The job of this timer is to disconnect thecompressor circuit and connect a resistive heatingelement located near the evaporator atregular time intervals. The defrost heater is thermostaticallycontrolled and is used to melt any frostformation on the evaporator. The defrost heater ispermitted to operate for some length of time beforethe timer disconnects it from the circuit and permitsthe

compressor to operate again. The defrost timer is operated by a single-phase synchronousmotor like those used to operate electricwall clocks, Figure 28 1. The contacts are operatedby a cam that is gear driven by the clock motor. A schematic drawing of the timer is shown inFigure 28 2. Notice that terminal 1 is connected tothe common of a single-pole double-throw switch. Terminals 2 and 4 are connected to stationary contactsof the switch. In the normal operating mode, the switch makes connection between contacts1 and 4. When the defrost cycle is activated, thecontact will change position and make connectionbetween terminals 1 and 2. Terminal 3 is connectedto one lead of the motor. The other motor lead isbrought outside the case. It should be noted that the schematic drawingcan be a little misleading. In the schematicshown, the timer contact can only make connectionbetween terminals 1 and 4, or terminals 1 and 2. Inactual practice, a common problem with this timeris that the movable contact becomes stuck betweenterminals 4 and 2. This causes the compressor anddefrost heater to operate at the same time. The schematic for the continuous run timer isshown in Figure 28 3. Notice in this circuit that thepigtail lead of the motor has been connected to terminal1, and that terminal 1 is connected directly tothe power source. Terminal 3 is connected directlyto the neutral. This places the timer motor directlyacross the power source,...

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