Carrier Understanding Timer Functions and Sequence Operation for Air Conditioners

CONDENSER

COIL

FUSIBLE

PLUG

SUCTION

LINE

	ACCUMULATOR
DISCHARGE	COMPRESSOR RAIL
LINE

Fig. 45 Ð Accumulator and Fusible Plug

Timer Functions Ð (See Timer Cycle, Fig. 47.) Each refrigeration circuit is controlled by an independent timer which allows for the independent operation of each refrig- eration circuit.

NOTE: Unit 38AH044-084 optional single-circuit units have one timer which controls the lead compressor. Lag compres- sor is controlled by CCPS (capacity control pressure switch).

SWITCH A Ð The timer is energized through contacts A-A1 or A-A2. This establishes the Time Guard function which prevents compressor short-cycling. Start of compressor is delayed approximately 5.5 minutes after shutdown.

SWITCH B Ð The compressor is initially energized through contacts B-B1.

SWITCH D Ð Contacts D-D1 provide a 21¤2-minute bypass of the low-pressure switch at start-up for winter-start con- trol. On 38AH044-084 optional single-circuit units, contacts D-D2 control start-up of compressor A2.

SWITCH E Ð Contacts E-E1 provide a 40-second bypass of the oil pressure switch at start-up. If oil pressure does not build to the required minimum pressure in 40 seconds, the compressor shuts down and the control circuit locks out.

On 38AH044-084 dual-circuit and 38AH094,104 units, lag circuit B start-up is delayed 60 seconds after a call for cool- ing is made to the circuit. This prevents compressor(s) in both lead and lag circuits from starting at the same time.

Control Circuit Reset Ð The control circuit locks out if the unit shuts down because of low oil pressure, high discharge gas temperature (DGT), or excessive high-side pres- sure. To reset the control circuit, open and close the fan circuit breaker (FCB). This resets the timer, and the unit restarts under Time Guard control. At start-up, if the low- pressure switch (LPS) does not close after 21¤2 minutes, the unit shuts down. When the pressure builds enough for the LPS to cut in, the control circuit is energized automatically and start-up proceeds under Time Guard control.

Fig. 46 Ð Pressure-Actuated Capacity Control Valve

CONTROL

Sequence of Operation Ð Units are controlled with electromechanical components. Each refrigeration circuit (except 38AH044-084 optional single-circuit units) is oper- ated by an independent timer which controls the operation sequence of each circuit.

On a call for cooling, ®rst stage cooling thermostat TC1 closes. Condenser fans and timer (TM) are energized. After approximately 7 seconds, timer contacts E-E1 close. Approximately 12 seconds after TC1 closes, normally-open timer contacts B-B1 close for 1 second. This energizes compressor A1 contacts CA1 and starts the compressor. At the same time, solenoid drop relays (SDRs) and liquid line solenoid valve no. 1 (LLS-A for 38AH044-084 dual-circuit units; LLS-A1 for all other units) open, and timer relay no. 1 (TR2) is energized. Normally open TR2 contacts close, completing a circuit around B-B1 and through compressor A1 contactors to maintain compressor operation when B-B1 contacts open. Contacts E-E1 remain closed for approxi- mately 40 seconds to bypass the oil pressure switch (OPS). If oil pressure is insufficient when contacts E-E1 open, the compressor stops, the timer cycles off, and the control cir- cuit locks out. At start-up, timer contacts D-D1 are closed, bypassing low-pressure relay contacts LPR-A for 21¤2 min- utes. This provides a winter start-up feature.

Approximately 21¤2 minutes after TC1 closes, timer con- tacts D-D1 open and D-D2 close. If pressure is insufficient to close the low-pressure switch, the low-pressure switch relay is open, the compressor shuts down, and the Time Guard control is initiated. (Time Guard control prevents compres- sor from restarting for 5 minutes after the demand for cool- ing is satis®ed.)

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