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B−Blower Compartment
Access panels can easily be removed for service.
1−Blower Wheel
Blower wheel size varies between models. See SPECIFI-
CATIONS.
2−Variable Speed Motor (B3)
15CHAX units use a three-phase, electronically controlled
D.C. brushless motor (controller converts single phase a.c. to
three phase D.C.), with a permanent-magnet-type rotor (fig-
ure 6). Because this motor has a permanent magnet rotor it
does not need brushes like conventional D.C. motors.
Internal components are shown in figure 7. The stator wind-
ings are split into three poles which are electrically connected
to the controller. This arrangement allows motor windings to
turn on and off in sequence by the controller.
A solid-state controller is permanently attached to the
motor. The controller is primarily an A.C. to D.C. convert-
er. Converted D.C. power is used to drive the motor. The
controller contains a microprocessor which monitors
varying conditions inside the motor (such as motor work-
load).
FIGURE 6
BLOWER MOTOR B3
MOTOR
CONTROLLER
SHAFT
J49
J48
BLOWER MOTOR COMPONENTS
FIGURE 7
STATOR
(WINDINGS)
OUTPUT
SHAFT
BEARING
ROTOR
The controller uses sensing devices to sense what position
the rotor is in at any given time. By sensing the position of the
rotor and then switching the motor windings on and off in se-
quence, the rotor shaft turns the blower.
All blower motors use single phase power. An external
run capacitor is not used. The motor uses permanently
lubricated ball-type bearings.
Internal Operation
Each time the controller switches a stator winding (figure 7) on
and off, it is called a pulse." The length of time each pulse
stays on is called the pulse width." By varying the pulse width ,
the controller varies motor speed (called pulse-width modula-
tion"). This allows for precise control of motor speed and allows
the motor to compensate for varying load conditions as sensed
by the controller. In this case, the controller monitors the static
workload on the motor and varies motor rpm in order to main-
tain constant airflow (cfm).
The motor controller is driven by the blower control
board (figure 4). The board receives its demand (PWM
signal or fixed 24 VAC or VDC signal) from optional controls
such as the Harmony zone control system, Lennox Signatu-
reStat
t
or a conventional thermostat.
Motor rpm is continually adjusted internally to maintain
constant static pressure against the blower wheel. The control-
ler monitors the static work load on the motor and motor amp-
draw to determine the amount of rpm adjustment. Blower rpm
may be adjusted any amount in order to maintain a constant
cfm as shown in Blower Ratings Tables (1, 2 and 3). The cfm
remains relatively stable over a broad range of static pressure.
Since the blower constantly adjusts rpm to maintain a specified
cfm, motor rpm is not rated. Hence, the terms cool speed" ,
heat speed " or speed tap" if used in this manual, on the unit
wiring diagram and on blower B3, refer to blower cfm regard-
less of motor rpm.
When Harmony is used, speed taps are overridden and a
PWM signal generated by the Harmony controller continuously
varies motor speed based upon zone demands.
Initial Power Up
When line voltage is applied to B3, there will be a large inrush
of power lasting less than 1/4 second. This inrush charges a
bank of DC filter capacitors inside the controller. If the discon-
nect switch is bounced when the disconnect is closed, the dis-
connect contacts may become welded. Try not to bounce the
disconnect switch when applying power to the unit.
Motor Start-Up
When B3 begins start-up, the motor gently vibrates back and
forth for a moment. This is normal. During this time the elec-
tronic controller is determining the exact position of the rotor.
Once the motor begins turning, the controller slowly eases the
motor up to speed (this is called soft-start"). The motor may
take as long as 10-15 seconds to reach full speed. If the motor
does not reach 200rpm within 13 seconds, the motor shuts
down. Then the motor will immediately attempt a restart. The
shutdown feature provides protection in case of a frozen bear-
ing or blocked blower wheel. The motor may attempt to start
eight times. If the motor does not start after the eighth try, the
controller locks out. Reset controller by momentarily turning off
power to unit.
The DC filter capacitors inside the controller are connected
electrically to the speed tap wires. The capacitors take
approximately 5 minutes to discharge when the disconnect
is opened. For this reason it is necessary to wait at least 5
minutes after turning off power to the unit before attempt-
ing to change speed taps.
