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CONSTANT PRESSURE SYSTEMS
SD & 2XD 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 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 constant pressure 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
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