ECM blower motors responding to static pressure--how?

[georgelass]

I understand Bryant Evolution systems and a certain Lennox,
(model G60UHV60C090X07) vary blower speed depending on,
I think, static pressure. For zoned systems, I believe these
units do not need bypass dampers. They can somehow sense
if one zone is open versus two or more, and then adjust blower
speed and CFM accordingly. I was hoping to determine whether
that sensing is done entirely by the blower motor electronics
or whether there is a sensor in the airstream somewhere and
maybe the main control board tells the blower motor to adjust.
Obviously, this knowledge would really help diagnosis in the event
a blower motor was responding inappropriately to static pressure.

For example, the motor slowing down to almost nothing every
time one of the zones opens and then speeds up when the
zone dampers are closed--I pulled the zone damper motor wires
in that case.

So this is not the typical ECM-motor-not-running problem. This motor
runs at various speeds. It just does not do it consistently. It USED
to do it fine, for years. Then it stopped. And then it supposedly started
working after I left the house when the homeowner called for
heat hours later.

Thank you for any enlightenment.

[perfectionist]

The way the motor knows how to adjust speed is done electronically. The motor communicates the RPM to the board and the board inturpets this for CFM. If the motor still ramps up and down I would look for changes in the duct/house that would affect the air flow.

[54regcab]

The motor determines amp draw and rpm then calculates CFM based on it's internal programming.

[georgelass]

So are you guys in agreement or at odds as to whether the board of the motor or the board of
the furnace would be the most likely cause of simply strange behavior? This unit stopped responding
appropriately a month ago, then started working fine, then, a month later, started acting weird
again, and then started working okay. The homeowner swears nothing is changing with respect
to registers or filters or ductwork. Nobody is touching anything. I visually checked the zone
dampers--peered at them via the open evap coil cover--so I know what they are doing. What I don't
know is how to troubleshoot these motors very well except for basic working/not-working. I have
a tester for that.

[Tony3696]

I had a similar issue a few months back on a York. Pull the ECM head off the back of the motor and look at the board. The Yorks have 2 capacitors in them and one of them was leaking. In some cases they sound quieter when a 2nd zone opens up but the blower speeds up. If that isn't the case, I would pull both boards and look at the back of to see if anything is fried/shorted. If you aren't 100% sure which board is bad, call tech support.

[rundawg]

So are you guys in agreement or at odds as to whether the board of the motor or the board of
the furnace would be the most likely cause of simply strange behavior?

The board in the motor ECM module is the cause of this behavior.

The motor, and the equipment manufacturers, develop tabular data to relate torque and RPM to mass airflow. This data is then stored in the ECM microcontroller, and used to control the motor to a constant airflow.