•3.5 second pulse = suction pressure is in the proportional band region and is > 70% and < 80% from the dead- band region.
•4 second pulse = suction pressure is in the proportional band region and is > 80% and < 90% from the dead- band region.
•4.5 second pulse = suction pressure is in the proportional band region and is > 90% and < 100% from the dead-band region.
•5 second pulse = suction pressure is beyond the proportional band region ( > 100% from the dead-band region.)
Auto-Cycle Setpoints
The auto-cycle setpoints define the control points in which the compressor will automatically cycle on and off when the compressor has been placed into “Auto” mode. These setpoints can be “enabled” or “disabled” using the check box. A delay can be entered to momentarily delay the start or stop from immediately occurring when the pressure reaches setpoint. If a compressor shut down is desired on a suction pressure drop and a manual reset is required, set the OFF value below the Low Suction Pressure safety trip value. This will shut down the compressor and a Reset will be required to restart it.
•[x] Enable = enables the Auto-cycle control. Uncheck the box to disable the Auto-cycle set-points.
•Start Pressure = When the suction pressure meets or exceeds this set-point, the compressor will start.
•Start Delay = delays the compressor from starting when the suction pressure meets or exceeds this set-point.
•Stop Pressure = When the suction pressure meets or falls below this setpoint, the compressor will stop.
•Stop Delay = delays the compressor from starting when the suction pressure meets or exceeds this set-point.
•Minimum Slide Position = The minimum capacity slide position that the compressor is allowed to run at.
NOTE: When the Pump-down Feature is enabled, the Auto-cycle setpoints are automatically disabled. Pump- down mode will cause the compressor to cycle off via the Pump-down Stop Pressure setpoint, and will not allow the compressor to start again.
VFD Settings (PID settings for Motor Speed Control)
When a VFD is used on the compressor motor, the speed of the VFD is controlled using a PID algorithm. The set- points are:
•P = Proportional (gain) set-point. Used to adjust the motor speed action in direct proportion to the differ- ence between the control set-point and the process variable (SP - PV error). This is a unit-less quantity and is used for coarse adjustment. Use an initial setting of 3.00. This set-point should be set to the lowest value that gives adequate control system response. Increasing the proportional setting increases the control system’s sensitivity to small process fluctuations and the tendency to hunt.
•I = Integral (reset) set-point. Used to adjust the capacity control action, integrating the error over time, to account for a small error that has persisted for a long time. This quantity is used for fine adjustment. Use and initial setting of 4.00 min. This set-point is used to smooth out process variations. This set-point should be set high enough to prevent hunting but not too high or it will cause control system overshoot.
•D = Derivative (rate) set-point. Used to adjust the capacity control action, accounting for how fast the error is changing, positively or negatively. A standard PID loop variable, it is not used for our applications. Use an initial setting of 0.00 min (Always).
