Secondary Limit; Blower Motor; Internal Operation; Initial Power Up - Lennox G71MPP SERIES Unit Information

B−Blower Compartment Figure 7
Power Choke

Blower Motor

(5 Ton Only)
To Remove Blower From Unit: Remove Integrated Control,
Door Interlock Switch, Bolts and Wiring Jackplugs.
Then Slide Out Front of Unit.
FIGURE 7
1. Secondary Limit (S21)
The secondary limits (S21) on G71MPP units are located in
the blower compartment on the back side of the blower hous-
ing. See figure 7. All G71MPP units are equipped with two sec-
ondary limits. 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.
2. Blower Motor (B3)
WARNING
During blower operation, the ECM motor emits ener-
gy that may interfere with pacemaker operation. In-
terference is reduced by both the sheet metal cabinet
and distance.
The G71MPP line uses two different motor sizes, a 1/2 HP
and 1 HP. The motor communicates with the integrated con-
trol via a 2−way serial connection. The motor receives all nec-
essary functional parameters from the integrated control and
does not rely on a factory program like traditional variable
speed motors. The wiring harness connecting the motor to
the integrated control includes a resistor which sets the motor
parameters the integrated control will use. See wiring dia-
gram regarding wiring harness. G71MPP units use a three-
phase, electronically controlled D.C. brushless motor (con-
troller converts single phase a.c. to three phase D.C.), with a
permanent-magnet-type rotor (figure 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 8. 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.
SECONDARY
LIMITS (2)
(B3)
IMPORTANT
Earlier ECM motors used on other Lennox furnace
models are not interchangeable with motors used on
the G71MPP furnace line.
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).
G71MPP BLOWER MOTOR COMPONENTS
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 G71MPP blower motors use single phase power. An
external run capacitor is not used. The motor uses per-
manently lubricated ball-type bearings..

Internal Operation

The motor is controlled via serial communication between
the ignition control on the furnace and the controller at-
tached to the motor shell. The messages sent back and
forth between the two controls serve to communicate rota-
tional direction, demand, motor size, current draw, torque,
and rpm, among other variables.
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.

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.
Page 28
STATOR
(WINDINGS)
BEARING
OUTPUT
SHAFT
ROTOR
FIGURE 8