The 6440 Series actuator from Caleffi accepts a "floating" (a.k.a. 3-position) control signal, which is a 3-wire method of doing simple modulating. Here is how it works:
On the 6440, there is a common wire (blu), an "open" wire (org), and a "close" wire (blk). When 24 VAC is applied to the common and "open" wires, the actuator drives open. When 24 VAC is applied to the common and "close" wires, the actuator drives closed. If neither, the actuator stays where it is. The 6440 is not a spring return (a.k.a. fail-safe) actuator so it "fails-in-place" when power is removed. It takes 40 seconds for the actuator to drive the Caleffi 6442 or 6443 Series ball valve full stroke (90 degrees). Keeping control power on the close or open wire is no problem; the actuator has an internal end switch which cuts off the power to the motor when fully open or fully closed.
A thermostat with single-pole double-throw (SPDT) contacts handles this control perfectly. See the image below:
It is also possible to "modulate" this actuator if the control source uses an SPDT contact with center off position (that's where the term "3-position" comes from). The actuator can drive in one direction for a certain amount of time, then the switch goes to the center position, power is removed and the actuator stops. This requires a thermostat or controller designed for floating control and the control must be configured to know the 40 second stroke time of the actuator. The same kind of floating control can be accomplished with controllers which use 24 VAC TRIACs (sourcing TRIACs).
The 6440 can also be controlled (on/off only) by 2-wires, which is commonly found in retrofit applications, with the addition of a 24 VAC relay. The power and the relay need to be located near the actuator if there are only 2 wires to the thermostat. See the image below:
Finally, the 6440 Series actuator has an integral single-pole, normally open auxiliary contact that will close when the actuator is more than 80% open. See the red and white wires in the above diagrams. The contact is rated at 5A at 24 VAC and is great for enabling boilers or pumps (or anything else).
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