Revision 6/F3501
©
Moffat Ltd, August 2006
E25 Convection Oven
-10-
3.3 EXPLANATION
OF
CONTROL
SYSTEM
The E25 and E25B Turbofan convection
ovens feature 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
power to all the controls of the oven. With the
power switch Off all functions of the oven are
inoperable.
NOTE:
The supply voltage is fed to the input
side of the heating element power relay on
E25B models whenever the electrical supply
is on.
With the power switch On (illuminated) power
is directly supplied to the 60 minute bake
timer, door microswitch, temperature control
circuit, and the light switch. The light switch
will turn the oven light on when the door is
closed, but only whilst the light switch is held
in.
The door microswitch on the
E25
oven
controls the oven light only. Accordingly the
light will come on when the door is opened but
the circulation fan and fan element will remain
on. The door microswitch on the
E25B
oven
however, controls the light, the circulation fan,
and the fan element. Hence, opening the
door on the E25B oven causes the circulation
fan and fan element to switch off, as well as
causing the oven light to come on. (The broil
function incorporated into the E25B oven
requires the fan to be inoperative when
broiling with the door open).
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 temperature control of these ovens is with
a capillary type thermostat which can be set to
a required cooking temperature. The E25B
also features a Broil position to provide top
browning or broiling in the oven.
The
E25
has an element coiled around the
circulation fan in the rear of the oven. Power
to the element is provided directly from the
thermostat. The control panel indicator light
above the thermostat knob cycles on and off
with the thermostat to indicate when the
element is on and the oven is heating. In the
‘Off’ position, the element relies on the
thermostatic control to prevent it switching on.
Accordingly, if the oven temperature drops
below approximately 20°C the thermostat and
element may cycle on at this setting.
The
E25B
features a Broil element (located in
the top of the oven) as well as the fan
element. The thermostat switch has a
separate switch assembled onto the front of
the shaft assembly. This auxiliary switch,
depending on the thermostat setting, selects
between the fan or broil elements, via a Broil
relay located at the rear of the oven. A
changeover contact on the broil relay
determines which element is provided with
power. In series with the broil relay is a
second relay which is controlled by the main
thermostat switch. This is the heating relay.
One pole of the heating relay switches power
to the fan element, the other pole switches
power to the broil element. When the
thermostat cycles, both poles are switched
open or closed at the same time. However,
since only one pole is being fed power by the
broil relay, only the selected element cycles
on.
When the
E25B
thermostat is set to a cooking
temperature, one set of auxiliary switch
contacts remain open. This isolates power
from the coil of the two pole broil relay. In this
state the normally closed contacts of the first
pole provide power directly from the phase
connection on the terminal block to the fan
element pole on the heating relay. (The
normally closed contact on the second pole is
unused with this thermostat setting). The
second set of auxiliary switch contacts is
closed to feed power from the door
microswitch to the main thermostat switch.
When the thermostat cycles on it in turn feeds
power to the coil of the heating relay, causing
the changeover poles to switch. This closes
the normally open contacts and provides
power to the fan element. When the
thermostat cycles Off it isolates power to the
heating relay coil. This causes the normally