[Amps] Here's a new one for me that I'd like to share

[Jim W7RY]

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 at juno.com
Sent: Tuesday, December 05, 2017 11:48 AM
To: amps at contesting.com
Cc: goodguyham at 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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