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A component, transformer and relay mounting control
panel is located along the right front control panel, along
the right side of the exterior jacket panel. This panel
contains a 75 VA transformer to drop 120 VAC to 24 VAC
for internal control operation, an optional electronic low
water cut-off, VFD, switching relays (gas valve relay,
pump relay, louver relay and VFD power relay), main
control circuit board, and ignition module.
VARIABLE FREQUENCY DRIVE
FIG. 47 Variable Frequency Drive
This appliance uses a variable frequency drive to control
operation of the combustion air blower motor. The variable
frequency drive is supplied with 120 VAC power. The
blower motor operates on 230 VAC 3 phase power. This
three phase voltage is generated by the variable frequency
drive (VFD) and supplied directly to the blower motor.
The variable frequency drive receives a signal from the
temperature controller based on water temperature to vary
the frequency of the voltage supplied to the blower motor
from 15 Hz up to 60 Hz. This varies the output of the
combustion air blower from 25% up to 100% of capacity
corresponding to the same variation in burner input. The
output from the temperature controller to the variable
frequency drive ensures that combustion air and gas are
always supplied in the proper ratio for clean combustion.
The variable frequency drive is driven to 100% during the
pre-purge portion of the start-up sequence. The variable
frequency drive is then provided a signal to operate at 50%
for initial burner ignition. After main burner ignition is
established, the temperature controller will use the variable
frequency drive to vary the blower speed based on desired
water temperature setpoint, the variation in actual water
temperature from the desired setpoint and the various
operating characteristics programmed into the control's
software.
When removing the variable frequency drive from the
appliance, disconnect the power wires to the combustion
air blower at the terminals on the variable frequency drive.
Do not disconnect the power wires at the blower motor.
Note the marking of the wires for proper reinstallation of
the three phase power wires to the VFD. Both the wires
and the VFD terminals are marked for proper location of
wire terminals. Incorrect installation of the wires may
reverse rotation of the blower motor resulting in major
operational problems or non-warrantable component
failures.
CAUTION:
The voltage output from the variable frequency
drive to the combustion air blower is 230 volt 3
phase. Avoid contact with high voltage wiring.
LOW AIR PRESSURE SWITCH
FIG. 48 Low Air Pressure Switch Location
A differential air pressure switch is used to prove operation
of the combustion air blower. The pressure switch sensing
points are installed across the point of pressure drop as the
air moves into the inlet of the burner. This switch measures
pressure drop points similar to those used by the ratio gas
valve to adjust manifold gas pressure. Correct differential
pressure across the sensing points of the pressure switch
proves operation of the combustion air blower to the
control. A fault alarm LED for Low Air will be indicated
in the Diagnostic Information Center and the appliance will
shut down operation when the differential pressure switch
detects a sustained low air condition.
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