2.2
MICROWAYE
GENERATER.
Microwave generator is the
heart
of microwave oven. The quality of a microwave oven mostly depends on the
quality of the microwave generator. A microwave generator is mainly composed of magnetron and its power
supply
circuit,
FIG.2-3
is the typical
circuit
diagram of the present used microwave
oven's
generator. The power
supply circuit is composed of rectifying circuit and filament circuit.
Usually, we adopt continuous wave magnetron.
It
can turn the direct energy which is applied to the magnetron
after
being high voltage rectified to microwave energy, the power supply circuit supply a direct
high
voltage
between
the cathode and anodę of the magnetron, a filament voltage to the cathode filament of the magnetron.
The working process of the magnetron: When the anodę volt-age gradually rises
from
zero, the anodę current is
approx.zero,
the power is
very
smali as
well.
When the anodę voltage rises to
"THRESHOLD"
voltage value, the
magnetron
starts
oscillating, and the anodę current
would
increase
obviously,
provided the anodę voltage rises a
little
morę,
the anodę current would increase a
lot
(FIG.2-4),
and would reach the
ratę
value
guickly.
If
the anodę voltage is
undulating,
it would cause the anodę
current swing fiercely, even
madę
the magnetron stop oscillating.
For keeping a steady output, the power supply circuit of the
oven
must
supply a stead direct current voltage. The filament
voltage of the magnetron must be supplied by an alternating
current voltage. For
simplifying
the circuit, it would be supplied
by the same leakage magnetic transformer with the anodę
power (high voltage power). The filament of the magnetron
which the present used microwave ovens
arę
all
treated through
some
special
technology,
and all
have
the
cold
start
character.
But when in cold start, there
still
is a very strong surge electric
field
attached
to the surface of the anodę, and would be harmful
to the anodę. In order to reduce the surge voltage, the filament
of the magnetron must be connected as the FIG.2-3 shown. In
this figurę, when the anodę current of the magnetron circulates,
the filament current should
flow
to the FA end from the
Iower
end.
2.3
COOLING
SYSTEM
4,OJ
100 200 300 400
anodę current
—
Ib(mA)
Fig.
2-4
In the working process of the microwave oven, the magnetron
often
makes the anodę temperaturę rise due to
anodę
loss
caused by the electronics strike the oven surface and the heat radiate of the cathode. To prevent the anodę
temperaturę rise too high, thus
affect
the working steady and its
life
span, it is
necessary
to
cool
the magnetron.
According to the
different
models and
ratę
output of the magnetron, compelling wind cooling and flowing
water-cooling
can be adopted. Usually, the
home
used microwave oven adopts the compelling wind cooling
method,
and all
arę
fixed with cooling
fin.
Cooling system includes fan
motor,
air
duet,
air entrance, air vent etc. The flowing direction of the cooling wind
should. Be parallel to the cooling fin of the magnetron.
Generally,
we adopt the method of air
blast
rather than air
absorb. And all the cabinet of the oven is with air entrance and air vent, the hot wind blowing through the
magnetron is guided with air
tubę
to improve the cooling
effect.
In the technical parameter chart of the magnetron,
it usually will give out the requirement of cooling wind. A
shortage
of cooling wind would damage the character of
the magnetron, even burn out the magnetron. The amount that the fan blasted should not be less than the
requirement. When fix the fan,
attention
must be paid to prevent the cool wind from blowing directly to the
glass
part
of the magnetron to avoid blasting.
2.4
ELECTRIC
CONTROL
SYSTEM.
To those mechanical control microwave
oven,
electric control system mainly composes of interlock switch, timer,
power distributor and thermal cutout, etc. The electric control system of those tough control microwave oven is
mainly composed of interlock switch, computer controller and thermal cutout, etc.
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