Engineering Manual o.. - HVAC.Amickracing

ELECTRIC CONTROL FUNDAMENTALSWhen the high-limit controller calls for less heat, the potentiometerwiper R (shown dotted) moves halfway from B to W.This causes an unbalance of 70 ohms (210 – 140) in the rightleg of the bridge as shown in the following table:Left Leg Right LegController potentiometer 140 0High-limit potentiometer 0 70Feedback potentiometer 0 140Total 140 210The 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 moves 35 ohms to the right. The tablethen appears as follows:Left Leg Right LegController potentiometer 140 0High-limit potentiometer 0 70Feedback potentiometer 35 105Total 175 175When 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(75 percent open).CONTROL COMBINATIONSThe following illustrates common applications of Series 90controls including low- and high-limit controls from anapplication viewpoint.DISCHARGE AIRLOW LIMITCONTROLLERROOMCONTROLLER135 135W R B W R BACTUATORHOT WATERW R BT1T2VALVETRANSFORMERTOCOILC2526DISCHARGE AIRROOMHIGH LIMITCONTROLLER CONTROLLER135 135LINEVOLTAGEFig. 25. Series 90 Circuit with Low-Limit Control.HIGH-LIMIT CONTROLFigure 26 illustrates a typical Series 90 circuit for a heatingapplication with a room controller, motorized valve, and highlimitcontroller located in the discharge air to the space. Thiscircuit is used when there is danger of the temperature risingtoo high. The high-limit controller takes over control of thevalve to the heating coil if the discharge air temperature risesabove a comfortable level. This circuit is similar to the lowlimitcircuit except that the high-limit controller is in the Bleg of the actuator circuit and drives the actuator toward theclosed position.W R B W R BLOW-LIMIT CONTROLFigure 25 illustrates a typical Series 90 circuit for a heatingapplication with a room controller, motorized valve, and a lowlimitcontroller located in the discharge air to the space. Thetemperature of the space can rise rapidly as a result of increasedsolar radiation, occupancy, or other conditions resulting in asudden decrease in heating load. The room controller is thensatisfied and closes the valve to the heating coil. If the systemuses outdoor air, closing the valve to the heating coil can causeair to be discharged into the room at a temperature lower thandesirable. To correct this, the low-limit controller causes thevalve to move toward open thus limiting the low temperatureof the discharge air.ACTUATORHOT WATERW R BT1T2VALVETRANSFORMERTOCOILLINEVOLTAGEC2527Fig. 26. Series 90 Circuit with High-Limit Control.ENGINEERING MANUAL OF AUTOMATIC CONTROL 107