750-392
CBEX-DE
3-19
Low Gas Pressure Switch
3.10.3 — Modulating Motor
The modulating motor has a 90º shaft rotation. The motor manufacturer also provides a 160º stroke model for
other applications. If a replacement is obtained from someone other than a Cleaver-Brooks Service or Parts rep-
resentative, it may have an incorrect stroke. To prevent damage, determine the 90º stroke prior to installing a
replacement.
The stroke may be determined by powering the motor and connecting terminals R-B to actually determine the
stroke as the motor drives to an open position.
The modulating motor contains either one or two internal switches, depending upon application. The micro
switches are actuated by adjustable cams attached to the motor shaft.
Factory replacement motors have the cams preset. The low-fire start switch is set to make the red and yellow
leads at approximately 8º on motor closing. The high-fire purge air proving switch (located in the modulating
motor) is set to make red and blue tracer leads at approximately 60º on motor opening. Normally, the settings
are left as is, but job conditions may require readjustment. If the cams require adjustment or resetting, follow the
instructions in the manufacturer’s technical manual.
3.11 — Low Gas Pressure Switch
Adjust the scale setting to slightly below the normal burning pressure. The control circuit will be broken when
pressure falls below this point. Since gas line distribution pressure may decrease under some conditions, shut-
downs may result if the setting is too close to normal. However, regulations require that the setting may not be
less than 50% of the rated pressure downstream of the regulator. Manual resetting is necessary after a pressure
drop. Press the reset lever after pressure is restored.
FIGURE 3-6. Gas Pressure Switches
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