Engineering Manual o.. - HVAC.Amickracing

ELECTRIC CONTROL FUNDAMENTALSBRIDGE CIRCUIT WITH LOW-LIMIT CONTROLIn a heating application, a low-limit controller moves a valveactuator toward open when the low-limit setting is reached. Todo this, the limit controller is wired into the left or W leg of thebridge. An increase in resistance in the left leg of the controllercircuit drives the actuator and feedback potentiometer towardthe open position.In Figure 23, when the controller and the low limit aresatisfied (both potentiometer wipers at the W ends), the actuatoris at the closed position and the bridge is balanced. This isshown in the following table:Left Leg Right LegController potentiometer 0 140Low-limit potentiometer 0 0Feedback potentiometer 140 0Total 140 140DISCHARGE AIRLOW LIMITCONTROLLERCONTROLLERhas to go on each side of the bridge to rebalance the circuit, thefeedback potentiometer moves 35 ohms to the left. The tablethen appears as follows:Left Leg Right LegController potentiometer 0 140Low-limit potentiometer 70 0Feedback potentiometer 105 35Total 175 175When 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(25 percent open).BRIDGE CIRCUIT WITH HIGH-LIMIT CONTROLIn a heating application, a high-limit controller moves a valveactuator toward closed when the high-limit setting is reached.To do this, the limit controller is wired into the right or B leg ofthe bridge. An increase in resistance in the right leg of thecontroller circuit drives the actuator and feedback potentiometertowards the closed position.W R BW R BW R BIn Figure 24, when the controller and high limit are satisfied(both potentiometer wipers at the B ends), the actuator is at theopen position and the bridge is balanced. This is shown in thefollowing table:DRIVE SHAFTELECTRONICRELAYOPENCLOSEFEEDBACKPOTENTIOMETERC2524Fig. 23. Bridge Circuit with Low-Limit Control.Left Leg Right LegController potentiometer 140 0High-limit potentiometer 0 0Feedback potentiometer 0 140Total 140 140CONTROLLERDISCHARGE AIRHIGH LIMITCONTROLLERWhen the low-limit controller calls for more heat, thepotentiometer wiper R (shown dotted) moves halfway from Wto B. This causes an unbalance of 70 ohms (210 – 140) in leftleg of the bridge as shown in the following table:W R BW R BW R BLeft Leg Right LegController potentiometer 0 140Low-limit potentiometer 70 0Feedback potentiometer 140 0Total 210 140The unbalance causes the electronic relay to trigger the righttriac, energize the cw motor winding, and drive the actuatordrive shaft toward open. Since half of the 70-ohm unbalanceELECTRONICRELAYOPENCLOSEFEEDBACKPOTENTIOMETERDRIVE SHAFTC2525Fig. 24. Bridge Circuit with High-limit Control.ENGINEERING MANUAL OF AUTOMATIC CONTROL 106