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5.2.1 PROPORTIONAL CONTROL (FOR USE WITH SSRS ONLY)
Proportional control on the HTC is implemented as follows:
When using
SSRs to directly control the power applied to a trace circuit, the output may be
switched on/off very rapidly. The controller implements proportional temperature control on a
cycle-by-cycle basis (50 or 60 Hz power line cycle). This algorithm monitors the temperature of
the heating circuit and compares it to the
CONTROL SETPOINT temperature. If the temperature of
the control sensor is at or below the
CONTROL SETPOINT temperature, power is applied to the
trace with a duty cycle of 100%—the controller output is full on. If the temperature sensed by
the control sensor is equal to or greater than the
CONTROL SETPOINT temperature + the PROPOR-
TIONAL BAND
setting, the controller output will have a duty cycle of 0%—the 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.
5.2.2 DEADBAND CONTROL (FOR USE WITH EXTERNAL CONTACTORS)
Deadband control on the
HTC is implemented as follows:
When using the HTC in an application where the controller is used to open and close a contactor,
proportional control cannot be used. In these cases a deadband control algorithm is used. The
output duty cycle is not controlled; instead, the output is either fully on or completely off. The
user can set the
DEADBAND value. The controller monitors the temperature of the trace circuit
and compares it to the
CONTROL SETPOINT temperature as in the proportional control. If the con-
trol sensor temperature is above the
CONTROL SETPOINT temperature by more than the DEAD-
BAND
value, the output is turned off. If the control sensor temperature falls below the CONTROL
SETPOINT
temperature the output is turned on. This is a simple control algorithm but it works
very effectively in heat trace applications where the temperature of a traced system changes rel-
atively slowly.
When the control sensor temperature is within the deadband, the output does not change its
state. Also, when using deadband control a contactor is not allowed to toggle faster than every
two seconds. 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.
5.2.3 PROPORTIONAL AMBIENT SSR CONTROL (FOR USE WITH SSRS ONLY)
When an
HTC using an SSR is used to control the output using the ambient temperature, this
control mode should be used.
Proportional ambient
SSR control on the HTC is implemented as follows:
• When using
SSRs to directly control the power applied to a heating circuit, the output may be
switched on/off very rapidly. The controller implements proportional temperature control on
a cycle-by-cycle basis (50 or 60 Hz power line cycle).
• This algorithm monitors ambient temperature and compares it to the
CONTROL SETPOINT
temperature. 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 is fully on.
Proportional Control Temperature Band
Control Sensor Temperature Duty Cycle
Setpoint + proportional band 0%
Setpoint + proportional band/2 50%
Setpoint 100%
Deadband Control Temperature Band
Control Sensor Temperature Output State
Setpoint + deadband Off
Setpoint On
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