Today's Message Index:
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1. 05:17 AM - Re: Hints for recycling furnace blowers . . . (Fran & Joe)
2. 09:16 AM - Re: Hints for recycling furnace blowers . . . (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Hints for recycling furnace blowers . . . |
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
Message 2
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| Subject: | Re: Hints for recycling furnace blowers . . . |
At 07:15 AM 7/15/2012, you wrote:
>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?
Work produced (pounds of air per second
accelerated to some new velocity) translates
to watts of power needed at the motor shaft.
If that demand (torque loading) is too great
then the induction motor cannot accelerate
"up-slope and over the hump' to reach it's
designed operating point.
The amount of current being drawn is simply
a companion artifact of where the motor is
operating on it's performance curve while being
overloaded.
If one were to build an enclosure around an
airplane propeller . . . a disk-shaped box
with just enough clearance to allow free
propeller movement . . . opening the
throttle fully would produce a much greater
than red-line RPM response from the engine.
This is because the engine is now spinning
a fixed amount of air in a close volume as
opposed to accelerating an unrestrained
mass of air through the propeller disk.
The series wound motor of a vacuum sweeper
doesn't have that hump in the operating curve
defined by Xc=R characteristic of the squirrel
cage rotor. But RPM of that motor is
still a function of load: force proportional
to the product of acceleration and mass.
And like induction motors, 4-cycle engines
have a "hump" in their performance curves.
Load them down too much and they 'fall down
the curve' and die. Brush-commutated motors
don't exhibit this characteristic.
Bob . . .
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