Installation Electrical
RTHD-SVX02H-EN 73
Soft Loading
Soft loading will prevent the chiller from going to full
capacity during the pulldown period.
The UC800 control system has two soft loading algo-
rithms running all of the time. They are capacity con-
trol soft loading and current limit soft loading. These
algorithms introduce the use of a Filtered Chilled Water
Setpoint and a Filtered Current Limit Setpoint. After the
compressor has been started, the starting point of the
filtered chilled water setpoint is initialized to the value of
the Evap Leaving Water Temperature. The filtered current
limit setpoint is initialized to the value of the Current
Limit Softload Starting Percent. These filtered setpoints
allow for a stable pull-down that is user adjustable in
duration.
They also eliminate sudden transients due to setpoint
changes during normal chiller operation.
Three settings are used to describe the behavior of soft
loading. The setup for softloading can be done using TU.
• Capacity Control Softload Time: This setting controls
the time constant of the Filtered Chilled Water Set-
point. It is settable between 0 and 120 min.
• Current Limit Control Softload Time: This Setting con-
trols the time constant of the Filtered Current Limit
Setpoint. It is settable between 0 and 120 minutes.
• Current Limit Softload Starting Percent: This setting
controls the starting point of the Filtered Current
Limit Setpoint. It is adjustable from 40 to 100 percent
RLA.
External Base Loading - Optional
Primarily for process control requirements, base loading
provides for immediate start and loading of a chiller up
to an externally or remotely adjustable current limit set-
point without regard to differential to start or stop, or to
leaving water temperature control. This allows the flex-
ibility to prestart or preload a chiller in anticipation of a
large load application. It also allows you to keep a chiller
on line between processes when leaving water tempera-
ture control would normally cycle the unit.
When the base loading option is installed through Trac-
er TU it will be controllable through TD7/TU, External
Hardware Interface or Tracer (if Tracer is installed). Order
for precedence for all setpoints, TD7/TU then External
then Tracer from lowest to highest priority. If one of the
higher priority setpoints drops out due to a bad sensor
or communication loss then base loading shall go to
the next lowest priority of command and setpoint. The
command settings and control setpoints associated with
base loading are explained below.
Base Loading Control setpoint
This setpoint has three possible sources, an External
Analog Input, TD7/TU or Tracer.
• TD7/TU Base Loading Control Setpoint
The range is 40 - 100 % Compressor Load (Max
%RLA). The default is 50%.
• Tracer Base Loading Control Setpoint
The range is 40 - 100 % Compressor Load (Max
%RLA). The default is 50%.
• External Base Loading Setpoint
This is an Analog Input that sets the base loading set-
point. This signal can be controlled by either a 2-10Vdc
or 4-20ma Signal based on configuration informa-
tion. The equations show the relationship between
input and percent compressor load:
If the input is configured as a 4 - 20 mA:
% Load = 3.75 * (mA Input) + 25
If the input is configured as a 2 - 10 Vdc:
% Load = 7.5 * (Vdc Input) + 25
Summit Interface - Optional
UC800 provides an optional interface between the chiller
and a Trane Summit BAS. A Communications interface
LLID shall be used to provide “gateway” functionality
between the Chiller and Summit.
LonTalk Communication Interface - Optional
UC800 provides an optional LonTalk Communication
Interface (LCI-C) between the chiller and a Building
Automation System (BAS). An LCI-C LLID shall be used
to provide “gateway” functionality between the LonTalk
protocol and the Chiller.
External Chilled Water Setpoint - Optional
UC800 will accept either a 2-10 VDC or a 4-20 mA input
(J9-4, J9-5) signal, to adjust the chilled water setpoint
from a remote location.
External Current Limit Setpoint - Optional
UC800 will accept either a 2-10VDC or a 4-20mA input
(J7-11, J7-12) signal to adjust the current limit setpoint
from a remote location.
Percent Condenser Pressure Output - Optional
UC800 provides a 2-10 VDC analog output to indicate
percent High Pressure Cutout (HPC) condenser pressure.
Percent HPC = (Condenser Pressure/High Pressure Cut-
out Setpoint)*100
Compressor Percent RLA Output - Optional
UC800 provides a 0-10 Vdc analog output to indicate %
RLA of compressor starter average phase current. 2 to 10
Vdc corresponds to 0 to 120% RLA.
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