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Function
Principle of
self-supervision
Prerequisites for
burner startup
Building Technologies Division
Infrastructure & Cities Sector
In contrast to conventional amplifiers, the signal delivered by the flame detector is
handled dynamically and not statically. The flame detector signal is converted to a
sequence of control pulses and then fed to the flame relay circuit. The latter is designed
such that the flame relay can only be energized by a flame signal of the described form.
If the pulses change due to a faulty detector or faulty detector cables, the relay will be
deenergized and the burner control triggers the required safety actions. In the case of
UV supervision, it must also be ensured that self-ignition of the UV tube (e.g. due to
aging) does not simulate a flame signal. For that reason, incident radiation at the UV
cell is periodically interrupted by a shutter. In addition to the self-checking facility, the
flame signal circuit is subjected to a functional test during the prepurge time. If it does
not operate correctly, the startup sequence will be aborted or lockout initiated.
Furthermore, if mains voltage drops to a level where safe operation of the burner
control is no longer ensured, the burner will automatically shut down. When mains
voltage returns to the normal level, the burner control repeats the startup sequence. If
the detector signals are only slightly above the minimum levels, such mains voltage
fluctuations can also give rise to burner lockout however.
The burner control is reset and in the start position (terminals 11 and 12 must
-
receive power)
The air damper is closed. End switch (z) for the fully closed position must feed
-
power from terminal 11 to terminal 8
All control contacts between terminals 12 and 5 (limit thermostat, control
-
thermostat, etc.) must be closed
A
Start
When control thermostat or pressurestat closes, the burner control's sequence switch
starts running. At the same time, the fan motor connected to terminal 6 (only
prepurging) receives power and, on completion of switch-on delay time, the fan motor
or flue gas fan at terminal 7 (pre- and postpurging) also receives power. On completion
of interval (t16), the control command to open the air damper is given via terminal 9.
During the running time of the motor, the sequence switch does not operate, as
terminal 8, via which the motor of the sequence switch first receives power, is not live
during that period of time. The sequence switch starts again and programs only after
the air damper is fully open and end switch (a) has changed over to feed power to
terminal 8.
t1 Prepurge time with air damper fully open (nominal amount of combustion air)
Shortly after the start of the prepurge time, air pressure switch must change over, thus
interrupting the current path between terminals 4 and 13. Otherwise, the burner control
would go to lockout (start of air pressure check). At the same time, terminal 14 must be
live since this current path is used to power the ignition transformer and the fuel valves.
t3´ With the LOK16..., an ignition transformer connected to terminal 15 is therefore
switched on at this point in time (long preignition). If there is no air pressure switch, the
ignition transformer receives power already with the start command. On completion of
the prepurge time, the burner control via terminal 10 drives the air damper into the low-
fire position, which is determined by the changeover point of auxiliary switch (m).
During the positioning time, the sequence switch stops again until terminal 8 receives
power from «m».
t5 Interval
On completion of interval (t5), terminal 20 receives power. At the same time, control
outputs 9 to 11 and input 8 are galvanically separated from the unit's control section, so
that the latter is protected against reverse voltages from the load control circuit. The
startup sequence of the burner control ends with the release of load controller at
terminal 20. The sequence switch switches itself automatically off, depending on the
time variant used, either immediately or after some so-called idle steps, that is, without
changing the contact positions.
13/24
CC1N7785en
06.03.2012
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