It seems counter intuitive, bit what Jim say's is true, block the
blower and it does not have to move the air so is doing "less work".
You can demonstrate this by monitoring the current on the vacuum
cleaner which will DECREASE when you block the input or output.
- Paul
- KW7Y
At 10:07 AM 12/5/2017, Jim W7RY wrote:
A vacuum cleaner motor also speeds up when you plug the suction or
the discharge.
That's the nature of a blower. Remove the load (moving air) and it speeds up.
73
Jim W7RY
-----Original Message----- From: w2cqm@juno.com
Sent: Tuesday, December 05, 2017 11:48 AM
To: amps@contesting.com
Cc: goodguyham@aol.com
Subject: [Amps] Here's a new one for me that I'd like to share
After some preliminary tests, I decided that a dual 8877
hombrew amplifier project would not be cooled adequately with a
previously installed low RPM tandem blower. Checked the "junk box"
for another candidate with higher speed and more volume and thanks
to a long forgotten hamfest purchase, from yesteryear(s), I
found a suitable replacement. The squirrel cage blower was larger
and the manufacturer's label indicated TWO speeds. Imprinted on
the motor were specifications including a 115VAC operating
voltage and a schematic showing the three wire hookup
sequence. Two color coded wires indicated line voltage input and
the third wire (black) depicted the installation of an AC starting
capacitor wired from that lead across to one of the color
coded AC leads. NO VALUE WAS GIVEN TO THE CAPACITOR. I initially
utilized a 1uFd cap for testing. I was disappointed since there
were no other wires that could be used for speed selection. To
test for suitabili
ty, I nevertheless increased the opening for the larger blower
mounting flange and installed it on the rear of the amp enclosure.
It started to work; but nowhere near that potentially higher value
of rotation stenciled on the motor body. I was a bit stymied at
that point! Not certain what possessed me, but I decided to reach
in and placed my hand over the blower output to restrict
flow. Mysteriously, the fan substantially increased its
speed. Removed the back pressure restriction caused by my hand
and it slowed down. Not certain if I can explain the phenomenon ,
but I did experiment with much higher AC capacitors values and
found the fan speed increasing but that resulted in the buildup
of abnormal heat around the motor. That was unacceptable. I
settled on the one and only low value 1 uFd capacitor I had
since the amplifier cooling system was now operating well with
adequate cooling volume. After replacing the tubes in the sockets and sealin
g off the pressurized lower cabinet, the fan increased under the two
tube cooling requirement restrictions and the blower motor heat
build up is no longer an issue. Obviously, the automatic speed
control has something to do with the value of the starting
capacitor, the normal back pressure of cooling, and to some extent
the resultant additional current draw. It's safe to assume that
it's not a ghost nor is there an air flow control. within the fan
body! Hopefully, someone can explain what's happening and share it
with me and the other AMPS subscribers? Thanks, Ron W2CQM/3 Note!
you can view some early digitals of this project with annotations
on my QRZ site. Click on SHUTTERFLY at the bottom of the page.
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