The command can be a manual alternation or an alternation event signal. If the alternation event is selected, the lead pump alternation takes place every

time the event occurs. Selections include whenever an alternation timer expires, at a predefined time of day or when the lead pump goes into sleep mode.

Staging is determined by the actual system load.

A separate parameter limits alternation only to take place if total capacity required is > 50%. Total pump capacity is determined as lead pump plus fixed

speed pumps capacities.

Bandwidth Management

In cascade control systems, to avoid frequent switching of fixed speed pumps, the desired system pressure is kept within a bandwidth rather than at a

constant level. Staging bandwidth provides the required bandwidth for operation. When a large and quick change in system pressure occurs, the override

bandwidth overrides the staging bandwidth to prevent immediate response to a short duration pressure change. An override bandwidth timer can be

programmed to prevent staging until the system pressure has stabilized and normal control has been established.

When the cascade controller is enabled and the drive issues a trip alarm, the system head is maintained by staging and de-staging fixed speed pumps.

To prevent frequent staging and de-staging and minimize pressure fluctuations, a wider fixed speed bandwidth is used instead of the staging bandwidth.

6.1.8 Pump Staging with Lead Pump Alternation

With lead pump alternation enabled, a maximum of two pumps are con-

trolled. At an alternation command, the PID stops, the lead pump ramps

to minimum frequency (f

min

quency (f

). When the speed of the lead pump reaches the de-staging

max

frequency, the fixed speed pump will be cut out (de-staged). The lead

pump continues to ramp up, then ramps down to a stop, and the two

relays are cut out.

After a time delay, the relay for the fixed speed pump cuts in (staged) and this pump becomes the new lead pump. The new lead pump ramps up to

maximum speed and then down to minimum speed. When ramping down and reaching the staging frequency, the old lead pump is now cut in (staged)

on the lines as the new fixed speed pump.

If the lead pump has been running at minimum frequency (f

contributes little to the system. When the programmed value of the timer expires, the lead pump is removed to avoid water heating problems.

6.1.9 System Status and Operation

If the lead pump goes into sleep mode, the function is displayed on the local control panel. It is possible to alternate the lead pump on a sleep mode

condition.

When the cascade controller is enabled, the operation status for each pump and the cascade controller is displayed on the local control panel. Information

displayed includes:

•

Pumps Status, a read-out of the status for the relays assigned to each pump. The display shows pumps that are disabled, off, running on the

adjustable frequency drive, or running on the line power/motor starter.

•

Cascade Status, a readout of the status for the cascade controller. The display indicates when: the cascade controller is disabled, all pumps are

off, emergency has stopped all pumps, all pumps are running, fixed speed pumps are being staged/de-staged, and lead pump alternation is

occurring.

•

De-staging at No-Flow ensures that all fixed speed pumps are stopped individually until the no-flow status disappears.

6-6

) and after a delay, it ramps to maximum fre-