Engineering Manual o.. - HVAC.Amickracing

ELECTRIC CONTROL FUNDAMENTALSSERIES 90 CONTROL CIRCUITSAPPLICATIONThe Series 90 low-voltage control circuit provides modulatingor proportional control and can be applied to:– Motorized valves.– Motorized dampers.– Sequence switching mechanisms.The Series 90 circuit can position the controlled device(usually a motorized damper or valve) at any point betweenfull-open and full-closed to deliver the amount of controlledvariable required by the controller.Proportional control, two-position control, and floatingcontrol have different operating limitations. For example:1. In modulating control, when an actuator is energized, itmoves the damper or valve a distance proportional to thesensed change in the controlled variable. For example, aSeries 90 thermostat with a 10-degree throttling rangemoves the actuator 1/10 of the total travel for each degreechange in temperature.2. In two-position control, when an actuator is energized itmoves the valve or damper to one of the extreme positions.The valve or damper position remains unchanged untilconditions at the controller have moved through the entirerange of the differential.3. In floating control, when an actuator is energized, it movesthe damper or valve until the controller is satisfied. Theactuator maintains that position until the controller sensesa need to adjust the output of the valve or damper.ACTUATORSA Series 90 actuator (Fig. 19) consists of the following:– Reversible drive motor.– Electronic relay.– Switching triacs.– Feedback potentiometer.– Gear train and drive shaft.– Rotation limit switches.– Optional spring-return mechanism.The actuator has a low-voltage, reversible-drive motor whichturns a drive shaft by means of a gear train. Limit switcheslimit drive shaft rotation to 90 or 160 degrees depending on theactuator model. The motor is started, stopped, and reversed bythe electronic relay.The feedback potentiometer is electrically identical to theone in the controller and consists of a resistance path and amovable wiper. The wiper is moved by the actuator drive shaftand can travel from one end of the resistance path to the otheras the actuator drive shaft travels through its full stroke. Forany given position of the actuator drive shaft, there is acorresponding position for the potentiometer wiper.All Series 90 actuators have low-voltage motors. A linevoltagemodel has a built-in transformer to change the incomingline voltage to low voltage for the control circuit and the motor.Low-voltage models require an external transformer to supplythe actuator (Fig. 19).Series 90 circuits combine any Series 90 controller with anactuator usable for proportioning action. Limit controls can alsobe added.TEMPERATURE CONTROLLER135 OHMSSENSINGELEMENTEQUIPMENTW R BACTUATORCONTROLLERSW R BTRANSFORMER– Temperature controllers.– Humidity controllers.– Pressure controllers.– Manual positioners.TRIACSWITCHDRIVESHAFTELECTRONICRELAYTRIACSWITCHT1T2LINEVOLTAGESeries 90 controllers differ from controllers of other seriesin that the electrical mechanism is a variable potentiometerrather than an electric switch. The potentiometer has a wiperthat moves across a 135-ohm coil of resistance wire. Typicallythe wiper is positioned by the temperature, pressure, or humiditysensing element of the controller.CLOSELIMITSWITCHCCWWINDING(CLOSE)FEEDBACKPOTENTIOMETER135 OHMSMOTOROPENLIMITSWITCHCWWINDING(OPEN)C2520-1Fig. 19. Series 90 Actuator Circuit.ENGINEERING MANUAL OF AUTOMATIC CONTROL 103