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Fixed Speed Compressor Operation
4-Compressor/4-Stage Unit
In this configuration there are four equally sized compressors
and two cooling circuits. In the following description,
compressor #1 is lead. However, if Auto Lead/Lag Staging is
selected on the keypad, the lead compressor is the one in the
lead circuit with the least number of run hours.
When the unit disconnect is closed, 115 V (ac) power is
supplied directly from control transformer T1 to the compressor
crankcase heaters, and motor protectors (MMP).
Cross Load
During a call for mechanical cooling, if HP1 is closed, then
DO1 on expansion module C closes, energizing the M1
compressor contactor. The M1 auxiliary brings on required
condenser fans, liquid line solenoid valve and de-energizes the
crankcase heater.
The second stage of cooling is controlled by DO1 on expansion
module D. Compressor 2 is on circuit 2 and is brought on in
the same manner as compressor #1, as well as the condenser
fans, solenoid valve and crankcase heater on the circuit.
The 3rd stage of cooling is controlled by DO2 on expansion
module C and brings on compressor 3.
The 4th stage of cooling is controlled by DO2 on expansion
module D and brings on compressor 4.
Lead Load
The loading and unloading process is similar except that both
compressors in the lead cooling circuit 1 energize before
energizing any compressors in lag circuit 2.
Phase Voltage Monitor
The phase voltage monitor protects against high voltage,
phase imbalance, and phase loss (single phasing) when any
one of three line voltages drops to 74% or less of setting. This
device also protects against phase reversal when improper
phase sequence is applied to equipment, and low voltage
(brownout) when all three line voltages drop to 90% or less of
setting. An indicator run light is ON when all phase voltages are
within specified limits. The phase voltage monitor is located on
the load side of the power block with a set of contacts wired to
the 115-volt control circuit to shut the unit down whenever the
phase voltages are outside the specified limits.
IM 1287 • REBEL APPLIED ROOFTOP
External Time Clock
You can use an external time clock as an alternative to (or in
addition to) the MicroTech 4 controller's internal scheduling
function. The external timing mechanism is set up to open
and close the circuit between field terminals 201 and 202.
When the circuit is open, power is not supplied to digital input
MCB- DI3. This is the normal condition where the controller
follows the programmable internal schedule. When the circuit
is closed, power is fed to DI3, the MicroTech 4 controller
responds by placing the unit in the occupied mode, over riding
any set internal schedule
VAV Box Signal/Fan Operation Signal
Digital Output #10 (MCB-DO10) may be selected as either the
Fan Operation output or the VAV output via the keypad. The
VAV/Fan Pop selection can be selected by accessing the Unit
Setup menu in the Extended Menu section. See
Wiring" on page 31
for details.
Fan Operation
The Fan Operation output is ON when the unit is not Off and
when both the unit is OFF and airflow is detected. It is off when
the unit is off and airflow is not detected.
VAV Box Output
In the Heating state, the VAV Output is turned OFF to indicate
that hot air instead of the normal cool air is being supplied to
the VAV boxes. The VAV boxes are driven to their Heating
Position when hot air is provided based on either the normally
open or normally closed contacts of the VAV output. The VFD
will continue to be controlled to maintain the desired duct
static pressure. This output is also OFF when the unit is in
the Startup or Recirculation states. If this output is in the Heat
(OFF) position when the unit enters the Fan Only state or
Minimum DAT Control state, the output remains OFF for an
adjustable Post Heat Time (while the unit VFDs are driven to
minimum speed) or until the VFD gets to its minimum speed if
the Post Heat Time is set greater than 0. The Post Heat Timer
can be adjusted via the keypad/display Timer Setting menu in
the Extended Menus.
During unoccupied operation, the VAV Box Output is in the
Cool (ON) position unless airflow is detected. When airflow is
detected, it switches to the Heat (OFF) position.
38
Unit Operation
"Field Power
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