I have been asked to repair a water pumping system for a complex of
about 10 house and need a bit of technical help. The system consists
of a very large water tank, a dual 3-phase pump water pumping system
and a galvanised water pressure tank. The pumps are suppossed to
Alternate between each other when water pressure is called for by the
water pressure switches fitted to the feed of the pressure tank. At
the moment, both pumps come on at the same time. I have narrowed down
the problem (I think) to what I have found to be on the internet, a
"Alternating Relay" I have never come across one of these before, so I
checked it out on the manufacturers web site, took the relay out of
the control box, put the correct voltage to its Coil Inputs, and
nothing happens. I metered out the contacts, and they are both open
circuit, which got me wondering, what is the "priciple of operation"
of the control box. I will swop out the relay with a new one, later
this afternoon. But in the mean time, I am struggling to understand
how any of the pumps come on, with this relay not working. There are
two pressure switches, one seems to be set to a higher water pressure
than the other. Can any kind soul tell me the priciple of operation,
is there any web sites that shows a circuit diagram of the wiring in
the control box, and once I have got the system running correctly
again, what pressure should I be setting the on-off pressures points
of the pressure switches? thanking you in anticipation.
Mark in Spain snipped-for-privacy@markXscotford.com - (remove the X to reply))
I don't know too much about your exact application, but a "dual pump, with
wear equalization" control system is pretty common in industrial
There are two pumps and two pressure switches. One pressure switch is set at
a lower pressure than the other. In one position of the alternating relay,
the low switch is connected to pump 1, the high switch to pump 2. When the
relay alternates, the low switch runs pump 2, the high switch pump 1. The
relay alternates every time the high switch trips.
If one pump can keep up with the flow, than the pumps simply alternate,
maintaining pressure. This is the normal case, and wear on the pumps is
If a single pump can't keep up with the flow, the low pressure switch trips,
turning on the second pump.
Hope this helps.
What Hershel describes is frequently called "lead-lag" operation.
My advice is either to get an integrated magic box:
or to hire an electrician. Pump controls are no joke, and if you do it
wrong, the cost to repair pump motors is pretty high. Not to mention,
when the water goes out, people just love to complain.
As to how the alternating circuits actually work, I have no idea. I've
seen the little doodads inside the control panels, but don't know how
they work. I presume that they are solid state.
I have however seen it set up so that a mechanical timer alternated the
relay that was energized to start the pump. This only alternated the
pumps, and didn't provide lead-lag. This was a nasty hack though.
As far as the cut-on/cutoff pressure, a good cut on is 45 psi, cutoff
at 70 psi. If you had a lead lag setup you could start the second pump
at 30 or 35 psi.
However, if the neighborhood has a lot of relief, you might get low
pressure above the pressure tank. I can't imagine a 10 home
neighborhood having that much relief though.
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