Trane ACONT900AC43UA Service Manual page 178

Silicon Nitride Ignitor
Ignitor Learning Routine
Figure 1
The ignitor is constructed of a tungsten heater ele-
ment and silicon nitride ceramic insulators. The volt-
age rating of the ignitor is 80 volts R.M.S., which is
less than the line voltage applied to the furnace; there-
fore, the control will reduce the number of cycles it
supplies to the SiNi ignitor per second. The control
checks the line voltage constantly and when a call
for heat is received the control sets the number of
cycles it is going to supply to the ignitor based on the
line voltage it is reading. This reduced number of
AC sine wave cycles (Figure 2) per second reduces
the effective voltage, R.M.S., applied to the SiNi igni-
tor per second. If the line voltage is low, the number of
cycles will go up, or if the line voltage is high, the
number of cycles is reduced.
Figure 2
GF - E 2
Number of cycles per second affects the voltage or
voltage R.M.S.
R.M.S. = Root--Mean--Square = The value assigned
to an alternating current or voltage that results in the
same power dissipation in a fixed resistance as DC
current or voltage of the same numerical value.
One or more cycles are turned off by the IFC control
to control the R.M.S. voltage supplied to the silicon
nitride ignitor (Figure 3.)
Figure 3
Learning Process
After each successful ignition, the number of AC sine
waves cycles are reduced. The lower number of AC
sine waves reduces the effective voltage applied to
the ignitor, and this lowers its maximum tempera-
ture. After each successful ignition attempt, the
burner lights, then the number of cycles are reduced.
During this learning program the number of cycles
will become too low and the gas fails to ignite (Figure
4.)
Figure 4
A retry is initiated by the IFC and the number of AC
sine wave cycles is raised four steps up to increase
the ignitor temperature.