8
AUXILIARY LOW WATER CUT-OFF
(See page 4 for Wiring Diagram)
Operation
Operation of this control is accomplished by sensing a minute AC
current flowing between submerged contact probe in the boiler shell.
When this minute AC current is conducted through an external cir-
cuit resistance up to 100,000 ohms or less, a signal of sufficient mag-
nitude is present to trigger the SCR and, in turn, energize the control
relay.
As the water level in the boiler drops below the level of the probe.
the AC circuit is broken and the control relay is de-energized. The con-
trol will not energize until sufficient water is present in the boiler.
Specifications
Input supply ....................120 vac/50-60 hz
Delectable Range............100.000 ohms
Probe Voltage..................24 vac
Probe Current..................10 milliamps
Control Relay ................Single pole double throw
CAUTION:
Control will not work with de-ionized or
demineralized water.
PROPORTIONING PRESSURE CONTROL (See Figure 17)
Typical Operation
Pressure variations cause the bellows to expand or contract.
Linkage between the bellows and the potentiometer wiper causes the
wiper to move across the windings on the potentiometer. This varies
the resistance between R and B, and between R and W, causing an
imbalance in the circuit connected to the controller.
A proportioning pressure control is used to regulate a motor driven
or solid state sequencer. The controller potentiometer, the feedback
potentiometer in the motor and a balancing relay in the motor form an
electric bridge circuit. As long as the pressure of the controlled medi-
um remains at the set point of the controller, the circuit is balanced;
i.e., equal currents flow through both sides of the balancing relay and
the relay contacts are open. When the circuit is balanced, the motor
does not run.
If the pressure of the controlled medium rises, the wiper in the con-
troller moves toward W. This unbalances the circuit, so a larger current
flows through one side of the balancing relay. The “close” contacts in
the relay make, causing the motor to drive toward its closed position.
As the motor runs, the wiper on the feedback potentiometer moves in
a direction balance, the balancing relay contacts open and the motor
stops.
Similarly, if the pressure of the controlled medium falls, the wiper
on the controller potentiometer moves toward B, and the “open” con-
tacts in the balancing relay make. The motor drives toward its open
position until circuit balance is achieved.
The slightest change in the pressure of the controlled medium will
cause a change in the number of elements energized to compensate for
it, thus keeping the pressure constant. This process is called modula-
tion.
PROPORTIONAL PRESSURE CONTROL ONLY
SUPPLIED WITH SEQUENCER
Main Setting - Turn the adjustment screw until the indicator is
opposite the low point of the desired throttling range. That is, if the
pressure is to be held to a minimum of 50 psi, set the indicator at 50
psi. The pressure will then be maintained between 50 psi and a higher
pressure equal to the 50 psi plus the throttling range.
THROTTLING RANGE SETTING L918
After setting the indicator for the minimum pressure, turn the throt-
tling range adjustment screw until the throttling range indicator points
to the desired throttling range on the scale. This scale is graduated from
“min” to “F”. The value of each division varies with the scale range of
the instrument.
Pressure
Value Each
Scale Rating
Division on Scale
0-15 psi
2.2 psi
5-150 psi
3.6 psi
Pressure scale rating will vary depending on pressure control supplied.
CHECKOUT
After the controller has been installed, wired, and set, it should be
tested with the system in operation. First allow the system to stabilize.
Then observe the operation of the controller while raising and lower-
ing its set point. Pressure should increase when the set point is raised
and decrease when the set point is lowered. Use accurate pressure test-
ing equipment when checking out the controller. Do not rely on inex-
pensive gauges. The controllers are carefully calibrated at the factory.
If the motor or actuator runs in the proper direction when the set
point is adjusted, it can be assumed that the controller is operating
properly. If it runs in the wrong direction, reverse the B and W wires.
Observe the action of the motor to see if it stabilizes. If the motor is
moving constantly, widen the proportioning range a little at a time,
until the system is stable.
IF A CONTROLLER SEEMS
TO OPERATE IMPROPERLY (See Figure 18)
If the controller is suspected of operating improperly, it may be further
checked as follows.
1.
Leave the controller installed where it is, but disconnect all power
to the controlled motor.
2.
Loosen the cover screw below the main scaleplate and remove the
cover.
3.
Disconnect the wires from the controller.
4.
Connect an ohmmeter between controller terminals 8 and W to
measure the resistance of the potentiometer in the controller. The
ohmmeter should read about 135 ohms on an L91B.
OPTIONAL EQUIPMENT FOR CHROMALOX STEAM BOILERS
Adjusting Screw
Differential
Adjusting Screw
R
W
B
Figure 17
(L91B) Proportioning Pressure Control Used with Sequencer Control