Download INDUCED DRAFT BLOWER MOTORS

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INDUCED DRAFT BLOWER MOTORS
PURPOSE;
The induced draft blower assembly
(fig.1) provides combustion air to the
furnace burners. 80% furnaces do not
need as powerful of a motor to drive the
induced draft blower motor assembly as
90% efficient models. 80% efficient
furnace models are draft furnaces and
use a shaded pole motor. 90% efficient
models use a PSC type of motor because
the furnace must use the induced draft
blower motor assembly to provide both fig. 1
combustion air and vent the products of
combustion. The motor will take the furnace through a pre-purge cycle, trial for
ignition cycle, burner cycle, post purge cycle and will be energized any time there
is a call for heat.
A TYPICAL INDUCED DRAFT MOTOR CIRCUIT
In most cases, the motor is energized by the
furnace IFC board. As you can see in this
wiring diagram (fig.2) of a typical furnace
induced draft motor circuit, the induced draft
motor receives 115 volt potential on the IND
H terminal of the IFC. The motor’s neutral
lead is connected to the IND N terminal.
There should be 115 volt potential present at
these terminals anytime there is a call for
heat.
If the furnace is a two stage model, the
induced draft motor will be a multispeed model with a low and high speed
tap (Fig.3). The IFC will control the
speed of the motor by either applying
115volts to the high speed tap or low
speed tap depending on which stage of
heat is being called for.
fig. 3
fig. 2
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INDUCED DRAFT BLOWER MOTOR CHECK
TOOL NEEDED;
MULTIMETER
PROCEDURE; SINGLE SPEED PSC MOTOR
1. Disconnect power to the furnace and make sure the induced draft motor is
cooled down to room temperature before beginning. IF THE MOTOR IS HOT,
THE MOTOR’S INTERNAL OVERLOAD MAY BE OPEN. ALLOW UP TO 2
HOURS FOR THE INTERNAL OVERLOAD TO RESET.
2. Unplug or disconnect the induced draft motor electrical terminals or leads from
the motor. The lead terminals may be at the motor, IFC board, or relay.
3. Using an ohmmeter, check for
resistance between the common and
run Windings, and between
common and start winding of the
induced draft blower motor. You
should measure resistance between
each winding (fig.4). If you
measure infinite resistance,
(AFTER A REASONABLE COOL
DOWN PERIOD) from both
common to run and common to
start windings, the internal overload
is open and the motor must be
replaced.
fig. 4
4. Check the resistance from the run
and start motor winding leads to the
motor case (fig.5). You should
measure infinite resistance. If you
measure any amount of resistance,
the motor is shorted to ground.
Replace the motor. If all of the
motor tests check out OK, proceed
to the next step.
5. Connect the induced draft motor
leads to their correct terminals on
the relay or IFC board.
fig. 5
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6. Using a voltmeter, check for 115
volts to the induced draft motor.
(fig.6). If the furnace uses a relay to
energize the induced draft motor,
check for 115 volts at the relay
terminals. If the furnace has a IFC
board that controls the motor, check
for 115 volts at the IFC board inducer
motor terminals. (fig.7) If no voltage
is present, check for any other safety
circuit lockout conditions that may be
preventing the IFC from allowing the fig. 6
induced draft motor from energizing.
fig. 7