Dunham-Bush HRSC - D, Operating & Maintenance Instructions

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Stage 2 Loading
Subsequent to the start of Compressor No.1 an approximate
five minute time delay is incorporated into the microcom-
puter. After this time delay, and a further call for cooling, the
second step of the staging thermostat (T1) will close. This
will energize relay (R2) and time delay (TD2). If all the safety
controls and switches are closed, (see “Safety Control’ sec-
tion for more details) the control circuit for compressor No.2
will energize, allowing the compressor to start. Power is sup-
plied to condenser fan interlock terminal CT20 and CT2 to
enable the remote condenser fan circuit. Relay (R6) will ener-
gize, allowing the liquid line solenoid valve (SOL2) to ener-
gize. Relay (R6) will energize the safety controls the same as
compressor No.1 circuit.
Time delay (TD2) will provide a 30 second jumper around
the low pressure control (LP2) and will energize the full un-
load solenoid (UL3-2) for 30 seconds then energize relay (R4).
Upon energizing relay (R6), the crankcase heater (CCH2) is
de-energized and the normally open contacts of relay (R4)
are closed, allowing the thermostat (T1) to energize the load
solenoid (UL2-1) and the unload solenoid (UL2-2) upon de-
mand. Relays (R1) and (R2) will interlock the water pump
starters.
SHUTDOWN SEQUENCE
Stage 2 Shutdown
After both compressors have unloaded fully and the leaving
water temperature is still lower than the set-point, the sec-
ond step of the thermostat (T1) will open and de-energize
relay (R2), providing an approximate five minute time delay,
again integral with the microcomputer to the start of Com-
pressor No.2. This will close the liquid line solenoid (SOL2)
condenser fan control contactor and turn off Compressor
No.2. Compressor No.1 will load and unload as required.
Compressor No. 2 will not be able to restart for five minutes
due to integral time delays.
Stage 1 Shutdown
If the leaving water temperature is still lower than the set-
point, the first step of the thermostat (T1) will open and de-
energize relay (R1). This will close the liquid line solenoid
(SOL1) condenser fan control contactor and turn off Com-
pressor No.1 as long as it has run for approximately three
minutes. Compressor No.1 will not be able to restart for five
minutes.
STAR-DELTA STARTING
OPEN TRANSITION
Time delay relay (TD1) closes, energizing start contactor (S1)
and the five second transition timer (TD9). Contactor (S1)
closes, tying the center legs of the motor windings together
into the “Star” (wye) configuration. (S1) N.O. auxiliary con-
tacts close and energize motor starter (C1), starting the com-
pressor motor. This motor starter is then “held in” by (C1)
N.O. auxiliary contacts.
After five seconds (TD9) N.O. contacts close, de-energizing
“Star’ contactor (S1) and energizing run contactor C2 through
(S1) N.O. auxiliary contacts. The compressor motor now runs
in the “Delta” configuration.
There is an instant of time (the “open transition”) between
the opening of (S1) power contacts and the closing of (C2)
power contacts, in which power across the motor windings
is interrupted.
MICROCOMPUTER CONTROLLER
1. General Description
Your Dunham-Bush HRSC is controlled by an NC25 micro-
computer system. The computer system is composed of a
main microcomputer board, several types of input/output (I/
O) boards, and sensors. The I/O boards are connected to the
microcomputer by ribbon cables. These hardware compo-
nents are controlled by the software program in the micro-
computer. The software determines the state of the output
relays based on the input values.
There are two types of inputs to the computer system. A
digital input indicates whether or not voltage is present at
the input. This could be used to determine whether or not a
contactor is pulled in, a water flow switch is made, or the
status of other on/off devices. Another type of input is an
analog input. An analog value is one that varies continu-
ously, such as temperature, pressure, current, etc. An ana-
log input must be converted to a digital value with an ana-
log to digital (A/D) converter board before the computer can
process the data.
The computer system outputs are relays that switch the 115
VAC control devices such as contactors and solenoids.
The software for the computer is stored in three different
types of computer memory chips. The two EPROM chips
(Electrically Programmable Read Only Memory) store gen-
eral operating procedures (algorithms) and are the same for
all packages. The EAROM chip (Electrically Alterable Read
Only Memory) stores the program that defines the logic for a
particular unit. This software is called the data base. Both of
these chips retain their content when power is removed. The
third type of memory is battery backed RAM (Random Ac-
cess Memory) and is used for temporary storage.
Information can be retrieved from the NC25 and displayed
on a video display terminal. The RS232 communications port
is used. Refer to FORM 6372 for more information.
2. Microcomputer Hardware Precautions
The following precautions must be taken while work-
ing with the microcomputer system.
2.1 Do not lay the solder side of a computer board on a
metallic surface. This may damage the battery on
the board.
2.2 Cover the computer components completely when
drilling sheet metal near the computer.
2.3 Strip wires away from microcomputer to prevent
strands from falling on components.
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