Section 7-Control Modes; Introduction; Switch Control Modes - nvent 910 Series Installation, Operating And Maintenance Instructions

SECTION 7—CONTROL MODES

7.1 INTRODUCTION

7.2 SWITCH CONTROL MODES

There are several different types of control modes in the controller. Some of these modes
require further explanation in order to fully understand and implement their operation.
This section describes the control modes available in the HTC and how to set their associated
parameters.
There are four different SWITCH CONTROL modes associated with the HTC. The following is
an explanation of their implementation in the controller and the differences between them.
7.2.1 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, then 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 PROPORTIONAL BAND setting, then 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.
Proportional Control Temperature Band
Control Sensor Temperature
Setpoint + proportional band
Setpoint + proportional band/2
Setpoint
7.2.2 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 since this would cycle the contactor too quickly. In these
situations, 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 may 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 mode. If the control sensor
temperature is above the CONTROL SETPOINT temperature by more than the DEADBAND
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 very simple control algorithm but it works very effectively in heat trace applications
where the temperature of a traced system changes relatively slowly.
Deadband Control Temperature Band
Control Sensor Temperature
Setpoint + deadband
Setpoint
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.
Duty Cycle
0%
50%
100%
Output State
Off
On
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