Pentair 920 Series Installation, Operating And Maintenance Instructions page 50

INDUSTRIAL HEAT TRACING SOLUTIONS
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.
• If the temperature sensed by the control sensor is equal to or greater than the CONTROL
SETPOINT 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.
proportional ambient ssr Control temperature band
Control sensor temperature
Setpoint
Setpoint - proportional band/2
Setpoint - proportional band
Important: 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
temperature 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 re-mainder of the CYCLE TIME.
• The duty cycle is calculated each time the output toggles, based on the ambient temperature,
PROPORTIONAL BAND setting, and the CON-TROL 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
SETPOINT 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.
proportional ambient Contactor Control temperature band
Control sensor temperature
Setpoint
Setpoint - proportional band/2
Setpoint - proportional band
Important: The load shedding fail-safe mode is not supported when using proportional ambient
contactor 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.
EN- Raychem920series-IM-H56874 05/15
duty Cycle
0%
50%
100%
duty Cycle
0%
50%
100%
50 / 82