Carrier Air Conditioner specifications Stepper Motor 12 VDC

Electronic Expansion Valve (EXV) (See Fig. 15) — Standard units are equipped with a bottom seal EXV. This device eliminates the use of the liquid line solenoid pumpdown at unit shutdown. An O-ring has been added to bot- tom of orifice assembly to complete a seal in the valve on shut- down. This is not a mechanical shut-off. When service is required, use the liquid line service valve to pump down the system.

High pressure refrigerant enters bottom of valve where it passes through a group of machined slots in side of orifice as- sembly. As refrigerant passes through the orifice, it drops in pressure. To control flow of refrigerant, the sleeve slides up and down along orifice assembly, modulating the size of orifice. The sleeve is moved by a linear stepper motor that moves in in- crements controlled directly by the processor. As stepper motor rotates, the motion is translated into linear movement of lead screw. There are 1500 discrete steps with this combination. The valve orifice begins to be exposed at 320 steps. Since there is not a tight seal with the orifice and the sleeve, the minimum po- sition for operation is 120 steps.

STEPPER

MOTOR (12 VDC)

ORIFICE ASSEMBLY (INSIDE PISTON SLEEVE)

LEAD SCREW

PISTON SLEEVE

Two thermistors are used to determine suction superheat. One thermistor is located in the cooler and the other is located in the cylinder end of the compressor after refrigerant has passed over the motor. The difference between the 2 ther- mistors is the suction superheat. These machines are set up to provide approximately 5 to 7 F (2.8 to 3.9 C) superheat leaving the cooler. Motor cooling accounts for approximately 22 F (12.2 C) on 30GTN,R units and 16 F (8.9 C) on 30GUN,R units, resulting in a superheat entering compressor cylinders of approximately 29 F (16.1 C) for 30GTN,R units and 23 F (12.8 C) for 30GUN,R units.

Because the valves are controlled by the EXV module, it is possible to track the position of the valve. Valve position can be used to control head pressure and system refrigerant charge.

During initial start-up, the EXV module will drive each valve fully closed. After initialization period, valve position is controlled by the EXV module and the MBB.

The EXV is used to limit the maximum cooler saturated suction temperature to 55 F (12.8 C). This makes it possible for the chiller to start at high cooler fluid temperatures without overloading the compressor.

Energy Management Module (Fig. 16) — This factory-installed option or field-installed accessory is used for the following types of temperature reset, demand limit, and/or ice features:

•4 to 20 mA leaving fluid temperature reset (requires field-supplied 4 to 20 mA generator)

•4 to 20 mA cooling set point reset (requires field- supplied 4 to 20 mA generator)

•Discrete inputs for 2-step demand limit (requires field- supplied dry contacts capable of handling a 5 vdc, 1 to 20 mA load)

•4 to 20 mA demand limit (requires field-supplied 4 to 20 mA generator)

•Discrete input for Ice Done switch (requires field- supplied dry contacts capable of handling a 5 vdc, 1 to 20 mA load)

See Demand Limit and Temperature Reset sections on pages 46 and 43 for further details.

Capacity Control — The control system cycles com- pressors, unloaders, and hot gas bypass solenoids to maintain the user-configured leaving chilled fluid temperature set point. Entering fluid temperature is used by the Main Base Board (MBB) to determine the temperature drop across the cooler and is used in determining the optimum time to add or subtract ca- pacity stages. The chilled fluid temperature set point can be au- tomatically reset by the return temperature reset or space and outdoor-air temperature reset features. It can also be reset from

Fig. 15 — Electronic Expansion Valve (EXV)

an external 4 to 20 mA signal (requires Energy Management Module FIOP/accessory).

With the automatic lead-lag feature in the unit, the control determines which circuit will start first, A or B. At the first call for cooling, the lead compressor crankcase heater will be deen- ergized, a condenser fan will start, and the compressor will start unloaded.

NOTE: The automatic lead-lag feature is only operative when an even number of unloaders is present. The 040-070 units require an accessory unloader to be installed on the B1 com- pressor for the lead-lag feature to be in effect.

If the circuit has been off for 15 minutes, and the unit is a TXV unit, liquid line solenoid will remain closed during start- up of each circuit for 15 seconds while the cooler and suction lines are purged of any liquid refrigerant. For units with EXVs, the lead compressor will be signaled to start. The EXV will re- main at minimum position for 10 seconds before it is allowed to modulate.

After the purge period, the EXV will begin to meter the re- frigerant, or the liquid line solenoid will open allowing the TXV to meter the refrigerant to the cooler. If the off-time is less than 15 minutes, the EXV will be opened as soon as the com- pressor starts.

The EXVs will open gradually to provide a controlled start- up to prevent liquid flood-back to the compressor. During start- up, the oil pressure switch is bypassed for 2 minutes to allow for the transient changes during start-up. As additional stages of compression are required, the processor control will add them. See Tables 5A and 5B.

If a circuit is to be stopped, the control will first start to close the EXV or close the liquid line solenoid valve.

For units with TXVs, the lag compressor(s) will be shut down and the lead compressor will continue to operate for 10 seconds to purge the cooler of any refrigerant.

For units with EXVs, the lag compressor(s) will be shut down and the lead compressor will continue to run. After the lag compressor(s) has shut down, the EXV is signaled to close. The lead compressor will remain on for 10 seconds after the EXV is closed.

During both algorithms (TXV and EXV), all diagnostic conditions will be honored. If a safety trip or alarm condition is detected before pumpdown is complete, the circuit will be shut down.

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