nvent 910 Series Installation, Operating And Maintenance Instructions page 52

52 | nVent.com
7.2.3 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, then 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, then 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.
7.2.4 PROPORTIONAL AMBIENT CONTACTOR CONTROL (FOR USE WITH
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 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, then power is applied to the trace with a duty cycle of 100%.
The controller output will be fully on for 1/30 of the CYCLE TIME setting before the duty
cycle is calculated again.
• If the temperature sensed by the control sensor is equal or greater than the CONTROL
SETPOINT temperature, then the output will have a duty cycle of 0%. The controller output
will be off for 1/30 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. Also note that 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.
Duty Cycle
0%
50%
100%
Duty Cycle
0%
50%
100%