The Defrost Circuit - Electrolux FROST- FREE Service Manual

defrost timer is in the run mode contacts (1) to (4)
are closed and powers applied to the overload
protector of the compressor. Current flows through
the overload protector applying powers to both
the start and run windings of the compressor. When
the compressor is trying to start the only path for
current flow is through the circuit formed by the run
winding and the coil of the start relay to neutral.
Because of the relatively high starting resistance
of the compressor. The current through the coil is
high enough to energize the relay. This closes the
contacts of the start relay and connects the start
winding to neutral. The current through the start
winding induces a second, out-of-phase, magnetic
field in the stator and starts the motor. As the motor
speed increases, the run winding current is reduced.

2. The Defrost Circuit.

The defrost cycle, operates for 30 minutes out of
every 12 hours of run time, to remove the frost from
the evaporator. The defrost circuit is made up of the
cold control, contacts one and two of the defrost
timer, defrost thermostat and the defrost heater.
When the defrost cycle is in operation, the cold
control contacts are closed applying power to
terminal (1) of the defrost timer. Terminal (1) is then
closed to terminal (2) of the defrost timer, which
applies power to the defrost thermostat.
The defrost thermostat is a bimetal device that
protects the evaporator from overheating during the
defrost cycle and damaging the cabinet. The
contacts of the defrost thermostat are closed at
normal freezer temperatures. Power passes to the
contacts of the defrost thermostat and is applied to
the defrost heater.
L1
COLD
CONTROL
DEFROST
TIMER
1
2
DEFROST
THERMOSTAT
20
At a predetermined condition, the run winding
current, which is also the current through the relay
coil, drops to a value below that necessary to hold
up the relay armature. The armature drops and
opens a start relay contacts and takes the start
winding out of the circuit.
The evaporator fan motor is connected between
terminal (4), of the defrost timer, and neutral. This
connects the evaporator fan electrically in parallel
with the compressor. This is helpful when
troubleshooting because, if either one of the
components operate, this indicates that the cold
control and defrost timer contacts (1) to (4) are
good and at the problem is in the other circuit. If
neither operate then the cold control and defrost timer
should be checked first.
Electrical Current Flow
Since the defrost timer contacts are electrically
after the cold control, the contacts of the cold
control must be closed before the defrost cycle
can operate.
With the contacts of the cold control close, when
the cam of the defrost timer closes contacts (1) to
(2), powers is applied to defrost heater through the
normally close contacts of the defrost thermostat.
The other end of the defrost heater is connected to
neutral, which completes the circuit and allows
current flow through the defrost heater.
The specifications of the defrost thermostat are that
it closes at 10°F and opens at 50°F. Although the
defrost timer provides power to the circuit for 30
minutes, the heating element provides enough heat
to open the contacts of the defrost thermostat in
about 10 to 12 minutes.
The rest of the time is used to allow the moisture to
drip off the evaporator so it does not refreeze when
the compressor starts.
DEFROST
HEATER
N