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Variable speed drives control duplex pumps

Many dual pump systems, as shown in the figure, are in operation today. One pump acts as a primary pump and a second pump is provided as back up. When one pump fails, the backup pump is enabled, either manually or automatically. These systems usually rely on operators to periodically alternate which is the primary pump and which will be be standby.

With the reduced costs of controls and variable speed drive, variable speed drives may now be cost effectively applied to duplex pumps to provide energy reductions and to provide automatic pump sequencing.
Variable speed drives control duplex pumps

Advantages

Reduction in maintenance costs. Variable speed drives are inherently soft start, increasing motor and linkage life.
Pumps can be alternated more frequently, say every 24 hours, increasing the life of the equipment.
On pump failure, the standby pump is automatically started with alarm indication.
Start-Stop and reset of pumps can be done by the variable speed drives.
Optional manual selector switch overrides the automatic pump alternation sequence.
In applications, where increased flow is required , on a temporary basis, the lead or primary pump may remain at maximum flow with the second back up pump enabled and varied to meet the system demand.