West Control Solutions Pro-EC44 - Manual (Page 91)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 84
14 Valve Motor Drive / 3-Point Stepping Control
When directly controlling the motor of a modulating valve or damper, set the Control Mode to
VMD in configuration mode to enable the 3-point stepping Valve Motor Drive control
algorithm.
The term
“3-point stepping” is used because there are 3 output states, open valve, close
valve or stopped (no action). Switched outputs move the valve further open, or further closed
when a control deviation error is detected. If the error is reduced to zero, no further output is
required until the load conditions change.
VMD mode
doesn’t allow on-off control (the minimum proportional band equates to 0.5% of
the scaled input span) and usually requires PI control, where the derivative parameter is set
to OFF.
Note: Some modulating valves have positioning circuitry to adjust the valve
position. These require a DC linear mA or voltage output and use the standard
control algorithm (Set Control Mode to Standard).
Special Wiring Considerations for Valve Motor Control
Valve motor drive mode must have two identical outputs assigned to position the valve. One
to open and one to close the valve. These outputs can be two single relays, two triacs, two
SSR drivers or one dual relay, but it is recommended to use two single relays (SPDT
change-over contacts), and to interlock the wiring as shown. This prevents both motor
windings from being driven at the same time, even under fault conditions.
CAUTION: The windings of a valve motor effectively form an
autotransformer. This causes a voltage doubling effect when power is
applied to either the Open or Close terminal, causing twice the supplied
voltage at the other terminal.
Switching actuators directly connected to the valve motor must only be used up to half of
their rated voltage. The internal relay and triac outputs in this instrument are rated at 240VAC
Therefore, the maximum motor voltage when using them is therefore 120V unless
interposing relays are used. Interposing relays or other devices used to control the valve
must themselves be rated for twice the motor supply voltage.
“O
pe
n”
R
elay
R
e
lay
“Close” Relay
120V Supply
Open Valve Winding
Close Valve
Winding
Valve Common