Download microwaves - Electrical

 Microwaves are the form of E.M. waves having range
from 1m to 10mm having frequency ranges between
300MHz to 300GHz
 They are small than waves used in radio broadcasting
 High power M.waves sources are vacuum tubes such
as magnetron klystron and gyrotron
 Low power M.waves includes field effect transistors
tunnel diodes and gun diodes
 M.WAVES are used for
 Communication
 Navigation
 Radar
 Radio astronomy
 Heating and power application
 Spectroscopy
 EFFECTS ON HEALTH:
M.waves can cause carcinogenic effect.
 Exposure to M.waves can damage tissue
and can cause serious burns.
 It is a common kitchen appliance that heats and cooks
food by exposing to M.radiation in E.spectrum
 This induces polar molecules in the food to rotate and
produce thermal energy known as dielectic heating
and it heats food quickly and efficiently.
 Food items having high water content are cooked
quickly than those having low water content
 They have a limited role in professional cooking
however additional heat sources can be added to
M.ovens to produce more heating effects
1.During world war-2, two scientists invented
the magnetron , the tube that produces
microwaves.
2.It was discovered that microwaves can also
cook food.
3.The first Raytheon commercial M.oven was
the 1161 radarange which was marketed in 1954
rated at 1600W,which was large and heavy as
refrigerator.
4.In 1967,Amana division of Raytheon
introduced its domestic radarange M.oven and
later stages its prices were decreased and their
capabilities were increased.
 A Microwave Oven uses Microwaves(Radio waves)
to heat food. These waves will have frequency
roughly 2,500 MHz . The waves in this frequency
range have two interesting properties .
 The first is : they are absorbed by water, fats and
sugars. When they are absorbed they are converted
directly into atomic motion and motion is
converted into heat.
These radio waves penetrate the food and excite
water and fat molecules pretty much evenly
throughout the food
As all the molecules are all excited together there
is heat everywhere. Radio waves penetrate unevenly
in thick pieces of food . Therefore food is heated by
microwaves instead of heat conduction.
 The second is : they are not absorbed by most
plastics, glass or ceramics. And metal reflects
microwaves, which is why metals cause spark in a
microwave oven.
.The reason that metal reflects microwaves is that no
electronic waves resident in inside of conductor
because as we know conductor’s conductivity is
infinity .
. By looking at this in detail, Molecules of all food are
consist of a dipole and have positive charge in one side
and have negative charge in another side. If we put
electromagnetic fields in this, all molecules are
rearranged : +charge is to negative pole and –charge is
to positive pole.
•In this process molecules heat is produced by friction.
The microwave having frequency 2,500 MHz change the
direction of electromagnetic fields 2,500,000,000 times in
1 sec. Consequently the heat efficiency of a microwave
oven is greatly high..
•A common misconception is that microwave ovens cook
food "from the inside out", meaning from the center of the
entire mass of food outwards . But this is not true.
• In most cases, in a homogenous food item, microwaves
are absorbed in the outer layers of the item at a similar
level to that of the inner layers. Depending on water
content, the depth of initial heat deposition may be
several centimetres or more with microwave ovens.
 A microwave oven converts only part of its electrical
input into microwave energy. In an average , it
consumes 1100 W in producing 700 W of microwave
power, an efficiency of 64% .
 Such wasted heat, along with heat from the product
being microwaved, is exhausted as warm air through
cooling vents. The other 400 W are dissipated as heat,
mostly in the magnetron tube
.Additional power is used to operate the lamps, AC
power transformer, magnetron cooling fan, food
turntable motor and the control circuits, although the
power consumed by the electronic control circuits of a
modern microwave oven is negligible (< 1% of the
input power).
.For cooking or reheating small amounts of food, the
microwave oven may use less energy than a cook stove
due to the reduced heat mass of the food's container.
. The amount of energy used to heat food is generally
small compared to total energy usage in typical
residences in the United States
 Microwave oven mainly consists of two sections:
 1.Control section and
 2.high voltage section.
 Control section consists of switches and relays for controlling
and safety purposes.
 High voltage section consists of transformers,capacitors,diodes
to convert 230V household voltage to high voltage for the
working of microwave oven.
 Control section consists of many electronic components and
sensors for controlling of the oven.
 High voltage section mainly works for conversion of electrical
energy into heat energy(microwaves).
Let us now see the basic circuit diagram of a oven
 This is the circuit diagram of first microwave oven.
 Typical microwave oven circuit diagram will be like
 control section consists of electromechanical and electronic
relays and sensors for controlling of the oven , like triac which is
a electromechanical device which receives the information from
sensors and gets ‘on’ to send current to high voltage section .
 High voltage section mainly consists of five parts i.e.,1. high
voltage transformer , 2.high voltage capacitor and rectifier diode,
3.magnetron tube , 4.waveguide and 5.the cooking chamber.
 High voltage transformer : Basically the microwave oven requires
high voltage than household equipments .
So a high voltage transformer is used to
convert 230V to around 3000V.
 High voltage Capacitor and Diode : Along with the High voltage
transformer , a high voltage
capacitor and diode are used to
convert low voltage to high voltage.
 Magnetron tube : This is the main
component of microwave oven . It converts the electrical energy
received from the transformer to microwaves which are used for
cooking food in cooking chamber.
 Wave guide : It is used to guide the micro
waves produced by magnetron towards
the cooking chamber.
Cooking chamber
Wave guides
 Cooking Chamber : Cooking chamber is the area that is used for
cooking the food in the oven . It is so designed that the
microwaves are reflected again and again within the chamber to
produce more heat for cooking of food.
 Frame : as the microwaves are harmful for humans , a frame is
constructed around cooking chamber to ensure that the waves
does not come out.
 There are also motors and fans used in oven for cooling of the
magnetron tube and other parts of oven.
•
Electricity from the wall outlet
travels through power cord and
enters the microwave oven through
a series of fuses and safety
protection circuits which include
thermal protectors to deactivate
the oven in case of overheating
conditions or short circuit.
•If all the systems are normal ,
the internal circuit is get closed and
electrical path is established through a
series
of safety interlock switches and setting the timer control
for the oven to cook operation which extends this voltage
path to the control circuits.
There are control units that include Triac which generates
signal to activate ,thereby producing voltage path to HV
transformer.
• The control system can govern the application of voltage
to the HV transformer , thereby controlling on-off ratio of
magnetron tube and thus output power of the microwave
oven.
• Along with HV transformer , a special diode and
capacitor is used to increase the household voltage (115V)
to high amount of approx 3000V…..!!!!
That HV is unhealthy , even deadly for humans , but
magnetron tube needs HV to convert into undulating
waves of electromagnetic cooking energy.
This microwave energy is transmitted into metal
channel called WAVEGUIDE , which feeds the energy
into cooking area where it encounters the slowly
revolving metal blades of stirrer blade.
• The energy is dispersed throughout all areas of
cooking compartment uniformly in all directions.
• All microwave energy remains inside the cooking
cavity. When the door is opened, or the timer reaches
zero, the microwave energy stops just as turning off a
light switch stops the glow of the lamp.
 The main advantage is cooking time in microwave oven
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reduces to one fourth of the time taken on gas stoves. So,
the energy and time are saved.
Loss of nutrients is minimized. 3-carotene and vitamin C
are better retained by microwave cooking compared to
pressure cooking .
It reduces the consumption of oil, thereby allows prepare
low fat food.
Baking can be done only in microwave ovens.
Heating frozen foods through microwave oven takes less
time.
 The major disadvantage is that some studies had proved that
microwaved food is hazardous to your health causing diseases
and increasing the risk of causing cancers as it emits cancer
causing agents. Also, it causes the degeneration of the immune
system and abnormal changes in the human blood.
 Foods that require deep frying like puri’s, bonda’s etc. cannot be
made in microwave oven. Also, it is not possible to make
chapatti’s and roti’s.
 Only specially made microwave bowls should be used.
Otherwise, there may be a chance of transferring unwanted,
harmful chemicals from the plastic cookware into the food.
 The short cooking time may not give a chance of blending of
flavours as in conventional methods.
 The operator should be careful in operating the microwave oven
since any exposure to microwave oven causes physiological
abnormalities.
 If the food is greater than 80 mm the central portion is out of
range of the microwave radiation will only heat by the normal
slow process of conduction. It will be relatively uncooked while
the exterior accessible to microwave is cooked in minutes or
seconds.
 Microwaved meals change blood chemistry
 Cholesterol levels increased rapidly.
 Hemoglobin decreased significantly (creating anemic
tendencies.)
 Milk heated to 72 degrees lost a full 96% of all immunoglobulinA antibodies, which fight invading microbes.
Thank you…
Presented by
 K . Sai vamshidhar Reddy(BT14EEE037)
 K . Sahithi(BT14EEE038)
 K . Rajashekar(BT14EEE039)
 K . Likitha(BT14EEE040)
 Hitesh.K(BT14EEE041)