Electrical Operation (Electric Dryers Models); Electronic Control Board Circuits; Drive Motor Circuit; Temperature Sensing Circuit - Frigidaire Dryer Servicing

Section 3 Electronic Control Testing
Electrical Operation
When the dryer is connected to electrical power, line 1
is connected to one side of the thermal limiter and the
COM terminal of the heater relay RL2 that is mounted
on the electronic control board. (See Section 6 Wiring
Diagrams or Wiring Schematic provided with the unit)
Power is applied to the control board through thermal
limiter to pin 1 of the eight pin plug. The electronic con-
trol board controls the operating temperature of the dryer
and the length of the cycle by either sensing the amount
of moisture in the
clothes or a fi xed amount of time when timed dry is used.

Electronic Control Board Circuits:

Line 1 is applied to the control board at pin 1 of the eight
pin plug and neutral on pin 2 of the eight pin plug. The
control board receives inputs from the selector knob and
push buttons which programs the control, the contact
sensors that senses the amount of moisture in clothes
and the control thermistor which senses temperature in
the dryer. The control board uses these inputs to signal
the interface board, control the drive motor circuit by
opening and closing the contacts of relay RL1 and the
heater circuit by opening and closing the contracts of
relay RL2.

Drive Motor Circuit:

When the door is closed, the COM terminal is connected
to terminal NO of the door switch. From terminal NO,
power is applied to terminal J4-2 of the motor relay RL1
on the control board. When the control board closes
relay RL1, power is applied through terminal J4-1 to
terminal M4 of the drive motor.
Terminal M4 is connected inside the motor to one side
of the thermal overload (The thermal overload protects
the motor from being damaged by overheating.) The
other side of the thermal overload is connected to one
end of both the run winding and the start winding of the
drive motor. When the motor is not turning, the other
end of the startwinding is connected internally to terminal
M5 of the motor through the NC contact of the motor
centrifugal switch. The other end of the run winding is
also connected internally to terminal M5 which is
connected to neutral.
When the motor is not turning, the start winding and
the run winding are connected in parallel. When the
contacts of relay RL1 are closed, with the dryer door
closed, line 1 and neutral voltage are applied across both
the start and run windings of the drive motor.
3-16
(Electric Dryers Models)
With power applied to both the start and run windings,
the motor starts to turn.
When the speed of the motor reaches about 80% of its
normal run speed, the contacts of the centrifugal switch
remove power from the start winding thus removing the
start winding from the circuit.
The drive motor performs two tasks in the dryer. A pulley,
attached to one end of the motor shaft, uses a belt to
drive the dryer drum. The blower wheel is attached to
the other end of the motor shaft to pull the air through
the clothes and force it out the exhaust vent.

Temperature Sensing Circuit:

The temperature in the dryer is controlled by the control
thermistor and the electronic control board. The control
thermistor, mounted in the blower fan housing, is a
negative coeffi cient thermistor that decreases in
resistance as the temperature increases.
The electronic control board reads the resistance of the
thermistor and converts it into temperature. It compares
the reading from the thermistor to the temperature
setting for the cycle.

The Heating Circuit:

The electronic control board applies power to the heating
circuit through the contacts of heater relay RL 2. When
the electronic control senses the temperature in the drum
is below the programmed temperature, it closes the
contact of RL 2. This applies line 1 power through the
high limit thermostat that is mounted on the heating
element assembly.
The high limit thermostat is a safety device that prevents
the dryer from overheating if the contacts of the RL 2 fail
closed. The contacts of the high limit thermostat,
normally closed and are set to open at a temperature
above the preset temperature specifi cations of the
electronic control board. From the output terminal of the
high limit thermostat, line 1 is connected to one side of
the element and to pin 3 of the eight pin plug. The other
side of the heating element is connected to line 2 through
the contacts of the second centrifugal switch in the drive
motor. This switch prevents power from being applied to
the element if the motor is not running. The connection
to pin 3 allows the control to monitor the contacts of the
high limit thermostat. An error code will show if contacts
open above a preset number of times in a cycle.