Carrier JK034-074 Air Conditioner User Manual


 
Compressor Operation
CONTROL RELAY (CR) 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 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 — RESET LIMIT — 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 — ECONOMIZER MINIMUM POSITION — This po-
tentiometer controls the set point for the minimum position
of the economizer.
P6 — MORNING WARM-UP TEMPERATURE — This po-
tentiometer controls the morning warm-up temperature set
point.
Single-Step Demand Limit 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.
A3-wire, 5 to 20 Kohm, field-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 field-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) The
DLCM provides a 2-step demand limit control using an ex-
ternal switch. The first 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 first 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.
P1 SUPPLY AIR
SET POINT
POTENTIOMETER
TWO-DIGIT
DISPLAY
DISPLAY
BUTTON
Fig. 6 Display/Set Point Board
7