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CONSTANT PRESSURE SYSTEMS
Essential Series drives can be configured as constant pressure (CP) water systems to
maintain a constant pressure under variable flow conditions. A CP water system includes a
pressure tank, a pressure gauge to observe system pressure, and either a digital pressure
switch or a 4-20 mA analog pressure sensor.
7.1 Control Principles of Constant Pressure Systems
In a CP system, a target pressure for the system is set through the keypad. A signal from
the pressure sensor interacts with firmware in the drive controller to control the motor
speed and maintain a constant water pressure.
In a strict definition of a CP system, the pump would never turn off. If the pressure
differential between pump-on and pump-off was actually zero, the noise fluctuations of the
sensor 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
in order to prevent either continuous running of the motor, or constant on/off cycling of the
motor.
Three basic conditions must be met in order for the pump to shut down and enter sleep
mode:
1. The pressure in the system must be at the pressure control point set by the parameter
psi SETPOINT.
2. The pump speed has slowed to a frequency below the value determined by SHUTOFF
FREQ + MIN FREQ. For example, if SHUTOFF FREQ is 10 and MIN FREQ is 30, the
pump will enter sleep mode at 40 Hz.
3. The time expired since the pump started after the last OFF cycle must be greater than
parameter T1ON or T2ON.
If the system pressure goes above the parameter OVERPRESSURE psi the pump will
immediately shut down. OVERPRESSURE psi is equal to psi SETPOINT plus
OVERPRESSURE psi. For example, if psi SETPOINT is 50 and OVERPRESSURE psi is
20 the pump will shut down at 70 psi.
Disable Sleep Mode
In some applications the user may choose to prevent the pump from entering sleep mode.
This is accomplished by setting the parameter SHUTOFF FREQUENCY to a value of zero.
In low or no flow conditions the pump will slow to the speed determined by MIN
FREQUENCY and remain at that speed indefinitely.
CAUTION: When sleep mode is disabled in low flow conditions, the drive will slow
down to minimum frequency and continue to run. Inadequate water flow in this condition
may overheat and damage the pump.
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) in order to conserve energy and preserve the motor. The adjustable parameters
TOFF, T1ON, T2ON, SHUTOFF FREQUENCY, OVERPRESSURE psi, DRAWDOWN
PSI, and BOOST AMOUNT control when the motor is turned off, how long it is off, and also
prevent short cycling of the motor at low water flow rates. More information including
default values for these parameters can be found in Table 13, Constant Pressure
Parameters.
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