Convection Cooking System; Damper Open-Close Mechanism; High Voltage Fuse - Sharp R-990KS Service Manual

[13] CONVECTION COOKING SYSTEM

This oven is designed with a hot air heating system where food is not
directly heated by the convection heater, but is heated by forced circu-
lation of the hot air produced by the convection heater. The air heated
by the convection heater is circulated through the convection passage
provided on the outer casing of the oven cavity by means of the con-
vection fan which is driven by the convection motor. It then enters the
inside of the oven through the vent holes provided on the left side of
the oven. Next, the hot air heats the food on the turntable and leaves
the oven cavity through the vent in the oven cavity left side wall. With-
out leaving the oven, this hot air is reheated by the convection heater,
passes through the convection passage and enters the inside of the
oven cavity again, in a continuing cycle. In this way, the hot air circu-
lates inside the oven cavity to raise its temperature and, at the same
time, comes into contact with the food being cooked. When the tem-
perature inside the oven cavity reaches the selected temperature, the
convection heater is de-energized. When the temperature inside the
oven cavity drops below the selected temperature, the convection
heater is energized again. In this way, the inside of the oven cavity is
maintained at approximately the selected temperature.When the con-
vection time reaches 0, the convection heater is de-energized and the
convection fan stops operating and the oven shuts off. Upon comple-
tion of the cooking time, the audible signal will sound, and oven lamp,
turntable motor, cooling fan motor and convection motor are de-ener-
gized. At the end of the convection cycle, if the cavity air temperature
is above 116°C, the circuit to RY6will be maintained (by the thermistor
circuit) to continue operation of the cooling fan motor until the temper-
ature drops below 116°C, at which time the relay will be de-energized,
turning off the fan motor. Relay RY5 will how-ever, open as soon as
the convection cycle has ended, turning off the convection fan motor.
This will now cool and allow the damper door to open.

[14] DAMPER OPEN-CLOSE MECHANISM

Usually, the damper is in the open position except during convection
cooking.Damper position is set automatically by damper motor,
damper switch, motor cam and damper shaft.These components are
operated by a signal that judges if microwave cooking or convection
cooking operation is selected by the CPU unit.
1. Microwave Cooking:
Damper is in the open position, because a portion of cooling air is
channelled through the cavity to removes team and vapours given off
from the heating foods.It is then exhausted at the top of the oven cav-
ity into a condensation compartment.
2. Convection Cooking:
Damper is in the closed position, so that no hot air will be allowed to
leak out the oven cavity.
3. Damper Operation
1. When power supply cord is plugged in:
1) When power supply cord is plugged in, a signal is sensed in the
control unit, and operates shut-off relay(RY4).
2) Contacts of shut-off relay (RY4) close, the damper motor is
energized, opening the damper door.
3) When the damper is moved to the open position by the damper
cam, damper switch is closed (ON position).
4) The signal of damper switch is re-sensed in the control unit and
shut-off relay (RY4) is turned off.
5) The rated voltage to the damper motor is stopped and the motor
turns off.
2. When oven is microwave cooking:
Damper is in the open position
3. When oven is convection cooking:
1) Damper motor is energized by touching the convection, temper-
ature and START pads.
2) When damper is in the closed position (dampers witch is OFF),
its signal is sensed by the control unit, and shut-off relay (RY4)
is de-energized.
3) The damper is held in the closed position during the convection
cooking operation.
4) .At the end of the convection cooking, shut-off relay (RY4) is
energized, and the damper is returned to the open position.
NOTE: If the damper door is not in the proper position, closed during
convection or open during microwave, the control unit will stop
oven operation after 1 minute.
Cooking Mode
Microwave cooking
Convection cooking
Sensor cooking
Automatic mix cooking
Grill cooking
Slow cooking
DAMP E R S HAF T
Figure D-2. Damper Mechanism

[15] HIGH VOLTAGE FUSE

The high voltage fuse blows when the high voltage rectifier or the mag-
netron is shorted.
6 – 2
R990K(W)
Operation of Damper
OPEN
CLOSE
OPEN
CLOSE
CLOSE
CLOSE
DAMP E R DUC T
DAMP E R
DAMP E R C AM
DAMP E R S WIT C H
DAMP E R MOT OR