Specifications

19.3 PID CONTROL - DIRECT AND REVERSE ACTING

The PID Setpoint Control software can be direct or reverse acting, and must be set to match

the system. This is accomplished with Parameter 70 - PID MODE. When set to NORMAL

(for direct acting systems), the PID unit will command an increase in motor speed if the

process variable feedback decreases, and a decrease in motor speed if the process variable

feedback increases. When set to REVERSE (for reverse acting systems), the opposite is true.

NOTE: The REVERSE setting for Parameter 70 - PID MODE is not used to compensate

for reverse acting feedback devices. If a reverse acting feedback device is used, program

Parameters 75 and 76 as described above.

An example of a direct acting system is one that maintains duct pressure. Duct pressure is

the process variable that is monitored by the feedback device. As duct pressure rises, motor

speed needs to be decreased to maintain the pressure. A direct acting PID control will

command a decrease in motor speed in response to an increasing process variable feedback.

An example of a reverse acting system is one that maintains water temperature in a cooling

tower. Water temperature is the process variable that is monitored by the feedback device.

As the water temperature rises, motor speed needs to be increased to lower the temperature

of the water. A reverse acting PID control will command an increase in motor speed in

response to an increasing process variable feedback.

19.4 SETPOINT REFERENCE SOURCES

The setpoint reference input can be one of the following:

1. Keypad

2. 4-20 mA signal at TB-5B

3. 0-10 VDC signal at TB-5A

4. Potentiometer signal at TB-5A

In order to use TB-5A or TB-5B for a setpoint input, one of the TB-13 terminals must be

programmed for the appropriate signal. Closing the TB-13 terminal to TB-2 will then select

that signal as the setpoint reference. If the contact closure is not made to TB-2, the setpoint

reference source will default to the keypad. See Parameters 47 - TB13A, 48 - TB13B, and

49 - TB13C.