Engineering Manual o.. - HVAC.Amickracing

ELECTRIC CONTROL FUNDAMENTALSCONTROLLERPOTENTIOMETER35 105CONTROLLERPOTENTIOMETER140SENSINGELEMENTSENSINGELEMENTW R BW R BWRBWRBELECTRONICRELAYELECTRONICRELAYDRIVESHAFTDRIVESHAFTOPENCLOSE70 70FEEDBACKPOTENTIOMETER C2522Fig. 21. Bridge Circuit on Increasein Controlled Variable.OPENCLOSE70 70FEEDBACKPOTENTIOMETER C2523Fig. 22. Bridge Circuit on Decreasein Controlled Variable.The unbalance causes the electronic relay to trigger the lefttriac, energize the ccw motor winding, and drive the actuatordrive shaft toward closed. Since half of the 70-ohm unbalancehas to go on each side of the bridge to rebalance the circuit, thefeedback potentiometer will move 35 ohms to the right. Thetable then appears as follows:Left Leg Right LegController potentiometer 35 105Feedback potentiometer 105 35Total 140 140When the feedback potentiometer reaches the new position(shown dotted) the bridge is rebalanced, the left triac turnsoff, and the actuator drive shaft stops in the new position(25 percent open).The unbalance causes the electronic relay to trigger the righttriac (Fig. 19), energize the cw motor winding, and drive theactuator drive shaft toward open. Since half of the 140-ohmunbalance has to go on each side of the bridge to rebalance thecircuit, the feedback potentiometer will move 70 ohms to theleft. The table then appears as follows:Left Leg Right LegController potentiometer 140 0Feedback potentiometer 0 140Total 140 140When the feedback potentiometer reaches the new position(shown dotted) the bridge is rebalanced, the right triac turnsoff, and the actuator drive shaft stops in the new position(100 percent open).BRIDGE CIRCUIT ON DECREASE IN CONTROLLED VARIABLEFigure 22 illustrates the bridge circuit in an unbalancedcondition on a decrease in the controlled variable. The controllerpotentiometer wiper R has moved all the way to the B end andthe feedback potentiometer wiper is at the center. This causesan unbalance of 140 ohms (210 – 70) in the left leg as shown:Left Leg Right LegController potentiometer 140 0Feedback potentiometer 70 70Total 210 70BRIDGE CIRCUIT WITH LIMIT CONTROLSLimit controls are commonly used to prevent the dischargeair temperature of a central fan system from becoming too lowor too high. Figures 23 and 24 illustrate limit controls in aheating application. These controls add resistance in the bridgecircuit and drive the actuator toward the open position if a lowlimitcondition is approached (Fig. 23) or toward the closedposition if a high-limit condition is approached (Fig. 24).Limit controllers can have either a 140- or a 280-ohmpotentiometer. The 140-ohm potentiometer can drive an actuatoronly half-way open (or closed) since it adds resistance into oneleg of the bridge but does not subtract resistance from the otherleg. If 100 percent control is required from a limit controller, a280-ohm potentiometer device should be used. The followingexamples are for limit controls with 140-ohm potentiometers.ENGINEERING MANUAL OF AUTOMATIC CONTROL 105