2242Conventional Electric also OnOff or Two State

Electricity is used as the power source. Control is discrete (on-off, high-low, cut-in/cut-out, etc.). Contact closures are used to implement control logic. This principle of control has widespread use, varying from simple and familiar control to complex interlocks and Boolean Logic.

Examples:

• A basic home heating thermostat cuts in at 67 degrees F and cuts out at 69 degrees F, thereby maintaining a room temperature of approximately 68 degrees F.

• A well pump controller cuts in when the storage tank pressure is 50 psig and cuts out when the storage tank pressure is 70 psig, thereby maintaining a system pressure of approximately 60 psig.

• An interlock circuit that prevents an exhaust fan from starting until the associated make-up air damper is first proven open.

The differentiator between Conventional Electric controls and Analog Electronic Controls is the discrete (two-state) nature of the inputs and outputs; analog controls have varying rate inputs and outputs.

22.4.2.1 Floating Control.

A variation of this two-state control is "floating control." Technically still "on-off," this unique control method has a system control action similar to analog (modulating) control. With floating control, whenever the signal is sufficiently off set point, the motor actuator (moving a valve or damper) is energized, providing correction to the process. What makes floating control different is that once the feedback measurement indicates the process has returned to set point, the actuator is de-energized but the device (valve or damper) holds its last position—which may be 1/4 open, 1/2 open, or any midrange point. Because of this, floating control can achieve tighter control than simple on-off control, approaching that of true modulating control. Floating control is less expensive to implement than modulating control, and is used for small terminal HVAC control valves and dampers, as well as large machinery like chiller inlet vanes. See also Control Modes for a diagram that illustrates floating control action.

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