Blower Compartment; Secondary Limit; Internal Operation - Lennox G61MPV Service Manual

B−Blower Compartment Figure 7
Power Choke
Blower Motor
(5 Ton Only)
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
FIGURE 7
Secondary Limt (S21)
The secondary limits (S21) on G61MPV units are located in
the blower compartment on the back side of the blower hous-
ing. See figure 7. All G61MPV units are equipped with two sec-
ondary limts. When excess heat is sensed in the blower
compartment, the limit will open. If the limit is open, the furnace
control energizes the supply air blower and closes the gas
valve. The limit automatically resets when unit temperature re-
turns to normal. The switch is factory set to open at 125°F
and cannot be adjusted.
Blower Motor (B3)
G61MPV 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 8). Because this motor has a permanent magnet rotor it
does not need brushes like conventional D.C. motors.
Internal components are shown in figure 9. 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. con-
verter. Converted D.C. power is used to drive the motor.
The controller contains a microprocessor which moni-
tors varying conditions inside the motor (such as motor
workload).
G61MPV BLOWER MOTOR B3
SHAFT
MOTOR
CONTROLLER
FIGURE 8
SECONDARY
LIMITS (2)
(B3)
J48
J49
Page 39
G61MPV BLOWER MOTOR COMPONENTS
FIGURE 9
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 G61MPV blower motors use single phase power. An
external run capacitor is not used. The motor uses per-
manently lubricated ball-type bearings.

Internal Operation

Each time the controller switches a stator winding (figure 9) 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
(figure 11), the controller varies motor speed (called pulse-
width modulation"). 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 control-
ler monitors the static workload on the motor and varies motor
rpm in order to maintain constant airflow (cfm).
The motor controller is driven by the Two−stage Variable
Speed Integrated control board. The board receives its
demand (PWM signal or fixed 24 VAC or VDC signal) from
optional controls such as the Harmony zone control system,
Efficiency Plus Humidity Control (CCB1) 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. The cfm remains rel-
atively stable over a broad range of static pressure. Since the
blower constantly adjusts rpm to maintain a specified cfm, mo-
tor rpm is not rated. Hence, the terms cool speed" , heat
speed " or speed tap" in this manual, on the unit wiring dia-
gram and on blower B3, refer to blower cfm regardless of mo-
tor rpm.
When Harmony is used, speed taps are overridden and a
PWM signal generated by the Harmony controller continuous-
ly varies motor speed based upon zone demands.
STATOR
(WINDINGS)
BEARING
OUTPUT
SHAFT
ROTOR