littelfuse Articles

Alternating & Duplexing Relays

Littelfuse 4/17/2018

Alternating & Duplexing RelaysAlternating:
Power must be applied at all times. When the level in the tank rises to the normal level, the Lead Float Switch closes. Pump "A" is turned on via Pump "A" contactor, and will remain in this condition until the Lead Float Switch opens. When the Lead Float Switch opens, the ARP relay contacts transfer. When the level in the tank rises again to the normal level, the Lead Float Switch closes, energizing Pump "B" via Pump "B" contactor. Pump "B" will remain energized until the Lead Float Switch opens. The ARP relay contacts then transfer back to their original position. The ARP's internal relay contacts transfer each time the Lead Float Switch opens. By alternating the lead pump for each successive operation, the total number of operating hours is similar.

Typical Connection Diagram for Alternating Relay

When an Alternating Relay is internally cross wired, the normal alternating operation is extended to include duplexing. If the Lead Float Switch cycles as previously explained, normal alternating operation will occur. If the Lead Float Switch and the Lag Float Switch close simultaneously, due to a heavy flow into the tank, both pumps A & B will be energized. The ability to alternate the pumps during normal work loads and then operate both when the load is high is called Duplexing. Duplexing relays can save energy in most systems because only one smaller pump is operating most of the time; yet the system has the capacity to handle twice the load.

Timing Diagram for Duplexing Relay

Duplex Panel with Latching Pump Down Operation

Many dual pump, duplex pumping applications, require two or more float switches to properly operate the system. The ARP Series of alternating (duplex or cross wired) relays are designed to equalize run time for two loads by automatically changing the "lead pump" and "lag pump" sequence at the end of each cycle. The ARP assures approximately equal wear on both loads, plus the duplexing models allow both pumps to operate simultaneously. This application can be used for water and wastewater pumping; and for circulating and distribution pumping of various liquids.

The diagram depicts a typical drain pumping application. The OFF, Lead and Lag float switches are connected as shown. As the liquid level rises, first the OFF and then the lead float switches close; pump "1" energizes. The liquid is pumped down by pump "1" until the OFF switch opens because of the latching action of the P1 auxiliary contacts on the pumps contactor. As the OFF switch opens, pump "1" turns OFF and the ARP toggles making pump "2" the lead pump. This operation continues with the pumps alternating lead/lag order on each successive cycle. If the flow is too heavy for one pump, the lag float switch eventually closes. Now, both pumps operate until the OFF float switch opens. A benefit of this connection method is the elimination of rapid cycling of the pump motors caused by float switch bounce.

Duplex Panel with Latching Pump Down Operation

Timer Replaces Expensive Float Switch

In this application, a TDM delay on make time delay replaces the OFF float switch. In a duplexing pump controller, an OFF switch is installed at a level below the Lead Float Switch. The lead pump starts when the Lead Float Switch closes, and stops when the OFF switch opens. The difference in the position of the switches produces a time delay that prevents rapid cycling of the pump. Because of the installation and maintenance expense associated with all float switches, this solution replaces the OFF float switch with a no maintenance TDM time delay relay.

In the figure when the input flow exceeds the capacity of a single pump, both pumps operate. Unless the lag pumps contactor is latched ON, the lead pump will operate continuously and the lag pump will cycle ON and OFF as the lag switch opens and recloses. Remember the lag switch only closes when the fill rate exceeds the capacity of the lead pump. As the Lag Float Switch opens, the lag pump is turned OFF. Because of peak flow, the level immediately rises and turns the lag pump back ON; rapid cycling it.

Operation with the time delay installed:
The diagram is shown with the Lead Float Switch already closed. Pump A (lead pump) does not start until the TDM delay on make timer energizes. When the TDM energizes, relay contacts 1 to 3 and 8 to 6 close energizing pump A. The TDM remains energized until the Lead Float Switch opens. When the level rises and closes the Lag Float Switch, the lag pump (pump B) energizes immediately. Pump contactor auxiliary contacts, PC A and PC B latch the lag pump on. Both pumps operate until the Lead Float Switch opens; the TDM de-energizes and the contactor's auxiliary contacts open. The ARP duplexing relay transfers to position B, making pump B the lead pump for the next cycle. Typically, the level rises again, re-closing the Lead Float Switch. The lead pump does not restart again until the TDM time delay times out. The TDM prevents rapid cycling of the lead pump by providing the time delay typically created by the OFF and Lead Float Switches; at a fraction of the cost.

Operation Time Delayed Install Diagram