This presentation includes the working mechanism of microwave oven.

1. COOKING MECHANISM IN MICROWAVE
OVEN
PROJECT BY:
PRUDHVI GANESH
MALEA,
ROLL.NO:K17SMB29

2. WHAT IS MICROWAVE RADIATION?
• Microwaves are a form of electromagnetic energy,like lightwaves or
radio waves.
• Its frequency ranges from 300 MHz(0.3GHz) to 300 GHz.
• Microwaves are used extensively in communications.
• Good for transmitting information because it can penetrate haze,light
rain,snow,clouds and smoke.
• Also used in radars and in detecting speed cars.
• Microwave has become most familiar as the energy source for
cooking food.

3. ELECTROMAGNETIC SPECTRUM
• Electromagnetic radiation exists in a range of frequencies
called the electromagnetic spectrum.
• Each frequency has a specific wavelength and as the frequency
decreases, the actual length of the wave gets longer.
• The frequency used in microwave oven is 2.45GHz.
C=f(lamda)

4. HISTORY OF MICROWAVE OVEN
• In 1945, Percy Spencer, working for Raytheon to develop magnetrons
for active radar signals, noticed a chocolate bar in pocket melted
while standing infront of an operating magnetron.
• He then tested popcorn infront of the magnetron and it quickly
popped all over the room.
• In 1947, Raytheon made the first microwave oven Radarange (6 ft
tall,3000W power and $3K) but did not sell well. In 1965, a counter
top version was developed for $495.

5. HOW DOES MICROWAVE OVEN COOK
FOOD?
• Microwave ovens selectively make liquid(water) hot. Most food,
even dry foods, have water in them.
• Water(H2O) is a polar molecule with 2 hydrogen atoms being
more positive then the single oxygen atom.
• In liquid water, the molecules are in constant motion and are
normally oriented.

6. CONTD…
• Glass, paper, ceramic or plastic containers are used in
microwave cooking because the microwaves pass through
them.
• Metals reflect microwaves so unsafe to have metal pans,
aluminium foil in oven, may damage oven.

7. WHAT HAPPENS WHEN WATER MOLECULES
ARE IN ELECTRIC FIELD?
• The molecules will experience in a torque from the electric field
and become aligned with direction of the field.
• Therefore, water molecules are oriented by the electric field.

8. WHAT HAPPENS TO WATER MOLECULES IN
OSCILLATING ELECTRIC FIELD?
• The water molecules follow the oscillations of the electric field.
• They collide more frequently with the molecules around them.
• The molecules move faster and faster and the temperature
increases(heating).

9. WHY DO THEY HAVE TURNTABLES?
• Microwaves are reflected by the oven wall, forming “standing waves”
in the cooking chamber.
• Standing Waves: Waves that oscillate in place.
• Node-Zero amplitude(no oscillation) at all times.
• Anti-nodes-Locations with maximum amplitude.
• Inside a microwave oven, the electromagnetic waves also form
standing waves from reflections at the walls.
• Without a turntable, the food will not be cooked uniformly.

10. INTERACTION OF MICROWAVES WITH
METALS
• Microwaves, incident on the metal walls of the oven, behave
similarly to visible light hitting a mirror.
• The microwaves are absorbed very effectively, since the electric
fields of the waves interact very slowly and strongly with nearly
free electrons of the metal. These accelerated electrons re-
radiate electromagnetic waves at the same frequency and in
phase, hence the microwaves are perfectly radiated.

11. MAGNETRON
• Magnetron is an electronic device which generates high energy
microwaves. It is the core element in a microwave oven.
• Working Principle: When a charge/charge particle accelerates in
space, it generates electromagnetic waves.
• This statement is the derivation of Maxwell’s law which states
that a classical electromagnetic radiation is ultimately
generated when a charged particle is accelerated through
space.

12. PARTS OF MAGNETRON
Cathode
Anode
Permanent Magnet

13. CATHODE
• Cathode in magnetron is a tungsten filament which emits
electrons on the principle of thermionic emission when an
optimum voltage is applied.

14. ANODE
• Anode in magnetron is a copper cup like structure which is a
little bit modified to perform more functions than a simple
anode does.

15. PERMANENT MAGNET
• The purpose of the permanent magnet is to apply a continuous
magnetic field across the cathode area.

16. WORKING OF MAGNETRON
• Electrons from a hot filament would travel radially to the
outside the ring if it were not for the magnetic field. The
magnetic force deflects them in the sense shown and they tend
to sweep around the circle. In so doing, they “pump” the natural
resonant frequency of the cavities. The currents around the
resonant cavities cause them to radiate electromagnetic energy
at that resonant frequency.

17. THEORY
• In a magnetron, the sources of electrons is a heated cathode located
on the axis of an anode structure containing a number of microwave
resonators. Electrons leave the cathode and are accelerated towards
the anode, due to the DC field established by the voltage source E.
• The presence of a strong magnetic field B in the region between
cathode and anode produces a force on each electron which is
mutually perpendicular to the DC field and the electron velocity
vectors, thereby causing the electrons to spiral away from the
cathode in paths of varying curvature, depending upon the initial
electron velocity at the time it leaves the cathode.

18. CONTD..
• Inside the anode there are a number of cavities designed to
resonate at 2.45GHz. A voltage of several KV is applied
between the electrodes and a magnetic field is applied parallel
to the axis such that electric field and magnetic fields are
perpendicular to each other.

19. THEORY OF OPERATION OF MICROWAVE
OVEN
• In the high-voltage section of a microwave oven, the diode and
the capacitor function together to effectively double the
already-high voltage. This is called Voltage-doubler Circuit.
• Voltage-doubler circuits are fed up with the stepped-up AC
voltage from the high voltage transformer’s secondary(output)
winding. Typically a transformer would step-up 220 volts to
about 2000 volts, which would have an approx. peak value of
2800 volts.

20. THE HALF-WAVE DOUBLE VOLTAGER
• During the first positive half cycle, which is designated on the
sine wave graph as T1, the voltage from the transformer
increases accordingly with the polarity shown.
• The transformer secondary and the charged capacitor and the
sum voltage of 5600 volts is applied to the magnetron cathode.

21. CONTD..
• There are two fundamental characteristics of this 5600 volt
output
• First, because a voltage doubler is also a rectifier, the output is a DC
voltage.
• Second, the resulting output voltage that is applied to the magnetron
tube is actually a pulsating DC voltage. This is because the doubler
generates an output only during the negative-half cycle of the
transformer’s output(secondary) voltage.
• So, the magnetron tube is infact pulsed on and off at the rate of 50 or
60 times per second, depending on the frequency of the line voltage.

22. RADIATION AND ITS TYPES
• Radiation is the release of the energy from any substance(source) and
travel through space in the form of Electromagnetic Waves.
• There are many different sources of energy around us. For Example,
our bodies give off heat, which is a form of energy. Energy is also
released from everyday things such as:
a) Household electrical appliances
b) Heaters
c) The Sun
d) X-ray machines.

23. • Not all radiations are harmful. It depends on the type of
radiation and how much exposure to it you have. There are
several types of radiation. All of which can be grouped under
either
Ionizing Radiation
Non-ionizing Radiation.

24. IONIZING RADIATION
• Radiation that has enough energy to remove tightly bound
electrons from atoms, thus creating ions. This is the type of
radiation that people usually think of as ‘radiation’. For
Example:
Alpha Particles
Beta Particles
Gaama Rays
X-rays.

25. NON-IONIZING RADIATION
• Radiation that has enough energy to move atoms in a molecule
around or cause them to vibrate, but not enough to remove
electrons, is referred as “Non-ionizing Radiation”. For Example:
UV rays from Sun
Sound Waves
Radio waves
Radiation waves given off from household electrical appliances, heaters,
mobile phones with or without headsets, and computers.

26. BIOLOGICAL EFFECTS
• Non-ionixing radiation can produce non-mutagenic effects such as inciting
thermal energy in biological tissue that can lead to burns. Recently, the IARC
from WHO released a statement indicating that radiofrequency
electromagnetic fields(including microwave) are possibly carcinogenic to
Humans.
• Certain body organs are particularly sensitive to the thermal effect of
microwaves. For Example, if the lens of the eye were exposed to excessive
heat from microwaves, its circulatory system would be look like the white of
an egg.
• Exposure to high levels of microwaves can cause cataracts. Also, the
stomach, intestines and bladder are especially sensitive to thermal damage
from high levels of microwaves.

27. SAFETY STANDARDS
• Safety code 6 part-III(Microwave Ovens) of the Radiation
Emitting Devices Regulations(C.R.C.C 1370) specifies the limit
for the leakage radiation at 5cm from the surface of the
microwave oven to be 5.0 mW/cm2.
• ANSI/IEEE-C95.1-1991 the power density should not exceed
1.6mW/cm2 at 2450 MHz(microwave oven frequency) for
human exposure in uncontrolled environments.

28. SAFETY TIPS
• Do not operate oven when empty.
• Exercise extreme caution if you have a pacemaker impact.
• Keep out of reach of children. Do not permit young children to
operate the oven.
• Do not put face close to door window area when oven is
operating.