Table of Contents
Advertisement
If the demand for water is still greater than the
capacity of these two pumps, then Combo 4
starts. As occurred in the previous Combo, the
inverter (VFD) speed immediately ramps down
to the "speed when lag pump starts" setting.
The second lag pump starts up and goes to
full speed, and the VSP ramps back up to try
and maintain setpoint pressure.
This sequence of events repeats for all
available Combos.
When the flow decreases, the VFD (inverter)
slows down to maintain a constant discharge
pressure. Eventually as the speed decreases,
a setpoint is reached where the VSP is not
pumping any water. The XL pump(s) are
supplying all the water required. If the flow
continues to decrease, the discharge pressure
increases above the setpoint. Slowing the
VFD (inverter) does not reduce the pressure
because the VSP is not currently pumping any
water. Once the pressure reaches 1 or 2 PSI
above the setpoint pressure, the Overpressure
Accumulator starts counting. Once the count
reaches 750, the last XL pump is stopped. If
the pressure is well above the setpoint, or
increasing rapidly, the pump can stop quickly;
in 1 to 2 seconds. If the pressure remains 1 or
2 PSI above setpoint, the pump turns OFF in
approximately 20 seconds. When an XL
pump stops, the inverter (VFD) speed
immediately ramps up to the "speed when lag
pump stops" setting. This prevents the system
pressure from dropping excessively due to the
XL pump shutting off.
Once an XL pump has been stopped, the VSP
waits for the time entered in the "delay time to
start" before the XL pump is permitted by the
program to restart. This feature minimizes
pump cycling and pressure surges. The
generic value for Combo 3 and up is 10
seconds. If the conditions for starting an XL
pump are met after this waiting period,
normally 10 seconds, the pump is restarted.
If the flow continues to decrease, each XL
pump is stopped in the manner described
above, until only the VSP is running. Stopping
the last VSP is accomplished by using "Speed
Test," which uses a stop method different from
stopping an XL pump. Because the inverter
speed keeps lowering as the flow decreases,
the discharge pressure would never get above
the setpoint. In this situation, the VSP would
"idle," pump no water, and would never turn
off. To shut down the VSP, the system
initiates the "Speed Test" whenever the flow is
below a predetermined value for 15 seconds.
The program then lowers the setpoint by 5
PSI. If the VFD speed falls below a
predetermined value and the pressure
remains above the reduced setpoint for 15
seconds, the VSP turns OFF, and the setpoint
returns to normal. Speed Test returns the
station to "System Charging" mode until a new
demand for water is created.
15
Advertisement
Table of Contents
