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Multiple Setting Modulating Motor
In some burner configurations, there are different ideal
settings for oil and gas firing, especially when higher
turndown is desired. This can be accommodated with an
optional modulating motor that has different low fire and
high fire positions for gas vs oil.
This optional modulating motor uses 4 to 8 internal
switches. One switch is used to prove the high fire purge
position during pre-purge. A second is used to prove the
fully closed position. This is the position of the motor
when the burner is off. A third switch is used to prove the
Figure B-18 Siemens Mod Motor Adjustments
ignition position – the point at which the burner will light.
A fourth switch is the low fire position. This is the position
of lowest firing. It can be different from the ignition
position. If the burner is a combination gas-oil burner,
two additional switches may be used. These do the
same function as the third and fourth switches already
listed, but can be set up to allow for different ignition and
low fire positions for gas and oil operation. There is also
a 7th and 8th switch that can be used to accommodate
two different high fire settings. See the burner wiring
diagram to determine switch numbers and functions.
Oil Limiting Potentiometer
The fan is sized for air at rated capacity plus the quantity
of FGR required for gas NOx emissions. When firing oil,
the FGR rate is usually reduced, providing a larger fan
capacity than desired. To prevent the burner from over
firing on oil, a limiting potentiometer is used to limit the
oil rate. In this mode, the modulating motor is restricted
in its travel to something under 90
This potentiometer is located in the control panel and is
adjusted at startup to provide the correct oil firing rate.
Parallel Positioning System (Linkageless)
The Posi-Control system is a parallel positioning system
(linkageless) that uses individual actuators for each
control valve and a computer controller that directs each
actuator to provide the input change from minimum to
maximum capacity. The control provides more flexibility
in setting each fuel rate (Figure B-2).
JBE(X) Manual
Removable
Cover
Drive
Arm
Modulating
Motor
o
.
Page 15
8. Electrical Controls
Control Panel
The control panel (Figure B-6) contains the flame
safeguard control, relays, terminal strips for electrical
connections and other components required for unit
control. Other components may be included for operation
of the boiler – a low water cutout relay, for example.
Flame Safeguard
The flame safeguard (Figure B-6) provides operational
control and safety sequencing for the burner. Safety
limits are tied to the unit and it controls the operation of
the fuel valves. The flame scanner is part of this and can
detect a flame failure, causing a shutdown. There are
several different flame safeguards available with different
features and costs. They can provide fault annunciation
and communications with other controls. The details of the
control used in the burner are supplied with the unit.
On-Off Switch
This switch is used to start and stop the burner by opening
or closing the limit circuit to the flame safeguard control.
Manual-Auto Switch and Potentiometer
The Man-Auto switch is used to select which signal source
is used for modulation control of the burner. With the switch
in the "Man" position, the burner firing rate is determined by
the position of the manual potentiometer. With the switch
in the "Auto" position, the burner firing rate is determined
by the signal from the boiler modulating controller. When
in the "Auto" position, the manual potentiometer can limit
the firing rate of the burner from anywhere between low fire
and high fire. The modulating motor will always drive open
and closed during pre-purge, regardless of the position of
the Man-Auto switch and potentiometer.
Fuel Transfer Switch
This switch selects the proper fuel for firing. It has a center
"off" position that prevents it from moving from one position
to the other, without stopping in the "off" position first.
Power On light
Indicates that power is applied to the control panel.
Call For Heat light
Indicates the burner On-Off switch is closed and the boiler
limits are closed.
Fuel On light
Indicates the main fuel valve circuit has been energized.
Alarm light
Indicates the flame safeguard control is in a safety
shutdown and lockout condition. The flame safeguard
control reset button must be pressed before the burner can
operate again. On some burners, the Alarm light may also
be used to indicate other failure conditions such as low
water or high limit. See the wiring diagram for details on
what other controls may be wired to the Alarm light.
Section B - Components
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