Bell & Gossett 10-001-275 XLS Integrated Pump Controller - Manual (Page 20)

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technician to copy values from one PID group to
the other. To copy, select the PID group to copy
from and click “Read”, then select the PID group
to copy to and press “Write”.
Speed Test:
Speed test is a method to shut down the lead
VFD when only 1 VFD is running. The PID value
is artificially reduced and the system is
monitored for PID response (pressure drop).
Speed Test shutdown is initiated when flow
falls below “Flow” for “Delay” seconds.
“Speed” is the speed to which the PID must drop
to pass speed test and shut down the pump.
“Period” is the time between steps for the speed
test routine to lower the PID output, and “Step
Size” is the amount of PID drop per step.
If the speed test routine detects the PID ramping
up during the test, speed test will abort and
operation will return to normal until flow is again
below the “Flow” setpoint for “Delay” seconds.
Discussion of Proportional and Derivative
settings, high and low flow:
Configuring the proportional and derivative
values is essentially a trade-off. Both these
values have an impact on the system
response. A high proportional value causes the
system to respond faster, thus reaching the
setpoint faster. At the same time, this faster
response means that the system will easily
overshoot the desired setpoint. This can be
checked by lowering the proportional value,
and increasing the derivative (d-Term) value.
The derivative term controls the systems
response to rapid changes in pressure,
regardless of the value of the pressure. This
system uses this to begin reducing VFD output
speed when pressure is rising quickly, helping
to avoid overshoot. The system can also
detect fast pressure drops to begin increasing
VFD output speed before large errors are
detected, increasing the systems
responsiveness.
Good starting values are 7 for proportional and 4
for derivative. Increase proportional when the
system lags too long changing the speed when
pressure is away from setpoint but relatively
steady. Decrease proportional and increase
derivative to reduce overshoot. Decrease
derivative when the system becomes unsteady
near setpoint under steady demand. Generally
speaking, only use enough derivative to reduce
overshoot.
Low and high flow settings:
The low flow and high flow settings are used
together to calculate the proportional and
derivative terms at any given time. The low flow
settings are what the proportional and derivative
terms would be if flow were 0. The High Flow
settings are what the proportional and derivative
terms would be if flow were greater than or equal
to “Max Flow for Low/High Flow”. The value to use
for each parameter is calculated based on the flow
at the time.
Example: Low Flow Proportional is 7, and
High Flow Proportional is 9, “Max Flow for
Low/High Flow” is 2000. At 1000 gpm, the
proportional value will be calculated to be
8. At 1500 gpm, the proportional value will be
calculated to be 8.5.
Minimum Speed:
This is the minimum speed that the system will
operate the VFD under PID control. This speed
should equal the minimum speed that the pump
will flow water at setpoint pressure. Setting this
value too high can cause over pressurization at
low demands. S e t t i n g this value too low can
cause the PID to hunt since the speeds below the
“true” min speed are ineffective and can result in
unnecessary delay in the PID when operating in
these speed ranges. Note that allowances must be
made for varying inlet pressure and adjustable
setpoints. Generally speaking, a low Min speed is
better than a too-high min speed, but the closer to
“true” min speed this is set, the better the system
will operate.
Control Deadband:
This value controls how far from setpoint the
pressure must be before the PID responds. The
units here are in 0.01 psi, so 25=0.25 psi (one
quarter-pound). This smooths the PID response
near setpoint. However, too large a value here will
cause the system to delay response and could
cause the system to “get behind” in response to a
large change in demand.
Transition Control
Starting Speed:
This is the initial speed of the Main VFD when
the Main Pump first starts. Should be high
enough to “kick start” the pump since it is
starting under unsupplied demand, but care
must be taken not to force the system to
overshoot under low demand situations. Start
the system with demand just greater than the
PM Pumps (or Jockey Pump, as supplied)
capacity and adjust for minimal overshoot.