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Revision 5/F3534
©
Moffat Ltd, April 2005
Turbofan Convection Oven
3.3
EXPLANATION OF CONTROL
SYSTEM
The E311 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
power to the controls of the oven. With the power
switch Off all functions of the oven are
inoperable.
NOTE:
The supply voltage is fed directly to the
input side of the power relay whenever the
electrical supply is on.
With the power switch On (illuminated) power is
directly supplied to the 60 minute bake timer,
steam (water injection) switch, door microswitch,
light switch, and the hold switch circuit. (On units
manufacture prior to serial number 232053 power
is also supplied directly to the temperature control
circuit). Accordingly the oven circulation fan will
operate when the door is closed, and with door
open the oven light will come on and the circula-
tion fan will shut down, as these are controlled via
the door microswitch. The control panel light
switch will turn the oven light on when the door is
closed. On units from S/N 260862 the switch is
latching and will activate the light while latched in
the down position. On units prior to S/N 260861
the switch is momentary and will need to be held
in to activate the light.
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.
US/Canada only:
The steam (water injection)
switch on the control panel can be operated
whenever the power switch is On. The switch is
momentary and when depressed, will operate the
electric solenoid valve at the rear of the oven and
inject water into the water trough at the bottom of
the left hand side rack in the oven. Releasing the
steam button will close the solenoid valve. This
feature is used to instantaneously add moisture
into the oven cavity, or to introduce a reservoir of
water in the water trough to maintain humidity in
the oven chamber and minimise product’s
moisture loss during cooking.
For units manufactured from serial number
232054, power to the temperature control circuit
and elements is supplied via a power relay
located near the terminal block at the rear of the
unit. Although the power relay has two poles with
changeover contacts, only the normally closed
contact on one pole is used - the other is
redundant. When the power switch on the control
panel is switched on the power relay is energised
and the normally open contact is closed,
providing power to the temperature control circuit.
The temperature control of this oven is with a
capillary type thermostat which can be set to a
required cooking temperature, or set to the Grill/
Broil position to provide top browning or grilling/
broiling in the oven.
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, one set of contacts is
closed to switch power from the oven thermostat
to the bottom heating element in the oven. The
second switch contact of this switch assembly
remains open in this setting and isolates power
from the top element inner coil. The top element
outer coil is directly switched on from the oven
thermostat. Accordingly only the top outer
element and the bottom element are used when a
cooking temperature is selected. The control
panel indicator light above the thermostat knob
cycles On and Off with the thermostat to indicate
when the elements are on and the oven is
heating.
When the thermostat control is set to the Grill
position the first switch contact opens and the
second switch contact closes. This arrangement
allows power to be provided to the top element
inner coil when the thermostat is On, and isolates
power from going to the bottom heating element.
The top element outer coil which is directly fed
from the oven thermostat switch is therefore On
in conjunction with the top element inner coil
when the thermostat is set to the Grill position.
As both the top inner and outer element coils are
thermostatically controlled, the elements will cycle
On/Off with the door closed as the thermostat
setting in the Grill position is 300°C (600°F). If the
Grill position is selected with the door open, the
elements will remain On continuously and the
