Sharp R-372 Service Manual ▷ View online
THE HEATING PRINCIPLE OF MICROWAVE
Microwave is one kind of radio wave whose wave length is very short, frequency is very high. Therefore, it is called ultrahigh
frequency electromagnetic wave. Microwave can heat food mainly result in the mutual affect of the food in the microwave field and
microwave field itself.
Under the affect of microwave field, the thermal effect mechanism produced from the mutual affect of the microwave and the food
includes two aspects: One is Dielectric loss of polar molecule, the other is conductive loss of ion.
Usually, food is constitute of organism (plant and animal).The organism is formed by all kinds of polar water molecule, polar
protein molecule, and all sorts of salt ion. The center of gravity of the positive and negative charge in the molecule is not coincide.
In normal condition, the molecule is in irregular order due to its thermal action, thus the food do not appear polarity (FIG.1-
1a).Under the action of outer electric field, the positive end of the polar molecule trend to the negative electric field, the negative
end of polar molecule trend to the positive electric field, and somewhat arrange in order through the direction of the electric
field(FIG.1-1c).This phenomenon usually be called “TORQUE POLARITY”. When the outer electric field apply for the opposite
polarity, the polar molecule then arrange an opposite direction order accordingly (FIG.1-1b).If the direction of the outer electric field
changed repeatedly, the polar molecule would repeatedly sway accordingly. During the swaying, it understands that the polar
molecule would produce heat due to somewhat similar friction among them.
When the electric field is applied for ultrahigh frequent microwave field from the outside, its direction would change tens billion
times per second, so do the molecule. This kind of molecule swaying producing similar frictional heat from the interference and
block of the action strength among the molecule, and changed to microscopic microwave heating. Microwave heating not only
concerned the nature of the matter itself, but also closely connected with the electric field strength and frequency. When the
frequency is low, the molecule swaying rate and the acute degree of the mutual friction among the molecule is low, and would
produce much heat. When the frequency is too high, as the swing of the polar molecule is with rotating inertia, it made the swing
do not in line with the changing rhythm of the electric field because of the friction drag, thus, actually lowed the polar molecule
swaying speed. The friction dragging degree is concerning about the magneto electric wave frequency, polar molecule shape, and
the matter’s sticky degree. To different matter’s molecule, there are different special frequency zone. Those ado
absorb microwave
energy from these zone are most capable to turn microwave energy to heat energy.
Apart from the above said action, there is another action which is electric ion under the action of microwave field, act fiercely
accompanied with the acceleration of electric field. The positive ion transfer to the negative polarity of the field while the negative
ion do opposite. Accompanying with the changing electric field, the electric ion changing accordingly. During the transferring, heat
produced with the crash among the ion. This kind of action take the main effect to those microwave heating of high salt molecule.
No matter it is the polar molecule swaying or the ion transferring, they both are turning the microwave energy which the heating
matter got from the microwave field to hear energy. From the analysis of theory, we can draw such a conclusion that the power
which a unit of volume matter absorbed from the microwave field as the following formula:
Pa=K·E· F·Er·tgδ
Pa Stands for the power the heated matter absorbed from the microwave field.
K Stands for a constant
E Stand for microwave field strength
F Stands for the microwave frequency.
Er Stands for relative dielectric constant of the heated matter.
tgδStands for loss angle tangent of the heated matter
2-1
INSTALLATIONS
INSTALLING
1. Empty the microwave oven and clean inside it with
a soft, damp cloth. Check for damage such as
misaligned door, damage around the door or dents
inside the cavity or on the exterior.
2. Put the oven on a counter, table, or shelf that is
strong enough to hold the oven and the food and
utensils you put in it. (The control panel side of the
oven is the heavy side. Use care when handling.)
3. Do not block the vent and the air intake openings.
Blocking vent or air intake openings can cause
damage to the oven and poor cooking results.
Make sure the microwave oven legs are in place to
ensure proper air flow.
4. The oven should not be installed in any area where
heat and steam are generated, because they may
damage the electronic or mechanical parts of the
unit.
Do not install the oven next to a conventional
surface unit or above a conventional wall oven.
5. Use microwave oven in an ambient temperature
less than 104°F(40°C).
6. Place the microwave oven on a sturdy and flat
surface at least 10 cm(4 inches) from the wall.
7. Place the microwave oven as far away as possible
from TV, RADIO, COMPUTER, etc., to prevent
interference.
BEFORE YOU BEGIN, READ THE FOLLOWING INSTRUCTIONS COMPLETELY
EARTHING INSTRUCTIONS
This microwave oven is designed to be used in a fully
earthed condition.
It is imperative, therefore, to make sure it is properly
earthed before servicing
WARNING-
THIS APPLIANCE
MUST BE EARTHED
IMPORTANT
The wires in this mains lead are colored in
accordance with the following code:
Green-and-yellow: Earth
Blue: Neutral
Brown: Live
Green-and-yellow: Earth
Blue: Neutral
Brown: Live
As the colors of the wires in the mains lead of this
appliance may not correspond with the colored
markings identifying the terminals in your plug,
proceed as follows.
The wire which is colored
green-and-yellow must be
connected to the terminal in the plug which is marked
with the letter
E or by the earth symbol (
) or
colored
green or green-and-yellow.
The wire which is colored
blue must be connected to
the terminal in the plug which is marked with the letter
N or colored black.
N or colored black.
The wire which is colored
brown must be connected
to the terminal in the plug which is marked with the
letter
L or colored red.
3-1
SCHEMATIC DIAGRAM
CIRCUIT DESCRIPTION
As the door is closed, the contact of MONITOR
SWITCH opens. This switch creates the short circuit to
blow fuse during operation under abnormal condition.
(ie, should the contacts of primary and secondary
switch fail to open the circuit)
• The latches are secured by latch board. The oven light
turns on while the oven is in operation.
WHEN MICROWAVE POWER LEVEL
AND COOKING TIME SET
AND COOKING TIME SET
• The contacts of the timer switch changes to ON
position when the timer knob is rotated.
(While power control knob is set )
• The contacts of primary and secondary switch close
the circuit.
• Input Volts AC. is applied to the high voltage
transformer through V.P.C.
• Turntable rotates.
• The fan motor rotates and cools the magnetron by
blowing the air (coming from the intake holes on the
baseplate) over the magnetron.
• The air is also directed into the oven to exhaust the
vapor in the oven through the upper plate.
• Timer starts rotating.
• 3.2 volts AC is generated from the filament winding of
the high voltage transformer. This 3.2 volts is applied
to the magnetron to heat the magnetron filament
through two noise-preventing choke coils.
A high voltage of approximately 2100 volts AC is
generated in the secondary of the high voltage
transformer which is increased by the action of the
diode and charging of the high voltage capacitor.
The negative DC voltage is then applied to the filament
of the magnetron
WHEN THE DOOR IS OPENED DURING
COOKING
COOKING
• Both primary switch and secondary switch open to cut
off the primary winding voltage to the high voltage
transformer to stop microwave oscillation.
• Timer, turntable and fan motor stop.
• As the door is opened, if the contact of primary switch
fail to open, the fuse opens due to the large current
surge caused by the monitor switch activation which in
turn stops magnetron,oscillation.
4-2
Click on the first or last page to see other R-372 service manuals if exist.