Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 27)

General Software Features
3.7
Optimizing Your Process System with the WATLOW Series 988
Sample Application
The Series 988 controls the lower platen of a compression molding press that contains five, 10-
ampere heaters. A 50A:50mA current transformer is used to monitor heater current.
Set the input 2 prompt
to current , the range low 2 prompt to 0 and the range high 2
prompt to 50. Find the range high 2 value with the following equation:
range high 2 =
(maximum CT primary current (load current))
x 50mA
(maximum output from CT secondary (input))
The application uses a Watlow current transformer (CT) part# 16-0233, which has a maximum input of 50 amperes, which corre-
sponds to a maximum output of 50mA.
range high 2 =
(50Amps)
x 50mA
Solving for rH2 gives you 50. This is the range high 2 setting.
50mA
Heater Current
Overview
The heater current feature measures and
responds to heater current in a system. This is
an ideal method for detecting heater loss in
multiple heater applications. The current is
measured when output 1 is ON. For instance, if
a system has five, 10-amp heaters, the heater
current input measures 50 amps regardless of
the percent output.
To view the heater current press the DISPLAY
key and advance to the process 2 prompt
.
The upper display indicates the last valid
current reading.
The input 2 prompt under the Input
menu can be set to current or loop
error detect . Current allows you to
monitor heater current and set alarm set
points based on high and low heater current
values. Alarms can only be configured as
process alarms (see Alarms, page 5.2). Setting
to loop error detect
enables monitoring
and alarm functions, and also triggers an error
and shuts OFF all outputs if current is present
with output 1 OFF or when no current is
present and output 1 power is more than zero.
There are limits associated with this feature:
•To obtain a reading, the output ON-time must
be a minimum of 0.3 seconds. To calculate
this, multiply the percent output by the cycle
time setting. Example: With 30-percent output
and a 2.0 second cycle time, the on-time would
be: 0.30 x 2.0 = 0.6 seconds. This would yield a
valid reading. If a valid reading is not possible,
the 988 will display the last valid reading.
•It will not function with burst-fire outputs. This
does not necessarily apply to the loop error
detect feature. If enabled, any current detected
with no output triggers an error.
•It will not function when the 988 has a process
output for output 1. A known cycle time is
required to detect the current. There is no cycle
time associated with process outputs.
The maximum signal the input can accept from
the current transformer secondary is 50mA.
So, you must calculate the output range of the
current transformer before wiring the system.
Requirements
Choose the heater current option for input 2.
Output 1 cannot be used as a process output.
Heater current monitors only output 1.
TL
TL
W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
10A heaters
input 1
input 2
Compression Molding Press
temperature
sensor
output 1
current
transformer
limit
sensor
limit