CHAPTERS
D.
MODULATING
MOTOR
SWITCHES
-
LOW
FIRE
AND
HIGH
FIRE
The modulating motor contains either one
or
two
internal switches depending upon application.
These
single-pole,
double-throw,
micro-
switches are actuated by adjustable cams
at·
tached to the motor shaft.
Factory replacement motors have the cams
preset. The low fire start switch
1s
set to make the
red and yellow leads at approximately
s·
on the
motor closing. The high tire purge air proving
switch (M954B motor)
is
set to make red and
blue tracer leads at approximately
60. on motor
opening. Normally these settings are left as
is
but
job
conditions may require readjustment.
If
the cams require adjustment
or
resetting, fol-
low the instructions in the manufacturer's Tech-
nical Manual.
E.
BURNER
OPERATING
CONTROLS
-
GENERAL
In general, when adjusting controls check
to
see
that they are level, especially those containing
mercury switches.
On temperature sensing con-
trols, make sure that the bulb
is
properly bot-
tomed in its well and that connecting tubing
is
not kinked.
Controls are carefully calibrated during their
manufacture
and
normally
do
not
require
recalibration. The dial settings generally are
quite accurate although it
is
not
unus~al
to have
a slight variation between a scale
setting and an
actual pressure gauge
or
the~ometer
readi~g
and to readjust the control settmg to agree wtth
these readings. This
is
predicated, however, on
pressure· gauges and thermometers being ac-
curate.
75()...150
ADJUSTMENT
PROCEDURES
Burner controls properly set to match load
demands will provide operational advantages
and achieve the following desirable objectives:
(a)
The
burner will operate in low fire
position prior
to shut down.
(b)
The
burner will operate at low fire for
a brief period on each start during normal
operation.
(c) Frequent bumeron-offcycling will be
avoided.
Figure 5
..
3 depicts a typical relationship
of
the
setting
of
the operating limit control
.an~
the
modulating control.
Please note that
thlS
IS not
drawn to any scale. The burner will be
"on"
whenever the pressure
or
temperature
is
below
point B and
"off'
whenever pressure
or
tempera-
ture
is
above point A The distance between
points
A and B represents the "on-off'' differen-
tial
of
the ·operating limit control.
In normal operation, the burner will shut down
whenever the pressure
or
temperature reaches
setting A. The switch in the operating limit
con-
trol will open. As the pressure
or
temperature
drops back
to B, the switch closes and the burner
will restart. The modulating control will
be
call-
ing for the modulating motor to be in
lo~
fire
position at this point.
If
the load exceeds this low
fire input, the modulating control will respond
to
increase the firing rate proportionately as pres-
sure
or
temperature falls toward the point D. The
modulating motor will stop at any intermediate
point between C and D whenever the fuel input
balances the load requirement.
As the load re-
quirement changes, the firing rate will change
accordingly. This
is
referred to as modulated
firing.
Point D represents the maximum firing rate
of
the
burner. In the event pressure
or
temperature
drops while the burner
is
firing at
its
maximum
input, this
is
indicative that the load exceeds the
generating rate
of
the boiler.
CHAPTER 5-S
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