Schreiber Chillers 1055 New - Manual (Page 51)

fan speed. The band is the value around the setpoint of which controls the speed signal. If the head
pressure is below the setpoint by the value of the band or more, the controller sends the minimum
output of 0 volts DC. If the head pressure is greater than the setpoint by the value of the band or
more, the controller sends the max output; 10 VDC.
If the head pressure falls between the setpoint value plus or minus the band, the speed is adjusted
accordingly.
To keep the motor from running at too low of a frequency, a minimum frequency is specified. If the
controller computes the required frequency to be 15 Hz, and the minimum frequency is 20 Hz., the
controller will open the fan relay and turn off the fans. When the head pressure rises the fans will turn
on @ 20 Hz, and increase in speed as required. Minimum off and minimum on times prevent the fan
form short cycling when close to the minimum frequency.
Sensor Failure, Variable Speed Enable/Disable.
If there is a failure of either of the head pressure transducers, the system will automatically disable
the variable speed feature. Variable speed can also be manually disabled in the FACTORY
SETTINGS section.
Condenser Fan System Parameters
These parameters are located in the factory setting sections and are password protected. Refer to the
menu tree in the CONTROLLER OPERATION section.
Minimum Frequency
If the required frequency computed by the controller is below this value, the fan will shut off. It should
not be set lower than 20 Hz, which is the fan motors minimum allowable frequency.
Maximum Frequency
This is the is the maximum frequency value (voltage)that the controller will send out to the variable
speed drive.
VFD Maximum
This is the max maximum frequency output of the drive, when the controller is sending out the
maximum signal, or 10VDC. This is programmed into the controller and the VFD.
VFD Minimum
This is the minimum frequency output of the drive, when the controller is sending out the minimum
signal, or 0 VDC. This is programmed into the controller and the VFD.
Setpoint
The head pressure that the controller will attempt to maintain by varying the fan speed.
Band