GF - B 2
Induced Draft Motors
Introduction
Late model *UY/DY and all *UH/DH, except UH1/DH1,
furnace models feature variable speed three phase in-
duced draft blowers. These motors receive three phase
non-variable frequency/voltage signal from the furnace
control board to achieve optimum system performance.
The furnace control board (Figure 1) will increase fre-
quency and voltage to increase the motor speed. The
service technician cannot measure this voltage accu-
rately.
Figure 1
Electrical circuit
The induced draft motor receives voltage from a 4
pin plug located on the furnace control board. A 4
pin connector connects the induced draft blower
motor to the furnace control board and a separate
connector connects the three phase motor to the elec-
trical harness (Figure 2.)
Figure 2
Sequence of operations
This vent motor is used on multi-stage furnaces. The
furnace control board will run a learning routine to
determine the best induced draft blower speed to
operate at based upon the vent and intake pipe pres-
sure drop. The furnace control will monitor the posi-
tion of the furnace pressure switches during the learn-
ing routine and vary the speed of the induced draft
blower to open and then re-close the pressure
switches. This process allows the furnace to learn
how slow it can run the induced draft blower, yet keep
the pressure switches closed.
Once the furnace control has performed the learning
routine, the furnace control will increase induced draft
blower motor speed if it detects a pressure switch
has opened during run operation. If the motor can-
not close the open switch, the furnace control will
make the necessary decision whether to revert to
first stage operation only, or whether to shut the fur-
nace off and lock out. A detailed operating sequence
follows:
First stage learning routine
This induced draft blower motor is a three phase
variable speed type; its speed will be adjusted to
optimize system performance. The motor performs
Variable Speed Three Phase Induced Draft Motors
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