Control Modes
CM-2201/CM-2202
©2018 Nelson Heat Tracing Systems
www.nelsonheaters.com
GA2497 Rev.6
8
6.0
Control Modes
The CM-2201/CM-2202 allows the user to select different
control modes based on their individual process control
parameters.
6.1
On-Off Control
This control method simply energizes the cable until the
actual monitored temperature rises to the setpoint value
plus half the deadband value (upper limit). The cable is then
de-energized until the actual monitored temperature drops to the
setpoint value minus half the deadband value. Note that this
type of control can result in some temperature “overshoot;”
this is because the cable is de-energized when the monitored
temperature reaches the upper limit. However, the residual heat
in the cable continues to transfer to the pipe, and this will cause
the pipe temperature to increase slightly above the upper limit.
Similarly, there can be some temperature “undershoot.”
6.2
Proportional Control
This control method uses the typical proportional
control algorithm wherein the cable is cycled on and off at a
rate proportional to the difference between the setpoint value
and the actual monitored temperature. As the difference
between the setpoint value and the actual
monitored temperature increases, the amount of time the
cable is energized increases proportionately. This helps
reduce the “overshoot” and “undershoot” commonly
associated with On/Off control.
6.4 Forced Control Feature
This control method simply allows the user to force the cable on or
off as desired using an external signal (e.g. +5 VDC or 24 VAC)
applied to the IN and G terminals or an external dry contact across
the +5V and IN terminals. The External Disable must be set to
“ON” to allow for external control, at which point the controller
operates as follows:
A)
If the Temperature setpoint is “OFF” or “None”, then the
heater gets energized by applying the external signal or closing the
external contact.
B)
If the Temperature setpoint is a specific value (ex. 55C),
then the setpoint will be maintained as per normal operation ONLY
when the external signal is present, or the external contact is
closed – otherwise, the heater will be disabled.
