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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