CHAPTERl
GENERAL DESCRIPTION AND PRINCIPLES OF OPERATION
Q.
MODULATING
FIRING
The modulating motor, through a linkage arran-
gement, controls the air damper and the butterfly
gas
valve or the oil metering valve to maintain a
·constant air-fuel ratio throughout the firing
range.
During burner operation, the motor
is
controlled
by
a modulating pressure control on a steam
boiler, or
by
a modulating temperature control on
a hot water boiler. A manually operated
poten-
tiometer is provided to permit the positioning
of
the motor at a desired burner firing rate. This
is
used primarily for initial
or
subsequent checking
and setting
of
fuel input. Normal operation
should be with the manual-automatic switch
in
the "automatic" position and under the control
of
the modulating control.
The modulating motor (commonly called a
·
damper motor) is reversible.
It
has an internal
limit switch that restricts shaft rotation to
90".
During nonnal operation the motor will move in
either direction
or
stop at any position within this
range.
The motor's potentiometer
is
electrically con-
nected to a
matching
potentiometer
in the
modulating control. Changing steam pressure or
water temperature alters the electrical resistance
CHAPTER 1
~22
of
the modulating controller's potentiometer.
This change
in
resistance compels an integral
balancing relay to start, stop,
or
reverse the motor
rotation. Rotation
in
either direction continues
until the resistance ratio
of
the two poten-
tiometers are equal.
When this occurs, the motor stops in a position
that allows the proper fuel and combustion air
flow
to meet operating demands.
A feature designed into the circuitry
is
that the
modulating motor must
be
in the low fire position
during ignition and remain there until the main
flame
is
established. A low fire switch, integral
to
the motor, is actuated by the rotation
of
the
motor. The switch must
be
closed to establish that
the damper and fuel metering valves are
in
low
fire position before the programmer commences
into the ignition period. During this time, neither
the manual flame control nor the modulating
control have any control over the damper motor,
regardless
of
their setting.
An optionally equipped boiler has a second in-
tegral switch used to
es~blish
th~t
.the
mo~or
has
driven the damper
to
an open
pos1t1on
dunng
the
pre-purge period. This switch closes, as high fire
position
is
approached,
to
complete an internal
circuit in the programmer and allow continuation
of
the programming cycle.
750-150
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