Principles of Operation of
the Module UCM-CLD
9
RLC-SVU02F-E4
Operational features
Entering evaporator water
temperature
When one or both compressors are
running, the UCM continually
monitors and compares the
entering and leaving evaporator-
water temperatures. If the
temperature of the entering water
drops more than 1°C below the
leaving water temperature for more
than
55°C- seconds, the UCM uses this to
indicate a loss of water flow
through the evaporator. This will
shut down that circuit's compressor
and will display MMR diagnostic.
Current Limit Setpoint (CLS)
The current limit setpoints for the
system are entered through the
Clear Language Display menus. The
current limit Setpoint for each
compressor is shown in Table 1.
Table 1 – Compressor current limit
Setpoint versus Chiller current
limit Setpoint
System
CLS
One
Two
120%
120
120
100%
120
100
80%
120
80
60%
120
60
40%
80
40
Based upon current levels received
at the UCM, the compressor slide
valve is modulated to prevent the
actual chiller current from
exceeding the CLS.
When a compressor is turned off,
the CLS for the remaining running
compressor shall be reset upward
immediately. When a compressor is
added, the CLS for the running
compressor shall be ramped
downward at a rate not less than
10% RLA per minute to the new
Setpoint.
Numbers of compressor in
operation
Electronic expansion valve
(EXV) test
This test can be performed only when
the “Stop“ key has been pressed. It
will confirm proper operation of the
electronic expansion valve and the
EXV module.
After the test has been initiated at
the Clear Language Display, the
UCM will:
1. Overdrive the EXV closed
(25 seconds)
2. Overdrive the EXV open
(25 seconds)
3. Overdrive the EXV closed
(25 seconds)
4. Reset the display to disable and
end the test
The EXV produces an audible
clicking sound when it is driven
against its end stops. Step 1 drives
the EXV to its closed position,
during which time service personnel
can move from the CLD to the EXV.
Note: A tool may be needed to aid
in hearing the clicking of the EXV,
such as a screwdriver held between
the EXV and the ear.
Current overload protection
The UCM continually monitors
compressor current to provide unit
protection in the event of an
overcurrent or locked-rotor
condition. Protection is based on
the phase with the highest current
and, if limits are exceeded, the UCM
will shut down the compressor and
will display an MMR diagnostic.
Leaving chilled-water temperature
control
If the Auto key is pressed and a
remote chilled-water Setpoint has
been communicated, the UCM will
control to that Setpoint. Otherwise,
it will control to the front-panel
Setpoint. Control is accomplished
both by staging compressors and
by modulating the slide valves on
each compressor.
Upon start-up, if the leaving chilled-
water temperature is dropping 0.8°C
per minute or faster, the chiller will
not load further.
