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nuckolls.bob(at)aeroelect
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 Posted: Sat Jul 14, 2012 1:35 pm Post subject: Hints for recycling furnace blowers . . . |
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For years, I've used salvaged the guts of furnaces
to fabricate utility blowers for carpet drying,
workspace comfort, paint drying, etc. Dr.
Dee occasionally uses an energetic blower outdoors
when gardening in hot, low-wind weather.
These blowers are compact (motor mounts inside
the blower wheel) and often include some variable
speed options. They can be had for very little
from HVAC companies that change out old furnace
and a/c units. I just picked one up at a garage
sale for $20 where the owner had already mounted
it to a base and fitted with a carrying handle
and line cord.
Most DIY conversions I've seen in the wild do
not account for the fact that these blowers
are designed to push LOTS of air through a maze
of ducts. I.e., they are intended to do more
WORK than just move xx cubic feet of air per
minute. There is back-pressure of the duct work
to consider.
Referring to an exemplar speed vs. torque and
current curve . . .
We can see that there's a 'hump' in motor's torque
curve. We also see that rated operating characteristics
for the motor are on the right-side of that hump.
This means that for the motor to start up and
accelerate past the hump, the load on the
motor MUST be less than the 'break down'
torque else the thing won't make it up the
hump to it's operating point.
I knew that the blower I just bought was
exceedingly crippled in performance . . . it's
outlet port was wide open. Sure 'nuf. Got it
home, plugged it in . . . it did indeed move
some air. The snap-on ammeter read 4.5 amps.
But operated completely open loop, it gulped
in and tried to accelerate too much air.
I took a scrap of plywood and blocked off
about 1/3 of the outlet. The motor accelerated
up over the hump, rpm rose dramatically and it
really began to move some air!
I've added a permanent baffle adjusted so that
the motor draws about 3.5 amps. That's 'cause
I'm too lazy to take it apart and see if the
motor frame is marked with rated load current. If
I knew what the rated current was, I'd adjust
the baffle to achieve that current. In any case,
it now draws 1 amp less than when I bought it
and moves a great deal more air.
This is something to remember about induction motor
characteristics . . . they must be loaded well
down the right side of the 'hump'.
If any of you have ever used an induction motor
lawn mower, you may have experienced what one might
call 'motor stall' when pushing too rapidly into
tall weeds . . . the speed is loaded down to where
the motor tumbles down the front side of the hump.
You have to pull back and let it recover.
For you techno-nerds, max torque (or breakdown torque)
from an induction motor occurs at a 'slip frequency'
in the squirrel cage where inductive reactance is
equal to the DC resistance of the cage conductors.
It's 105 out there today . . . added air motion
in the shop will be welcome.
Bob . . . [quote][b]
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user9253
Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan
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| Posted: Sun Jul 15, 2012 4:17 am Post subject: Hints for recycling furnace blowers . . . |
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Blocking off the air inlet has the same effect: less current draw because the motor is doing less work. This is contrary to what one might expect because, in most applications, hindering the process requires more force to accomplish the task. Have you ever noticed that a vacuum cleaner motor speeds up when the air inlet is blocked?
Joe
[quote][b]
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Joe Gores |
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