21
Procedure: Using the 920 Operator Console, enter the desired text. Refer to the separate
DigiTrace 920 Series HTC Operator Console—Installation and Operating Instructions (Tyco
Thermal Controls reference H56903) for
TAG entry information.
3.5.3 SWITCH CONTROL MODE
Purpose: This allows selection of the type of algorithm to be used by the
HTC to maintain the
CONTROL SETPOINT temperature. There are four different control algorithms available in the
HTC—proportional, proportional ambient SSR, proportional ambient contactor, and deadband.
See section 5.2 on page 50 for a complete explanation of these controlling techniques as imple-
mented in the HTC.
Setting:
PROPORTIONAL, PROPORTIONAL AMBIENT SSR (V3.11+), PROPORTIONAL AMBIENT CON-
TACTOR
(V3.11+), or DEADBAND
Procedure: Select the desired control technique. Note that deadband control and proportional
ambient contactor should be selected when using contactors or when precise control and
advanced current handling functions are not required.
Note: If deadband is selected, a
DEADBAND setting will be available in the HTC configuration
menu, else a
PROPORTIONAL BAND setting will be available. No MAXIMUM POWER, SWITCH CUR-
RENT RATING
or CIRCUIT BREAKER CURRENT RATING settings are available when the controller is
set to operate in either contactor mode. If proportional ambient contactor is selected, the
CYCLE
TIME
setting will also be available.
3.5.4 PROPORTIONAL BAND SETTING
(For use with the three proportional control modes only)
Purpose: When an
HTC equipped with SSRs is used to control a heating circuit, proportional or
proportional ambient
SSR modes are normally used, allowing for more precise temperature con-
trol. When using contactors, the proportional ambient contactor mode should be selected.
This programmable proportional band acts to vary the on to off time of the output based on the
difference between the measured control temperature and the desired
CONTROL SETPOINT
temperature.
Range: 2°F to 90°F (1°C to 50°C)
2°F to 630°F (1°C to 350°C) V3.2x and up
Procedure: Adjust the
PROPORTIONAL BAND setting to the desired differential from the CONTROL
SETPOINT
temperature.
Notes:
• See section 5.2 on page 50 for an explanation of how the three proportional modes use the
PROPORTIONAL BAND setting.
• When using series-type, constant wattage, or self-regulating tracers in an ambient tempera-
ture control application, significant energy savings may be realized by setting the
PROPOR-
TIONAL BAND
to match the expected range of operating ambient temperatures. Tracer design
is normally done assuming worst-case conditions, where 100% of the design output power
is required to maintain the desired minimum temperature. When the ambient temperature is
above the design minimum but some heat is still required, adjusting the
PROPORTIONAL BAND
width accordingly will allow only the amount of power required by the application to be con-
sumed, while maintaining the minimum required temperature.
Example: A water line must be protected from freezing when the ambient temperature falls
below 10°C. Either the proportional ambient
SSR or proportional ambient contactor mode is
selected as the control method (depending on the type of switch being used). The heater and
insulation are chosen to impart enough heat to the line to keep it from freezing at a worst-case
ambient temperature of –40°C. At 10°C, the heater should be completely off, since no heat is
required at this temperature to guarantee that the product will not freeze. It follows that the
amount of heat required by the water line decreases as the ambient temperature increases from
–40°C to 10°C (theoretically, at –15°C the heater output should be approximately 50%). Setting
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