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Constant Pressure Systems
DXL and LHX drives can be configured as constant pressure (CP) water systems using an analog pressure
transducer (analog constant pressure system). A CP water system includes a pressure tank, a pressure gauge
to observe system pressure, and pressure switches or sensors.
6.1 Control Principles of Constant Pressure Systems
In a CP system, a target pressure for the system is set through the keypad. Signals from the sensors interact
with firmware to control the motor speed and maintain constant water pressure.
In a strict definition of a constant pressure system, the pump would never turn off. If the pressure differential
between pump-on and pump-off was zero, the noise fluctuations of the transducer output would cause the
motor to cycle constantly between the on and off states. For this reason, most applications will want to accept
a small differential pressure in the system to prevent either continuous running of the motor or constant on/off
cycling of the motor.
Three basic conditions must be met for the pump to shut down and enter sleep mode:
1. The value (PSI, GPM, FT) in the system must be at the value control point set by the parameter 1.4.1
ANALOG SETPOINT 1 or at the value determined by the setting on the analog sensor.
2. The frequency is below 1.4.5 SLEEP FREQUENCY (as defined as 1.1.1 MIN FREQUENCY + input
value) or measured value is above 1.4.6 FORCE SLEEP THRESHOLD (defined as 1.4.1 ANALOG
SETPOINT 1 + the input value.
3. The time expired since the pump started after the last OFF cycle must be greater than parameter
1.4.23 T1 ON or 1.4.24 T2 ON.
Preventing Short Cycling During Low Flow Conditions
When a CP system is in a low flow state, it may be desirable to turn the motor off (sleep mode) to conserve
energy and preserve the motor. The adjustable parameters 1.4.23 T OFF, 1.4.23 T1 ON, 1.4.24 T2 ON, 1.4.5
SLEEP FREQUENCY, 1.4.6 FORCE SLEEP THRESHOLD, 1.4.7 WAKE UP THRESHOLD, and 1.4.8
BOOST AMOUNT control when the motor is turned off, how long it is off, and prevent short cycling of the
motor at low flow rates. More information including default values for these parameters can be found in Table
20.
1.4.7 WAKE UP THRESHOLD and 1.4.8 BOOST AMOUNT
The primary method to prevent short cycling is to allow a differential between the desired value (PSI, GPM,
FT) at which the pump turns off and the value at which it restarts. Two parameters, 1.4.7 WAKE UP
THRESHOLD and 1.4.8 BOOST AMOUNT, control this differential. 1.4.8 BOOST AMOUNT specifies the
increase in value above 1.4.1 ANALOG SETPOINT 1 just before the pump shuts down to enter sleep mode,
while 1.4.7 WAKE UP THRESHOLD specifies the value drop below 1.4.1 ANALOG SETPOINT 1 at which
the pump restarts.
For example, assume 1.4.1 ANALOG SETPOINT 1 = 50 psi, 1.4.8 BOOST AMOUNT PSI = 5, and 1.4.7
WAKE UP THRESHOLD = 5. When the pump is ready to enter sleep mode, the pump will boost to 55 psi,
then restart when the value falls to 45 psi.
These two parameters can be used together or independently to create a dead band in pressure control. 1.4.6
FORCE SLEEP and 1.4.5 SLEEP FREQUENCY can also be set to tell the drive when to enter sleep mode.
1.4.6 FORCE SLEEP THRESHOLD (analog CP only) is the system pressure at which the drive will go to
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