OK, here goes.....
I spent some time thinking and researching. I found this engineering
info sheet on the net. Read it and weep (see attached pdf).
I have tried to post this for a week and I guess you can not attach files.
Maybe some virus protection or something. My post just won't show after
sending. Instead of a file attachment, here is the relevant bits of the
The Mystery of Fans
A family of motor applications that tend to confuse people,
who are not regularly involved with them, is that of variable
torque loads. These loads represent a high percentage of
motor requirements so it is desirable to have a little extra
knowledge of the mysterious aspects of these loads.
Variable speed loads are fans, blowers, and centrifugal
There are two mysterious characteristics that these loads
have. The first is the way they act when speed is changed.
The rules that cover these characteristics are called the
"affinity laws". For example, we can discuss a fan. What
we find is that as the amount of air increases, the torque
required to rotate the fan goes up as a squared function of
speed (Speed2). Thus, increasing the speed causes the
torque required by the fan to go up, not directly with
speed, but in proportion to the change of speed squared.
For example, if we change the speed from 1160 to 1760
RPM the torque required will increase by the ratio of
(1760/1160)2. This would mean that the torque required
would go up 2.3 times to 230% of the original value. Also,
since horsepower (HP) is based on speed times torque,
and the speed has increased by 52%, the new value of HP
would be 2.30 x 1.52 or almost 350% of the HP required at
the original speed.
The dramatic increases in the horsepower needed to drive
these loads when speed increases is a little difficult to
understand but it is very important. It is important because
small decreases can result in great energy savings. For
example, decreasing the speed of a variable torque load
by only 20% will result in a driving energy reduction of
nearly 50%. This obviously has big importance when conservation
is considered. It also accounts for the tremendous
market that exists for variable frequency drives operating
Variable Air Volume (VAV) systems used in heating,
ventilating, air conditioning, and variable speed pumping
used in similar systems.
The second puzzling thing that occurs with variable torque
loads is that the motor load actually decreases as the output
or input to the fan is blocked off or restricted. There
was once a reported call from a motor user who had burnt
out a motor driving a blower on a heating system. The
motor was driving a blower that drew air through a filter
and fed it to a ducted distribution system. When they were
asked if there had been any changes in the system, he
stated, "Well, we extended the ducts into another room
and cut the end off to let the air flow, but that would have
made it easier for the motor not more difficult." When he
was told the opposite was true, he could not believe it. It
defies good judgement to think that adding a restriction to
the output of the blower would decrease the motor load. If
you don't believe it, here is a simple test. Take a vacuum
cleaner and listen to it carefully. You will notice that the
pitch of the motor goes up when the suction is closed.
What this means is that the load is being reduced on the
motor and it speeds up. If you still do not believe, you can
do the same test but with an ammeter on the motor. What
you will find is that the amps drop as the suction level is
section on page two under the mystery of fans, fourth paragraph "the motor
load actually decreases as the input or output is restricted." The apaprent
reason is that horepower is based on work done. If you are moving less air,
you are doing less work. We interpret that the fan is pushing harder to
move the air but it is rather provided less air to move and so works less to
maintain the same rpm. You basically get a fan that is spinning air around
in circles while letting some out to play with our fire.
Also, my exhaust blower on the roof of my shop is a 5 hp centrfugal
blower like the one Charles found. It is baffled after the blower and
before the venturi input device attached to the exhaust stack. My sales rep
at Quickdraft was the one who put me onto the efficiancy concept in the
first place because I was concerned at the energy usage.
I was thinking about it and of course the shut off on a organ would be
at the output as well.
Another thing is that these motors are all constant rpm motors. A non
controlled motor such as one in a vacuum cleaner will run faster when you
cut the input. I tried holding my hand over the output and heard the same
increase proving that the load is affected similarly via input or output
blocking. I also see this principle at work with my box fan that runs
overfast when placed against a wall.
OK ok I am done. I just have been thinkiong about this a lot while
driving to and from the most recent job we installed. The pictures are
crappy but the work was great. I will post a few on my site in a few
Let me know whats wrong with my reasoning when you get the chance.