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HVACR318 - Refrigeration II
Domestic Defrost
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Defrost Timer, General
• Many refrigerators in homes are frost free. The
brain of the frost-free refrigerator is the defrost
timer.
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Defrost Timer, Operation
• Operated by a single phase synchronous motor,
like the ones used in wall clocks.
• The contacts are operated by a cam that is gear
driven by the clock motor.
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Domestic Timer Operation
• The job of this timer is to disconnect the
compressor circuit and connect a resistive
heating element (located near the evaporator) at
regular intervals.
• The defrost heater is thermostatically controlled
and turned off after melting any frost formation
on the evaporator.
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Domestic Timer Operation
• The timer will continue to operate in defrost
until the preset time has elapsed.
• The defrost heater is permitted to operate for
some length of time before the timer disconnects
it from the circuit and permits the compressor to
operate again.
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Schematic of a Simple Defrost Timer
• Notice that terminal 1 is connected to the
common of a single pole double throw switch.
• Terminals 2 and 4 are connected to the
stationary contacts of the switch.
• In the normal operating mode,
the switch makes connection
between contacts 1 and 4.
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Schematic
• When the defrost cycle is activated, the contact
will change position and make a connection
between terminals 1 and 2.
• Terminal 3 is connected to one lead of the
motor.
• The other lead is brought outside the case. This
permits the timer to be connected in one of two
ways: continuous or cumulative run.
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Continuous Run Timer Schematic
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Continuous Run Timer
• Notice in this circuit that the pigtail lead of the
motor has been connected to terminal1, and that
terminal 1 is connected directly to the power
source.
• Terminal 3 is connected to neutral
• This places the timer motor directly across the
power source, which makes the motor run
continuously.
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Operation of Timer in Compressor Run
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Timer in Compressor Run
• Notice there is a current path through the timer
motor and a path through the timer contact to the
thermostat.
• This permits power to be applied to the
compressor and evaporator motor when the
thermostat closes.
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Operation of Timer in Defrost
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Timer in Defrost
• Notice there is still a complete circuit through
the timer motor.
• When the timer contact changes position, the
circuit to the t-stat is open and the circuit to the
defrost heater is closed.
• The heater can now melt any frost accumulation
on the evaporator.
• At the end of the defrost cycle the compressor
starts running again.
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Cumulative Compressor Run Timer
• The timer motor is permitted to operate only
when the compressor is in operation and the
thermostat is closed.
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Schematic of a Cumulative Run Timer
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Cumulative Run Timer
• Notice the leads of the clock motor have been
connected to terminal 2.
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Schematic During Compressor
Operation
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Compressor Operation
• The timer contact is making a connection
between terminals 1 and 4.
• This permits power to be applied to the
thermostat.
• When the thermostat contact closes, current is
permitted to flow through the compressor motor,
the evaporator fan motor, and the defrost timer
motor.
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Compressor Operation
• In this circuit, the timer motor is connected in
series with the defrost heater.
• The operation of the timer motor is not affected.
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Compressor Operation
• The impedance of the timer motor is much
greater than the resistance of the heater
• For this reason almost all the voltage of this
circuit is dropped across the timer motor.
• The impedance of the timer motor also limits the
current flow throughout the defrost heater to
such an extent that it does not become warm.
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Schematic of a Defrost Timer in
Defrost
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Defrost Timer in Defrost
• Notice in this circuit that the defrost heater is
connected directly to the power line.
• This permits the heater to operate at full power.
• There is also a current path through the timer
motor and the run winding of the compressor
motor in this circuit.
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Defrost Timer in Defrost
• The timer motor is connected in series with the
run winding of the compressor.
• As before, the impedance of the timer motor is
much greater than the impedance of the run
winding of the compressor.
• Almost all the voltage in this circuit to be
applied across the timer motor.
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Defrost Timer in Defrost
• At the end of the defrost cycle, the timer contact
returns to its normal position and the compressor
is permitted to operate.
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Testing the Timer
• An ohmmeter can be used to check the
continuity of the contacts and the motor
windings
• The cam can be manually turned to the position
so that the defrost cycle is started.
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Testing the Timer
• This can be checked with a voltmeter to
determine when full circuit voltage is applied to
terminal 2. It is then necessary to wait long
enough for the timer to open the contact to the
defrost heater and reconnect the compressor
circuit.
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Testing the Timer
• If the thermostat is closed, the compressor will
start when the timer contact changes position.
This test shows that the timer motor is operating
and that the contact does change position.
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A Common Problem
• The movable contacts become stuck between
terminals 4 and 2.
• Although 2 to 4 should never come together
• This causes the compressor and defrost heater to
run at the same time.