Appendix C: Heater Break Option - Fuji Electric PXV3 SERIES Operation Manual

Add Derivative Time
Do not add Derivative Time if the system is too dynamic. Start with a
small Derivative Time value which gives sluggish response to process
upsets and double the value. Analyze the process variable. If the
response to process upsets is still sluggish, double the value again.
Continue with the same procedure until the process starts to oscillate
at a quick constant rate. Decrease the Derivative Time value by 25%.
From a cold start, test and verify that the Derivative Time value allows
maximum response to process disturbances with minimum overshoot.
If not completely satisfied, fine-tune the value, up or down, as needed
and test until correct. Note that the Derivative Time value is usually
somewhere around 25% of the Integral Time value. The derivative
Time is now tuned.
Another tuning method is the closed-loop cycling or Zeigler-Nichols
method. According to J.G. Zeigler and N.B. Nichols, optimal tuning is
achieved when the controller responds to a difference between set-
point and the process variable with a 1/4 wave decay ratio. That is to
say that the amplitude of each successive overshoot is reduced by 3/4
until stabilizing at setpoint. The procedure is explained below.
1. Integral Time=0
Derivative Time=0
2. Decrease the Proportional Band to the point where a constant rate
of oscillation is obtained. This is the response frequency of the
system. The frequency is different for each process.
3. Measure the Time Constant which is the time to complete one cycle
of the response frequency. The Time Constant will be defined as
"T" when calculating Integral and Derivative Times.
Time
Constant
PV
4. Widen the Proportional Band until only slightly unstable. This is the
Proportional Band's Ultimate Sensitivity. The Proportional Band's
Ultimate Sensitivity width will be defined as "P" when calculating
the actual Proportional Band.
5. Use the following coefficients in determining the correct PID set-
tings
for your particular application.
Control
Action
P Only
PI
PID
Time
P I D
Setting Setting Setting
2P * *
2.2P .83T *
1.67P .5T .125T
APPENDIX C
Heater Break Option
The Heater Break Option is used to detect heater break conditions and
to energize an alarm relay when such conditions exist. In most cases,
the option is used to detect the failure of one or more zones in a multi-
zoned heater where all individual resistive heater zones are wired in
parallel. Failed heater zones would create cold spots in a system which
could hamper the process and even ruin the product. If cold spots in a
system are a problem, the Heater Break option is an effective way of
alerting the operator of a heater break condition, a cause of cold spots.
The PXW controller is able to detect a heater problem by analyzing the
current used by the heater. The actual sensing is done by a current
sensing transformer, sold separately, which is placed around the hot
lead going to the heater and connected to the controller. The signal
sent by the current sensing transformer is timed with the output of the
PXW. When the output is energized the signal sent from the current
sensing transformer is analyzed. When the output is de-energized the
signal sent from the current sensing transformer is not analyzed. This
eliminates the alarm condition turning on and off due to the output con-
dition of the controller. If the signal sent when the output is energized
indicates that the current level is below what the Heater Break alarm is
set for, the alarm is energized. The alarm is non-latching.
Notes:
1. The Heater Break Option is available on the PXW-5, 7, and 9
controllers only.
2. The Heater Break Option cannot be used on the PXW controller with
a 4-20mA DC output. The current sensing transformer would pick up
current changes due to fluctuating power output, between 0% and
100%, which would result in a heater break alarm condition even
though no such condition existed.
3. The Cycle Time must be set at 6 secs. or higher in order for the
controller to correctly analyze the signal sent by the current sensing
transformer.
4. The power supply used should be the same for the PXW and heater
to eliminate current fluctuations due to power differences between
different power supplies.
Wiring and Setting:
1. Choose the correct current sensing transformer based on the maxi-
mum current usage of the heater.
0 - 30 Amps (part # CTL-6-SF)
20 - 50 Amps (part # CTL-12-S36-8F)
2. Thread the hot lead going to the heater through the donut of the cur-
rent sensing transformer. Connect the wires of the current sensing
trans former to the current sensing transformer input terminals in the
back of the controller.
21
Connection to PXW
(Polarity not important)
Hot lead to Heater