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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.