SQ pumps on mod sine wave [RE-wrenches]

[Allan Sindelar]

Kirk,
We haven't worked with the SQ pump, but I would check the pressure tank
first. Your tank precharge pressure should be set at 1-2 psi below the
cut-in setting on the pressure switch. If it is set too low, the tank
pressure will drop quickly before the pump re-establishes pressure, which
seems to fit your description.

To check precharge, you must bring system pressure to zero or thereabouts,
then use an accurate tire gauge on the pressure tank's Schraeder valve.
Bring an air compressor or tire inflator.

Also, some pressure switches have an optional low-pressure cutoff built into
the switch, to provide dry run protection. If cut-in pressure is reached but
the pump doesn't recharge and pressure continues to drop, the low-pressure
cutoff will be reached. This requires a manual reset.

Allan at Positive E

----- Original Message -----
From: "Kirk Herander" <kirk at vtsolar.com>

> Group,
> I have a customer running a 1/2 HP Grundfos SQ series (soft start) pump on
> an old Trace U 2024 mod sine inverter. Occasionally, like once every two
> weeks, the pump won't fill the pressure tank when cut-in pressure(30 psi)
is
> reached. The pump turns on, but the pressure hovers around 10-15 psi.
There
> is also a dc pump plumbed to the tank from a spring. Once we switch over
and
> allow the DC pump to pressurize the tank, then switch back to the
Grundfos,
> it will work fine for a couple of weeks until this happens again. Any
ideas?
> Battery voltage is not low when this occurs.
>
> Also when cut-in is reached, the pressure will drop from 30 to 0 psi for a
> second, then rise normally as the pump pressurizes the tank. Is this
sudden
> pressure drop to be expected from a soft-start pump?

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