"Some Guy" wrote in message ...
Alan Sung wrote:

From what I can tell, all the electronics are inside the motor.
You hook up the main AC supply directly to the motor, just like
a conventional AC motor.

I don't think he was talking about the electronics of the motor.
Conventional motors were either ON or OFF and coincided with heat
or cooling on or off with a thermostatic delay. The newer ECM motors
have a control panel

As part of OEM equipment built into a furnace, an ECM motor will be
connected to a control panel.

As a replacement part bought over-the-counter, they most probably will
not come with any such panel.

There will most certainly be more connections on them for wires other
than household 120 AC power.

There may also be dip-switches or jumpers on them. Presumably it
would also come with a data sheet or wiring diagram.

that changes the speed of the blower but at the same time
controlling the furnace heat output or the A/C compressor
speed.

An HVAC maker can choose to use an ECM motor as a single-speed, or
maybe 2 speed, or continuously variable speed while at the same time
altering the heat output of the furnace (or changing the speed of the
compressor for AC) if they want to modulate the heat output of the
furnace and modulate the blower fan speed at the same time.

But it's still the case that an ECM motor is touted as being more
efficient *at all speeds and loads* compared to a PSC motor.

So even if I just use an ECM motor as a drop-in replacement for a PSC
motor, and wire the ECM motor for single-speed operation (which
theoretically shouldn't require an external controller board), then I
should realize some savings on my electrical bill.

And if I have the knowledge, I can rig my own control method such that
I can vary the fan speed at will. I could, for example, rig a pot or
a switch beside my thermostat so when I'm running the fan only for
circulation, I can turn down the speed (and realize even more savings
when compared to my PSC motor).

I don't think you'd get the full benefit by just switching motors
if it didn't match the control panel, burner and compressor.

If my current HVAC setup is operating such that I would not gain any
increase in "comfort" by having a variable-speed blower fan, then a
variable-speed motor would not be of any interest to me.

However, ECM refers to a particular type of motor technology. ECM
does not defacto mean variable speed. PSC motors can also be rigged
for variable speed operation too (with the appropriate external
controller).

The issue of a controller board keeps cropping up in this thread, and
it's a red herring.
There is obviously a lot of variation in ECM systems. My son's furnace uses
an ECM motor which is first rotated at a slow speed, then powered off and
the coast-down is timed to determine the air resistance of the duct system.
From that information the controller determines the RPM necessary for the
desired airflow and controls the motor at that airflow. The programmable
parameter is CFM instead of RPM.

When it went bad, the wholesale cost of the complete motor was about
$900.00. We located a replacement control module (which was a part of the
motor) for about $400.00. This controller board (and motor) is absolutely
required for this system to function as designed. Replacing the motor with a
different type would also require replacement of the entire furnace
controller board since the furnace controller depends on the RPM data from
the motor for its operation.

Don Young