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JOHNSON CONTROLS

104

FORM 150.62-NM6 (103)

SYSTEM LEAD/LAG

Lead/lag between systems may be selected to help
equalize average run hours between systems on chill-
ers with 2 refrigerant systems. This may be programmed
under the OPTIONS key. Auto lead/lag allows automatic
lead/lag of the two systems based on average run hours
of the compressors in each system. Manual lead/lag

selects specifically the sequence in which the micro

starts systems.

COMPRESSOR LEAD/LAG

The compressors within a system rotate starts in se-
quence 1, 2 or 1, 2, 3 with wraparound. The longest-off

compressor in a system will start first, and the longest-

running compressor in a system will turn off first. When

unloading, the system with the most compressors on,

unloads first. The lag system will shut down a com

pressor first when equal numbers of compressors are

operating in each system. The micro will not attempt to
equalize run time of compressors in a system.

Once the second system has started a compressor, the
micro will attempt to equally load each system. Once this
occurs, loading will alternate between systems.

If Soft Start is enabled on European models with this
option, compressor lead/lag will function as outlined in
Option 12 under the Options key.

ANTI-RECYCLE TIMER

The programmable anti-recycle timer assures that sys-
tems do not cycle. This timer is programmable under the
PROGRAM key between 300 - 600 seconds. Whenever
possible, to reduce cycling and motor heating, the anti-
recycle timer should be adjusted to 600 seconds. The
programmable anti-recycle timer starts the timer when

the first compressor in a system starts. The timer begins

to count down. If all of the compressors in a circuit cycle
off, a compressor within the circuit will not be permitted to
start until the anti-recycle timer has timed out. If the lead
system has run for less than 5 minutes, 3 times in a row,
the anti-recycle timer will be extended to 10 minutes.

ANTI-COINCIDENCE TIMER

This timer is not present on single-system units. Two
timing controls are present in software to assure com-
pressors within a circuit or between systems, do not start
simultaneously. The anti-coincidence timer assures there
is at least a one minute delay between system starts on
2-circuit systems. This timer is NOT programmable. The
load timers further assure that there is a minimum time
between compressor starts within a system.

EvAPORATOR PUMP CONTROL

The evaporator pump dry contacts (CTb2 – terminals
23 - 24) are energized when any of the following con-
ditions are true:

Unit Controls

Notes:

1. Step 1 is Hot Gas bypass and is skipped when loading occurs. Hot Gas bypass operation is inhibited during Pumpdown.

2. Step 3 is skipped when loading occurs.

3. Step 4 is skipped when unloading occurs.

TAbLE 26 –

RETURN CHILLED LIqUID CONTROL FOR 4 COMPRESSORS (6 STEPS)

*STEP

COMPRESSOR

COMPRESSOR ON POINT