Alternating Relays can be used in various ways in applications that need to control and balance the wear between pumps, compressors and other rotating machines.
The most basic of the relay family is the variant that simply requires one input to control the outputs which are connected to two loads.

Single Control Input, 2 Outputs

Fig.1 First initiation/closure of the control contact causes PUMP 1 to run

The control input simply consists of a voltage free contact (normally open) which could be a float switch, pressure switch, manual switch or other isolated contact. Upon closure of the contact, the first output relay contact closes which in turn runs the connected pump*. When the input contact opens, the pump stops. The next time the same contact is closed, the second output relay contact closes which in turns runs the next pump. Each opening and closing of the input contact will simply alternate the operation of both pumps.

Help function

If there is the need to have both pumps running simultaneously, there is the provision to connect a second contact to the inputs. Providing the the main control contact is already closed, closing the “Help” contact then allows the second Pump to run. This is often desirable when there is excessive demand from the application and where 1 pump is unable to cope.

Fig. 2 Both PUMPS running due to the closure of the Help contact

Key Product recommendation: LR44/2

* In most cases, the outputs from the unit will be connected to a form of contactor used to switch power on and off to the pump