Microwave oven with a motor driven electric heater

A motor drive for a heater in a microwave oven is disclosed which drives the rotation of the heater by the motor and controls the rotation of the motor with a control device to automatically position the heater suitably for cooking, thus preventing under or overcooking. A device is provided to permit the heater to rotate freely between a vertical position opposing a convection fan in the heating chamber and a horizontal position directly above a tray, a motor and transmission device are provided to drive the rotation of the heater, and a control device is provided to control the rotation of the motor.

BACKGROUND OF THE INVENTION 
This invention concerns a motor drive for a high frequency heater in the 
heating chamber of a microwave oven, which can freely rotate the high 
frequency heater between a vertical position for baking and microwave 
cooking, and a horizontal position for grilling. 
Heaters located vertically in microwave ovens, opposing a convection fan in 
the back of the heating chamber, with which baking and microwave cooking 
were effected, which could be lowered to a horizontal position, directly 
above a tray in the heating chamber for broiling were available with 
conventional units. However, with the heater in the conventional microwave 
oven, positioning of the heating element was effected by moving a heater 
drive lever on the control panel. With this type of positioning, food was 
in some cases not browned properly, or the temperature in the oven 
differed from that desired if it was not positioned properly, resulting in 
over or under cooking of food. 
SUMMARY OF THE INVENTION 
The object of this invention is to eliminate the above problems by 
providing a motor drive device which has a motor to rotate the heater in 
the microwave oven, and a control device to control the rotation of the 
motor in order to automatically position the heater suitably to provide 
for no-fail cooking. 
In order to achieve the above goal, means are provided to allow rotation of 
the heater in the heating chamber of the microwave oven from a vertical 
position opposing the convection fan, to a horizontal position directly 
above the tray; a motor drive is provided to rotate the heater; and a 
control device is provided to control the heater driving mechanism of the 
motor. 
In the motor drive for the heater in the microwave oven in this invention 
which is composed of the components stated above, the control device 
controls the rotation of the motor according to the type of cooking, 
locating the heater in a suitable position, eliminating over or 
undercooking of food due to user error.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, the microwave oven (1) is provided with a convection 
fan (3) on the front of the back wall of the heating chamber (2), and a 
tray (4) on the bottom of the heating chamber (2), which food to be cooked 
is placed on. A heater (5) which is bent back and forth is provided in the 
heating chamber (2), which can freely rotate by means of a pivot hole (6) 
provided in the side wall. This heater (5) rotates between a vertical 
position opposing the convection fan (3) and a horiztonal position 
directly above the tray (4). A door (7) is provided on the front of the 
microwave oven (1), and a control panel (8) beside it. A heater up button 
(9), heater down button (10), grill select button (11), grill auto button 
(12), heating button (13) and other buttons are provided on the control 
panel (8). 
Rotation of the heater (5) in the heating chamber (2) is driven by the 
motor (14) drive mechanism of FIG. 2 provided on the outside of the hole 
(6) in the side wall. This motor drive mechanism (14) comprises a motor 
(15) and transmission device (16) as shown in FIGS. 2 and 3. The motor 
(15) is an AC motor which can be reversed by a control device. The 
transmission device (16) transmits the rotation of the motor (15) to the 
heater. The rotation of the motor (15) is first reduced by the small 
pulley (17), belt (18) and big pulley (19), and transmitted to the worm 
gear (20) which is on the same shaft as the big pulley (19). The rotation 
which has been transmitted to the worm gear (20) is reduced and 
transmitted to the worm wheel (21). A clutch gear (22) is mounted on the 
rotation shaft of the worm wheel (21) which allows free rotation and 
sliding in the thrust direction. Clutch linings, (23), (23) with a high 
abrasion factor are firmly fixed to the surfaces of the clutch gear (22) 
and worm wheel (21) which face each other, and the clutch gear is strongly 
pressed against the worm wheel (21) by a compression coil spring (24) 
which is mounted on the worm wheel (21). The rotation that is transmitted 
to the worm wheel (21) is thus transmitted to clutch gear (22) shown in 
FIG. 3 at a torque transmission ratio of 1:1. The rotation that is 
transmitted to the clutch gear (22) is further reduced and transmitted by 
heater drive gear (25). The rotation that is transmitted to the heater 
drive gear (25) is finally transmitted to the heater (5). The heater (5) 
sticks out of the hole (6) provided in the side wall of the shown in FIG. 
3, and the end of the heater 26 that sticks out is bent perpendicular with 
respect to the rotation shaft. A heater choke (27) to permit penetration 
of the heater (5) and choke insulation (28) are provided around the 
outside of the hole (6), which serve as a bearing for the heater (5), 
prevents leakage of microwaves and serves as a double insulation for the 
heater (5). A heater drive angle (29) is fixed to the heater choke (27), 
and the heater drive gear (25) of the transmission device (16) is mounted 
to the heater drive angle (29) in such a way that it can freely rotate, to 
be integral with the rotation shaft of the heater (5). The heater drive 
gear (25) is provided with a sector gear (30) with a rotation angle of 90 
degrees or more, and a small projection (31) and L shaped projection with 
the end bent on the periphery other than the sector gear (30). The sector 
gear (30) transmits rotation to the heater drive gear (25) by meshing with 
the clutch gear (22). A hole (33) is provided in the end of the L shaped 
projection (32) which is bent, and the rotation of the heater drive gear 
(25) is transmitted to the heater (5) by fitting the end of the heater (5) 
in the hole (33). The motor (15) thus drives the heater (5) through the 
transmission device (16). If, for example, the rpm of the motor is 5000, 
and the reduction ratio of the transmission device is 1/1000, the heater 
can be rotated from the vertical to the horizontal position in about 3 
seconds. 
A microswitch (34) to detect the vertical position of the heater (5) and 
another microswitch (35) to detect the horizontal position of the heater 
(5) are provided around the rotating arc of the heater drive gear (25) as 
shown in FIGS. 4 and 5, with the small projection (31) on the heater drive 
gear (25) pressing against the vertical position detecting microswitch 
(34) to turn it ON when the heater is in the vertical position, as shown 
in FIG. 4, and the L shaped projection (32) of the heater drive gear (25) 
pressing against the horizontal position detecting microswitch (35) to 
turn it ON when the heater (5) is in the horizontal position. 
The control device which controls the rotation, stopping and direction of 
rotation of the motor (15), comprises a CPU (37) to which a memory (36) is 
connected, with signals from the control keys 9-13 on the control panel 
(8), and signals from the vertical position detecting microswitch (34) and 
horizontal detecting microswitch (35) taken in from the I/O interface 
(38), and a control signal output to the motor rotation changeover relay 
(39) to operate the motor (15). When the heater down button (10) is 
pressed, the control device drives the motor (15) in the forward direction 
to move the heater (5) towards a horizontal position from a vertical 
position. If the heater down button is released while the heater is being 
lowered, the rotation of the motor (15) stops, stopping the heater (5) at 
an intermediate position, permitting the heater (5) to be stopped at an 
arbitrary position. The heater (5) is moved until the horizontal position 
detecting microswitch (35) is turned ON if the heater down button (10) is 
held. Input of the heater down button (10) is not accepted while the 
horizontal position detecting microswitch (35) is ON. When the heater up 
button (9) is pressed, the control device causes the motor (15) to rotate 
in the reverse direction, moving the heater (5) towards a vertical 
position from a horizontal position. The rotation of the motor (15) also 
stops when the heater up button (9) is released, stopping the movement of 
the heater (5) at an intermediate position, allowing the heater (5) to be 
located at an arbitrary position. If the heater up button (9) is held, the 
heater (5) continues to move until it is stopped at the vertical position 
when the vertical position detecting microswitch (34) goes ON. Input of 
the heater up button (9) is not accepted while the vertical position 
detecting microswitch (34) is ON. 
The operation of the grill auto button (12) on the control panel (8) of the 
mcirowave oven (1) will now be explained, using FIG. 7. 
In step nl, when the user presses the heater button (13), microwave heating 
is commenced (n2). When microwave heating ends (n3), the control device 
causes the motor (15) to rotate in the forward direction, moving the 
heater (5) towards the horizontal position from the vertical position 
(n4). When the heater (5) stops at the horizontal position (n5), grilling 
is commenced (n6). While the door (7) remains closed (n7), the control 
device causes the heater (5) to be moved towards the vertical position 
(n9) when grilling ends (n8). When the heater (5) stops at the vertical 
position (n10), cooking is finished. Furthermore, if the door (7) is 
opened during grilling (n7), the heater (5) is turned off (n11), the 
control device goes to n9, and the heater (5) is raised to the vertical 
position. This prevents the possiblity of the user inadvertently touching 
the heater (5) and burning his/her hand. 
The transmission device (16) of the motor drive for the heater transmits 
the rotation of the motor (15) through the worm gear (20) and worm wheel 
(21). This prevents an variation in position of the heater (5) due to its 
own weight. However, in the transmission device (16) there is a slip 
clutch mechanism which comprises clutch linings (23), (23) fixed to 
opposing surfaces of the worm wheel (21) and clutch gear (22), which are 
held together by a compression spring (24). Rotation is transmitted 
through this slip clutch, so that when external force is applied, the slip 
clutch mechanism slips, allowing the heater (5) to be moved. This prevents 
the heater (5) from getting in the way by allowing it to be moved to an 
arbritrary position when the unit is unplugged in order to clean the 
inside of the heating chamber (2) of the microwave oven (1). 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.