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Revision 4/F3576 © Moffat Ltd, December 2004
3.2 EXPLANATION OF CONTROL
SYSTEM
The G1100 Turbofan convection oven
features multi-function operator controls for
which a correct understanding of their
operation is required before carrying out any
service or fault repair work. The control
device functions are explained as follows:
A power switch on the control panel isolates
all to the controls of the oven. With the power
switch Off all functions of the oven are
inoperable.
With the power switch On (indicator
illuminated) power is directly supplied to the
60 minute bake timer, steam (water injection)
switch, light switch, and the temperature
control circuit. The oven circulation fan will
operate only when the thermostat is turned on
and the oven door is closed. The control
panel light switch will turn the oven lights on
when the door is closed only when the light
switch is held in. With the thermostat
switched on, the oven lights will come on
automatically when the door is opened, as this
is controlled by the door microswitch.
The 60 minute timer is a mechanical timer and
can therefore be operated with the oven’s
power switch On or Off. However, only with
the oven’s power switch On will the switch
contacts of the 60 minute timer turn on the
time-up buzzer and illuminate the time-up
indicator on the control panel. The buzzer
and time-up indicator provide indication that
the time setting has run down to zero and at
this point will remain On continuously until the
60 minute timer has been manually set back
to the Off (vertical) position. The 60 minute
timer does not control any other part of the
oven’s operating system as this timer is
independent of the temperature control and
heating system.
The steam (water injection) switch on the
control panel can be operated whenever the
power switch is On. The switch is momentary
like the light switch and when depressed, will
operate the electric solenoid valve at the rear
of the oven and inject water across the oven
fan from the flat spray (vertical) nozzle
positioned at the rear of the oven. Releasing
the steam button will close the solenoid valve.
This feature is used to instantaneously add
steam into the oven.
The temperature control of this oven is with a
capillary type thermostat which can be set to a
required cooking temperature.
The thermostat switch has a separate switch
body assembled onto the front from the shaft
assembly and when the thermostat is set to a
cooking temperature, the switch contacts turn
on the oven fan. The switch is closed (fan on)
whenever the thermostat is not in the Off
(vertical) position. The control panel indicator
light to the right of the thermostat knob cycles
On and Off with the thermostat to indicate
when the gas burners are on and the oven is
heating.
The burners are automatically controlled by
the main oven thermostat or hold thermostat,
via an automatic ignition and valve control
device, which operates a gas solenoid valve
to provide gas to the burners as required to
maintain the set thermostat temperature.
On the thermostat calling for heat, the
thermostat will switch power to the ignition
control. The ignition control will then power
the gas control valve to open the solenoid and
provide gas to the burners. At the same time
the ignition control will generate a high voltage
ignition spark sequence conveyed to the
ignition electrodes through a high tension
ignition cable. The spark and earth ignition
electrodes will spark across the end gap
between these two to ignite the burners.
On successful ignition of the burners the third
electrode (a flame rectification rod) will be in
the burner flame. This provides the ignition
control with confirmation that the burners are
lit by establishing a micro-amp current from
the electrode to earth through the flame due
to combustion flames being ionised and
therefore able to conduct minute electrical
currents. The flame electrode is connected to
the ignition control with an insulated wire.
If this ignition sequence occurs correctly the
spark sequence will be stopped and the
burners will continue to operate until the
thermostat switches off and shuts down the
ignition control and consequently closes the
gas solenoid valve.
Should the ignition control not sense the
burner flame within a fixed ignition / spark
sequence period, the ignition control will
lock-out and turn off the gas solenoid valve.
Lock-out condition requires the oven
thermostat to be switched off before operation
can be restarted.