Compressor Operation

Carrier 50FK, JK034-074, 48FK specifications Compressor Operation

P1 SUPPLY AIR

TWO-DIGIT

DISPLAY

SET POINT

DISPLAY

BUTTON

POTENTIOMETER

Fig. 6 Ð Display/Set Point Board

CONTROL RELAY (CR) Ð This relay provides informa- tion to the processor about compressor operation (one con- trol relay per compressor). The relay controls and protects the compressor and also controls the crankcase heater.

A control signal to check the safety statuses and to start the compressor is sent from the relay board. This signal trav- els through all of the safeties: the high-pressure switch, and the internal protector (where used) and on to the control re- lay coil. Once the control relay coil has been energized, the control relay completes a feedback circuit for the processor, informs the processor of the status of the compressor safe- ties, energizes the compressor contactor coil, and deener- gizes the crankcase heaters. A fault will be detected by the processor if the control relay opens during operation or start- up. The processor will lock the compressor or the circuit off by deenergizing the appropriate relay(s) on the relay board and energizing an alarm signal.

Accessory Board Ð The accessory board is standard (factory supplied) in the VAV rooftop units. See Fig. 15. This board is located in the control box of each unit. Each board has a prewired connector supplied with it to connect directly to the processor board. It has 3 potentiometers: P3, P5, and P6.

P3 Ð RESET LIMIT Ð The processor board is programmed for occupied space temperature reset. In order for reset to work, the accessory temperature reset board must be used. Potentiometer P3 is the maximum set point temperature to which the supply air can be reset.

P5 Ð ECONOMIZER MINIMUM POSITION Ð This po- tentiometer controls the set point for the minimum position of the economizer.

P6 Ð MORNING WARM-UP TEMPERATURE Ð This po- tentiometer controls the morning warm-up temperature set point.

Single-Step Demand Limit Ð The single step de- mand limit provides a means to limit the capacity of the VAV unit using an external switch. Single step demand limit will limit the compressor displacement based on the ratio of the wiper arm to the full scale resistance. The exact percentage of capacity reduction differs depending on the number of ca- pacity steps.

A 3-wire, 5 to 20 Kohm, ®eld-supplied potentiometer (P4) is required for this option. The potentiometer should be wired to the processor J3 connections. In order to control the de- mand limit, the wiper arm of the potentiometer should be switched open and closed based on the demand limit requirement. The control switch is also ®eld-supplied and installed.

If the wiper arm wire is open, all capacity stages can be used. When the wiper arm wire is closed, the capacity is reduced by the amount set on potentiometer P4.

Demand Limit Control Module (DLCM) Ð The DLCM provides a 2-step demand limit control using an ex- ternal switch. The ®rst step is between 50% and 100% of the maximum compressor displacement. See Fig. 16. The sec- ond step is between 0% and 49% of the maximum compres- sor displacement. The exact percentage differs depending on the number of capacity steps.

Two adjustable potentiometers are used to set the 2 de- mand limit points. Potentiometer P1 is used to set a demand limit between 50% and 100% of the unit capacity. Potenti- ometer P2 is used to set a demand limit between 0% and 49% of unit capacity.

If no power is supplied to the demand limit control mod- ule, all capacity stages can be used. When power is supplied to terminal IN1 only, the ®rst step of the demand limit con- trol is energized and the capacity is reduced by the amount set on potentiometer P1. When power is supplied to terminal IN2 only, or to both IN1 and IN2, the capacity is reduced by the amount set on potentiometer P2.