On 6/9/15 3:38 AM, Uncle Monster wrote:
Manufacturers install a lower speed fan motor to keep the noise down
but use a fan blade with a higher pitch angle to get the same airflow
that would be obtained with a high speed fan motor and a blade with a
lower pitch angle. You can change the blade to match the motor and
the load on the motor will drop into an acceptable range. That's why
the current draw of fan motors should always be checked. If your
service tech installed a higher speed motor and used the fan blade
for the low speed motor, that will be the cause of the overload. No
competent service tech will install a mismatched motor and blade and
if he'd checked the current draw, he'd know something was wrong.8-)

[8~{} Uncle Blade Monster

In 1976, a church with high gas bills asked me to look at their
furnaces. There were six. I think they all blew into the same duct
system.

Each blower motor was 1/3 hp with an adjustable pulley. I think at
least one wasn't blowing at all.

It wasn't wired to make it easy to check current. How else could I set
the pulleys? I mounted an electrical box by each blower compartment with
a motor fuse and a neon light in parallel. Now each motor was fused, and
anyone walking by could spot a blown fuse by the neon light.

I bought an AC ammeter movement and mounted it in a portable box. I
could plug the box in, in place of a neon light. After the motor was
running, I could pull the fuse and read the current. That way, my meter
didn't have to handle the starting surge.

This allowed me to set each pulley for 1/3 hp. It had been necessary to
raise the thermostat 24 hours ahead of services, and people would still
be chilly. Without sufficient air flow, the heat was going to the
cathedral ceiling. With the pulleys adjusted, the furnace could warm
the church in an hour or so.