unit that would trip if my 3/4 HP
pump exceeded its specified
I think the most important
component in my system is the
voltage relay. It opens the control
circuit if line voltage is either too
low or too high. Trying to start the
pump under pressure with low
voltage will likely lead to a failure;
running it higher than the design
voltage is almost as likely to cause
The adjustable voltage relay
has hysteresis circuitry that
provides a ‘window’ of acceptable
voltages, but has a buffer above
and below the set point so it won’t
cycle or chatter if the voltage is on
either edge. Figure 4 shows how
my relay window is set to operate.
I set my trip points around
240 VAC because that is my
typical line voltage. If yours is
more typically 230 VAC, you may
want to set lower points. I was lucky to find my SquareD®
voltage relay on the surplus shelf of an electronics supply
for $10. SquareD no longer makes a 120 VAC unit, but
Omron® makes a similar unit. The Omron (listed in the
components list) requires a few more connections
because the power supply and relay terminals are
separated from the voltage measuring terminals.
The voltage relay is operating on the 120 VAC control
circuit (240 VAC units are available), but measuring the
AC voltage between neutral and L2 is okay. Adjusting your
set points around 120 VAC corresponds to the 240 VAC
line. Neutral to L1 voltage should be essentially the same
as neutral to L2. If not, you may
have a loose neutral — which can
spell disaster for appliances!
I installed a small solid-state
relay following the voltage switch.
Turn-on time is set via a resistor
and is adjustable from three to 60
seconds. The purpose of this
component is to be sure voltage
has stabilized before powering the
pump after an under- or over-voltage condition. When power
goes out in our rural area, there
are usually a few ‘false starts’
before it’s restored.
I set this relay for a 30 second
delay (with a 220KΩ resistor) to
clear the usual period of
fluctuation. Resistor R provides the
load required for the two-wire
solid-state relay to operate. I found
I needed a 2.15KΩ 10W unit for a
load, which draws 6.7W.
Using a three-wire relay such
as the one used for the time-out
function (following) negates the need for a load resistor.
I added an outboard relay (Figure 5) after I found the
well water very low one morning for no apparent reason. I
monitor the water level with a pressurized nylon tube that
I taped to the pump before lowering it into the well. The
problem turned out to be a small pipe leak that the pump
could keep up with, but the well could not.
As a result, the pump kept running for hours —
depleting the well. The time-out relay is adjustable and I
A normal run for this pump is
about seven minutes, so I only get
a shut-down if there’s a real
problem. The time-out relay resets
itself after every pump run. The
relay specified in the Parts List is
actually a time delay relay, but I’m
using the NC contacts so the
circuit opens (instead of closing)
after a 30 minute delay.
36 July 2016
■ FIGURE 3. The contactor takes the
place of the pressure switch.
■ FIGURE 4. These are the voltage points I set for my system (not to scale).