52
• If the temperature sensed by the control sensor is equal to or greater than the CONTROL SET-
POINT
temperature, the output will have a duty cycle of 0%—the controller output will be off.
• The temperature of the control sensor is constantly monitored and the output duty cycle is
adjusted proportionally according to where the temperature falls within the 0%–100% band.
Note: The load shedding fail-safe mode is not supported when using proportional ambient
SSR
control, since ambient temperature is being monitored rather than pipe temperature.
5.2.4 PROPORTIONAL AMBIENT CONTACTOR CONTROL (FOR USE WITH EXTERNAL
CONTACTORS)
When an
HTC using a contactor is used to control the output based on the ambient temperature
this control mode should be used.
Proportional ambient contactor control on the
HTC is implemented as follows:
• The output may not be switched on/off rapidly when using a contactor, so proportional tem-
perature control is implemented by applying the required duty cycle over the selected
CYCLE
TIME
.
• The output is fully on for a portion of the
CYCLE TIME as determined by the calculated duty
cycle, and it will be completely off for the remainder of the
CYCLE TIME.
• The duty cycle is calculated each time the output toggles, based on the ambient temperature,
PROPORTIONAL BAND setting, and the CONTROL SETPOINT temperature setting.
• The controller monitors the ambient temperature and compares it to the
CONTROL SETPOINT
temperature as in proportional ambient
SSR control. If the temperature of the control sensor
is at or below the
CONTROL SETPOINT temperature minus the PROPORTIONAL BAND setting,
power is applied to the trace with a duty cycle of 100%. The controller output will be fully on
for 1/30th of the
CYCLE TIME setting before the duty cycle is calculated again.
• If the temperature sensed by the control sensor is equal to or greater than the
CONTROL SET-
POINT
temperature, the output will have a duty cycle of 0%. The controller output will be off
for 1/30th of the
CYCLE TIME setting before the duty cycle is calculated again.
Note: The load shedding fail-safe mode is not supported when using proportional ambient con-
tactor control, since ambient temperature is being monitored rather than pipe temperature.
If an AC alarm with an alarm filter time greater than 0 is detected, the contactor will not toggle
until the alarm filter time has expired.
5.3 Load Shedding Control Mode
Load shedding is a control mode that can be programmed and initiated only by an external com-
municating device, or by the Model 780/GCC-9000 Group Communications Controller, which
overrides temperature control and forces the output of the controller OFF until reset by the 780/
GCC. When using a
GCC, load shedding is initiated by a contact closure (or opening) on one of
the four contact inputs. Each contact input initiates a load shedding command for the group of
Proportional Ambient SSR Control Temperature Band
Control Sensor Temperature Duty Cycle
Setpoint 0%
Setpoint - proportional band/2 50%
Setpoint - proportional band 100%
Proportional Ambient Contactor Control Temperature Band
Control Sensor Temperature Duty Cycle
Setpoint 0%
Setpoint - proportional band/2 50%
Setpoint - proportional band 100%
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