How Is A Microwave Oven Made?
You pop open the door to the microwave oven, slide in your frozen
dinner, push a few buttons and sit back down in front of the TV. When
the buzzer goes off you head back to the kitchen, grab your quickly
cooked food and head back to the easy chair. How convenient. That part
is easy, but did you ever wonder how a microwave is made?
There are several parts that make up a microwave oven. One of them is
the high voltage transformer. This takes the electricity that comes into
the microwave and hands it off to the next part, the magnetron. The
magnetron then converts the high voltage electricity turning it into
microwave radiation (this cooks the food).
There is also a magnetron control circuit with a microcontroller and a
wave guide (more about those in a minute) and of course the place where
you put the food, the cooking chamber.
Microwave ovens actually use frequencies similar to a radio to cook the
foods. There are some restrictions that are placed on those frequencies,
also called ISM bands, that must be followed by the manufacturer. They
must not be frequencies set aside for communication purposes. There are
bands set aside for microwave frequencies but these are not the same as
those used in microwave cooking.
They are 5.8 GHz and 24.125 GHz, which are cost prohibitive for
microwave cooking because the expense of generating power at these
frequencies, and 433.92 MHz, which is also not used for cooking because
of the cost to generate power and avoid interference outside the
frequency. Additionally, the latter is only available in a limited
amount of countries. For the regular kitchen microwave oven, the best
and most commonly used is the 2.45 GHz because it has worldwide
availability (915 MHz can be used, but is not worldwide).
The typical microwave oven gives you options to choose between different
power levels, including a high setting, a defrost setting and a reheat
setting. But don't let this confuse you: this doesn't usually make the
amount of power and radiation coming from the magnetron any more
intense, it just regulates the magnetron by turning it on and off every
few to several seconds.
You can tell this is happening when you hear the oven make a pause in
the humming sound for awhile then start back up (the time that the
microwave is actually cooking is called a 'duty cycle'). Sometimes you
can even see it when foods puff up and flatten back out as the magnetron
turns on and off. To do this on and off type of cooking the microwave
uses a linear transformer (an annular parallel connection of switches
and capacitors designed to deliver rapid high power pulses) which can
only be switched completely on or completely off; no in-between.
The newest microwave ovens have inverter power supplies (an inverter
converts direct current, DC, to alternating current AC - the AC can then
be at any required voltage and frequency by using the appropriate
transformers, control circuits and switching) which use PWM (pulse-width
modulation causes a power source to control the modulation of its duty
cycle [see above], which controls the amount of power sent to a load)
and this allows the oven to run on low power continuously.
Then there is the cooking chamber. It is a Faraday cage - an enclosure
formed by conducting material, or by a mesh of such material which
blocks out external static electrical fields and prevents the microwaves
from escaping the oven. The oven door has a usually has glass panel so
you can see the food.
Have you ever looked through an oven door and wondered what that wire
looking stuff is in the glass? This is the mesh material mentioned
above. The size of the perforations, or holes, in the mesh is smaller
than the wavelength of the microwaves. This keeps the majority of the
microwave radiation from passing through the door, but still allows you
to see inside the microwave. Light has a shorter wavelength and can pass
through the mesh.
Okay, got all that down? It truly seemed much simpler when you just
threw in a bag of popcorn and let the microwave oven do its thing,
right? |
