Lenze MC3000 Series Bedienungsanleitung PDF herunterladen (Seite 72)

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Example 1: A 0-100 psi transducer outputs a 4 mA signal at 0 psi and 20 mA at 100 psi.

Program Parameter 75 to 0.0 PSI, and Parameter 76 to 100.0 PSI (This

assumes that Parameter 31 - UNITS is set to pid PSI, and Parameter 33 -

UNITS DP is set to XXX.X).

For a reverse acting transducer, set Parameter 75 - FB @ MIN to the maximum process

variable feedback value, and set Parameter 76 - FB @ MAX to the minimum process

variable feedback value. In other words, Parameter 75 is set higher than Parameter 76.

See the example below:

Example 2: A 0-100 psi transducer outputs a 20 mA signal at 0 psi and 4 mA at 100 psi.

Program Parameter 75 to 100.0 PSI, and Parameter 76 to 0.0 PSI (This

assumes that Parameter 31 - UNITS is set to pid PSI, and Parameter 33 -

UNITS DP is set to XXX.X).

19.2 THE SYSTEM  DIRECT AND REVERSE ACTING

The entire system can also be direct or reverse acting. In a direct acting system, an

increase in motor speed causes an increase in the process variable. This is equivalent

to requiring an increase in motor speed in response to a decreasing process variable

feedback.

In a reverse acting system, an increase in motor speed causes a decrease in the process

variable. This is equivalent to requiring an increase in motor speed in response to an

increasing process variable feedback. Examples of direct and reverse acting systems

are described in the next section.

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

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