from the oven surface. The regulations also
require all ovens to have two independent, interlocking switches to stop
the production of microwaves the moment the latch is released or the door
In addition, a computer controlled scanner is used to measure emission
leaks around the door, window, and back of the oven. Other scanners check
the seating of the magnetron tube and antenna radiation. Each scanner
operation relays data to the next-on-line operation so that any problems
can be corrected.
Because of their speed and convenience, microwave ovens have become an
indispensable part of modern kitchens. Many developments in the microwave
market and allied industries are taking place fairly rapidly. For example,
foods and utensils designed specially for microwave cooking have become a
huge business. New features will also be introduced in microwaves
themselves, including computerized storage of recipes that the consumer
will be able to recall at the touch of a button. The display and
programmability of the ovens will also be improved, and combination ovens
capable of cooking with microwaves as well as by conventional methods will
become a standard household product.
Where To Learn More
Davidson, Homer L.
Microwave Oven Repair,
2nd edition. Tab Books Inc., 1991.
Gallawa, J. Carlton.
The Complete Microwave Oven Service Handbook: Operation, Maintenance.
Prentice Hall, 1989.
Microwave Oven Radiation.
U.S. department of Health and Human Services, 1986.
Pickett, Amold and John Ketterer.
Household Equipment in Residential Design.
John Wiley and Sons, 1986.
Cahners Publishing, 1985.
Klenck, Thomas. “How It Works: Microwave Oven.”
September, 1989, p. 78.
Roman, Mark. “The Little Waves That Could.”
November, 1989, p. 54.