microwave oven

1. LECTURE-4
Microwave Oven

2. WHAT ARE MICROWAVES?
 Microwaves are a form of electromagnetic energy,
like light waves or radio waves.
 Its frequency range is from 300 MHz (0.3 GHz) to
300 GHz.
 Microwaves are used extensively in
communications.
 Good for transmitting information because it can
penetrate haze, light rain and snow, clouds, and
smoke.
 Also used in radars and in detecting speeding 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.
c = fl
 The frequency used in microwave oven is 2.45 GHz

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 in front of an operating magnetron.
 He then tested popcorn in front of the magnetron
and it quickly popped all over the room.
 In 1947, Raytheon made the first microwave oven –
Radarange (6 ft tall, 3000 W power $3K) but did not
sell well. In 1965, a countertop version was
developed for $495.

5. CONTD…

6. HOW DOES A 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 than the single oxygen
atom.

7. CONTD…
 In liquid water, the molecules are in constant motion
and are normally randomly oriented.
 Glass, paper, ceramic, or plastic containers are
used in microwave cooking because the
microwaves pass through them.
 Metal reflects microwaves
– Unsafe to have metal pans/aluminum foil in
oven, may damage oven.

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

9. WHAT HAPPENS TO WATER MOLECULES IN
AN OSCILLATING ELECTRIC FIELD?
 The water molecules follow the oscillation of the
electric field.
 They collide more frequently with the molecules
(water and other) around them.
 The molecules move faster and faster, and the
temperature increases (heating).

10. 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.
 Nodes – zero amplitude (no oscillation) at all times
 Anti-nodes – locations with maximum amplitude

11. CONTD…
 In side 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.

12. 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 strongly
with the 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 reflected.

13. 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 says that a classical electromagnetic
radiation is ultimately generated when a charged
particle is accelerated through space.

14. PARTS OF MAGNETRON
 Cathode
 Anode
 Permanent magnet

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

16. 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.

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

18. WORKING OF MAGNETRON

19. RESONATOR

20. THEORY
In a magnetron, the source 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
toward 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 a the time
it leaves the cathode.

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

22. 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 a voltage-doubler circuit.
 Voltage-doubler circuits are fed with the stepped-up
AC voltage from the high-voltage transformer's
secondary (or output) winding. Typically, a
transformer would step up 220 volts to about 2000
volts, which would have an approximate peak value
of 2800 volts.

23. THE HALF-WAVE VOLTAGE DOUBLER
 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.

24. CONTD…
 During the capacitor charging time there is no voltage
to the magnetron because the current takes the
course of least resistance.
 The voltage across the capacitor will rise to the
transformer secondary voltage to the maximum 2800
volts.
 As the transformer secondary voltage begins to
decrease from its maximum positive value (at time
increment T2 on the sine wave graph), the capacitor
will attempt to discharge back through the diode. The
diode is like a one-way street in that it will not
conduct in this direction. Thus, the discharge path is
blocked, and the capacitor remains charged to the
2800 volts.

25. CONTD…
 Refer to Figure 2. At time T3, the transformer
secondary (output) voltage swings into the negative
half-cycle and increases in a negative direction to a
negative 2800 volts, with polarities as shown.

26. CONTD…
 The transformer secondary and the charged capacitor
are now essentially two energy sources in series. The
2800 volts across the transformer winding adds to the
2800 volts stored in the capacitor and the sum voltage
of 5600 volts is applied to the magnetron cathode.
 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.

27. CONTD…
 So, the magnetron tube is, in fact, pulsed on and off
at a rate of 50 or 60 times per second, depending
on the frequency of the line voltage.

28. RADIATION AND ITS TYPES
 Radiation is the release of energy from any source and travel
through space in the form of EM 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:
 Household electrical appliances
 Heaters
 The sun
 X-ray machines
 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

29. 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
 Gamma rays
 X-rays

30. 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 to as "non-ionizing
radiation”.
 Non ionizing radiation has enough energy to move things
around inside a cell but not enough to change cells
chemically. The radiation from a microwave oven is non
ionizing. Other examples include:
 Ultraviolet rays from the sun.
 Sound waves.
 Radio waves.
 Radiation waves given off from household electrical
appliances, heaters, mobile phones with or without
headsets, and computers and their screens.

31. SPECTRUM OF IONIZING AND NON-IONIZING
RADIATIONS

32. BIOLOGICAL EFFECTS OF MICROWAVES
 Non-ionizing radiation can produce non-mutagenic effects
such as inciting thermal energy in biological tissue that
can lead to burns. Recently, the International Agency for
Research on Cancer (IARC) from the WHO (World Health
Organization) 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 unable to provide sufficient
cooling, and it would cook 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.

33. 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 5 cm
from the surface of the microwave oven to be 5.0
mW/cm2
 ANSI/IEEE-C95.1-1991 - the power density should not
exceed 1.6 mW/cm2 at 2450 MHz (microwave oven
frequency) for human exposure in uncontrolled
environments.
 IRPA (International Radiation Protection Association)
recommends exposure limit of 5 mW/cm2 for RF workers
and 1 mW/cm2 for the general public. These exposure
limits are averaged over 6 minutes.

34. HOW FAR AWAY IS SAFE?
 One characteristic of microwaves is that they
disperse and dissipate very quickly in the
atmosphere.
 For example, If you are standing 2 inches away from
a microwave oven, and are being zapped by
5mw/cm2 of microwave energy, then you wisely step
back to a distance of 20 inches or roughly an arm’s
length. Your level of exposure would drop by a factor
of 100, (the square of the distance) to 0.05mw/cm2.

35. CONTD…
 Stay at least an arm’s length away from the
front of an operating oven.

36. SAFETY TIPS FOR OPERATION OF
MICROWAVE OVENS
 Do not operate oven when empty.
 Exercise extreme caution if you have a pacemaker implant.
Microwave radiation may cause pacemaker interference.
 Persons with pacemaker implants should not be near a
microwave oven unless they are sure that it is in good
operating condition and there is no leakage of microwave
radiation.
 Check to see that door seal and inside surfaces of door and
oven cavity are clean after each use.
 Keep out of the reach of children. Do not permit young
children to operate the oven.
 Do not put face close to door window when oven is
operating.

37. TROUBLESHOOTING AND MAINTENANCE
SAFETY
 Will be discussed during practical.