"Home Guy" wrote in message ...

As someone who's been living with and has experienced HVAC systems with
single-speed AC fan motors, I really can't appreciate the need for a
variable-speed fan motor. All this discussion about how PSC motor
efficiency drops to 15% - 30% when used at low speeds is a real mystery
to me - are there really furnaces out there that have the necessary
electronic controllers that will use PSC motors in such a variable-speed
capacity? Why no real discussion about the efficiency of 2-speed AC
motors?

PSC blower motors are not really multispeed at all. All the extra speed taps
are are taps on what acts as an internal autotransformer. High is rated
voltage and the lower speeds just effectively undervolt the motor. Run a
typical 1050 RPM 6 pole PSC blower motor on high sometime and measure the
voltage from the low tap to neutral, it is usually around 170 volts AC
depending on the motor.

There are true 2 speed induction blower motors that are wound as both a 4
pole and a 6 pole. The high and start windings are 4 pole so the motor
always starts on high but if it is hooked up as low it switches the power
from high to low as it starts using the same centrifugal switch that cuts
off the start windings. When not running these motors will show a direct
short between the 2 speed wires.

I also don't understand how running a fan at low speed is better at
humidity removal when the HVAC system is in A/C mode - yet ECM makers
make that claim.

Less airflow allows the evaporator to run colder hence it cools the air more
and the leaving air has a lower dew point hence a lower humidity % when
warmed back up by the house/building..