Watlow Ez-Zone PM4 - Cool Output Curve; NO-ARC Relay; Retransmitting a Process Value or Set Point

Watlow EZ-ZONE
®
PM PID Controller • 78 • Chapter 9 Features
12mA signal. This feature reduces the overall system
cost by using a single output to act as two outputs.
Outputs 1 and 3 can be ordered as process out-
puts. Select duplex [dUPL] as the Output Function
[``Fn] (Setup Page, Output Menu). Set the output to
volts [uoLt] or milliamps [`MA] with Output Type
[`o;ty]. Set the range of the process output with
Scale Low [`S;Lo] and Scale High [`S;hi].
NO-ARC Relay
A NO-ARC relay provides a significant improvement
in the life of the output relay over conventional re-
lays.
Conventional mechanical relays have an expected
life of 100,000 cycles at the rated full-load current.
The shorter life for conventional relays is due to the
fact that when contacts open while current is flowing
metal degradation occurs. This action produces un-
avoidable electrical arcing causing metal to transfer
from one contact to the other. The arcing conditions
continue on each subsequent contact opening until
over time the resistance through the contacts in-
creases causing the contacts to increase in tempera-
ture. Eventually, the contacts will weld together and
the relay remains in the on state.
The Watlow NO-ARC relay is a hybrid relay. It us-
es a mechanical relay for the current load and a triac
(solid-state switch) to carry the turn-on and turn-off
currents. NO-ARC relays extend the life of the relay
more than two million cycles at the rated full-load
current.
Although a NO-ARC relay has significant life ad-
vantages, a few precautions must be followed for ac-
ceptable usage:
Do not use:
• hybridrelaysforlimitcontactors.Alimitor
safety device must provide a positive mechanical
break on all hot legs simultaneously;
• dcloadswithhybridrelays.Thetriacsusedfor
arc suppression will turn off only with ac line
voltage;
• hybridswitchestodriveanyinductiveloads,such
as relay coils, transformers or solenoids;
• cycletimeslessthanfivesecondsonhybrid
switches;
• onloadsthatexceed264Vacthroughrelay;
• onloadsthatexceed15amperesload;
• onloadslessthan100mA;
• NO-ARCrelaysinserieswithotherNO-ARCre-
lays.
Retransmitting a Process Value or Set Point
The retransmit feature allows a process output to
provide an analog signal that represents the set
point or process value. The signal may serve as a
remote set point for another controller or as an in-
put for a chart recorder documenting system perfor-
mance over time.
In choosing the type of retransmit signal the op-
erator must take into account the input impedance
of the device to be retransmitted to and the required
signal type, either voltage or milliamps.
Typically applications might use the retransmit
option to record one of the variables with a chart re-
corder or to generate a set point for other controls in
a multi-zone application.
Output 1 can be ordered as process outputs. Select
retransmit [rMt] as the Output Function [``Fn]
(Setup Page, Output Menu). Set the output to volts
[uoLt] or milliamps [`MA] with Output Type
[`o;ty]. Select the signal to retransmit with Retrans-
mit Source [`r;Sr].
Retransmit
Retransmit Source
Output Scale
Range High
Range Low
Scale High
Scale Low
Set the range of the process output with Scale
Low [`S;Lo] and Scale High [`S;hi]. Scale the re-
transmit source to the process output with Range
Low [`r;Lo] and Range High [`r;hi].
When the retransmit source is at the Range Low
value, the retransmit output will be at its Scale Low
value. When the retransmit source is at the Range
High value, the retransmit output will be at its Scale
High value.
Cool Output Curve
A nonlinear output curve may improve performance
when the response of the output device is nonlinear.
If a cool output uses one of the nonlinear curves a
PID calculation yields a lower actual output level
than a linear output would provide.
These output curves are used in plastics extruder
applications: curve 1 for oil-cooled extruders and
curve 2 for water-cooled extruders.