Watlow Electric Integrated Controller User Manual Rev C EZ-ZONE PM - Manual (Page 60)

Wa t l o w E Z - Z O N E ® P M I n t e g r a t e d C o n t r o l l e r • 5 8 • C h a p t e r 9 F e a t u r e s
which allows its set point to be manipulated by an
external source. A common application would use one
ramping controller with a set-point retransmit out-
put to ramp multiple controllers using the remote set
point. Or you could use an analog output from a PLC
to send set point values to an EZ-ZONE
® PM.
The controller must have two process inputs to
use the remote set point feature.
You may select between local and remote set
points at the front panel, with an event input, from a
remote computer using the communications feature
or from an external switch using an event input.
Make sure all input and output impedances are
compatible.
Switch to the remote set point with Remote En-
able [`r;En] (Operations Page, Loop Menu). Select
whether the remote set point controls an open- or
closed-loop set point with Remote Set Point Type
[`r;ty].
Assign the function of switching to a remote set
point to a digital input with Digital Input Function
[``Fn] (Setup Page, Digital Input Menu).
Assign the function of switching to a remote set
point to the EZ Key with Digital Input Function
[``Fn] (Setup Page, Function Key Menu).
Outputs
Duplex
Certain systems require that a single process output
control both heating and cooling outputs. An EZ-
ZONE
® PM controller with a process output can func-
tion as two separate outputs.
With a 4 to 20mA output the heating output will
operate from 12 to 20mA (0 to +100 percent) and the
cooling output will operate from 12 to 4mA (0 to -100
percent).
In some cases this type of output is required by
the device that the EZ-ZONE ® PM controls, such as
a three-way valve that opens one way with a 12 to
20mA signal and opens the other way with a 4 to
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 relays.
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 uses
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 advan-
tages, a few precautions must be followed for accept-
able 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 relays.
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.
Outputs 1 and 3 can be ordered as process out-
puts. Select retransmit [rMt] as the Output Func-
tion [``Fn] (Setup Page, Output Menu). Set the out-
put to volts [uoLt] or milliamps [`MA] with Output
Type [`o;ty]. Select the signal to retransmit with
Retransmit Source [`r;Sr].