Flow switch circuit for water pump controller

[calzap]

Diagram and descriptive text are below.  The pump is 240 VAC, pumping from pond into pressure tanks; only 1 m suction head.  The 24 VAC control circuitry is non-negotiable ... a lot of existing controller and sensor parts are 24 VAC, some sensors are 10 m or more away in a electrically noisy environment, some auxiliary devices (lights, valves, etc.) are 24 VAC, etc.

All relays will have surge suppressors for their coils.  If you want to follow the action, start with the pressure and flow switches; then the relays in numerical order.

Would like comments on problems and improvements.

Mike in California
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Pressure switch closing initiates a pump cycle.  Assuming other safeties allow it, pump will start, and if water flows, flow switch should close within 2 seconds.  As pressure builds in tanks, flow slows, and flow switch will open before cutoff pressure is reached.  Circuit is designed to check if flow is present in the interval 2 to 4 seconds after pressure switch initiates a cycle.  If flow is not confirmed, then the system will be shutdown via the latching relay and require a manual restart.  After 4 seconds of pumping, flow switch plays no further role except as described below.

Pressure switch has a safety pressure below cut-in pressure at which, it will do a cut-out that requires a manual reset.  At very high flow rates, like use of a fire hose, the pressure may fall below the safety level.  Flow switch is wired through its relay in parallel with the pressure switch in order to maintain pumping at high flows regardless of pressure.

Normal operational position of the latching relay is 2.  In this position, its SPST contacts are open, which means the secondary DPDT relay will not be activated.  The NC contacts of the DPDT relay allow the contactor coil to operate.  The closed SPDT contacts of position 2 of the latching relay allow the flow switch  relay to cause the latching contacts to move to position 1 if the flow switch has not closed during 2-4 seconds of pump operation.   When the latching relay moves to position 1, the latching SPST contacts will be closed, activating the DPDT relay.  And change of position of the SPDT contacts of the latching relay means the flow switch relay will no longer be able to change the latching relay position until the latching relay is reset to position 2 by the reset switch.  The reset switch also serves as a test switch because successive presses will change the latching relay.
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