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Author Topic: Here's A New One On Me Requiring Some Assistance  (Read 451 times)

Posts: 2

« on: December 07, 2017, 04:46:37 AM »

After some preliminary tests,  I concluded that a dual 8877 hombrew  amplifier project  would not be cooled  adequately with a previously installed  low RPM  single motor 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 4+”squirrel cage blower was larger and with greater volume and  the manufacturer's  label indicated TWO  speeds that were substantially higher than the blower being replaced.  Imprinted on the motor were  specifications including  the  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 and I was confused about the two speeds listed as a specification.   To test for suitability, I nevertheless increased the opening  for the larger blower mounting flange and  installed it on the rear of the amp enclosure. It operated well and was quiet; but nowhere near that  potentially higher value of rotation stenciled on the motor body.  I was a bit stymied at that point! Why were  dual speeds listed with no apparent method to access the higher rotation?  Not certain what possessed me, but I decided to reach in and placed  my hand over the blower output  to restrict flow and to determine the extent of  pressure.  Mysteriously, the fan substantially increased its speed at least several hundred RPM’s.   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 capacitor values  and found the fan speed noticeably increasing; however,   that  resulted  in the buildup of  abnormal heat around the motor body.  That was unacceptable.  I settled on  the one and  only  low value  1 uFd  capacitor I had since the amplifier cooling system  appeared to be  operating  well with adequate cooling volume and low noise parameters.   After replacing the tubes in the sockets, installing the chimneys, and sealing off the pressurized lower cabinet, the fan speed increased resulting from  the two tube cooling requirement  restrictions. The blower motor  heat build up was  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 with the tubes in place, and to some extent the resultant (minor)  additional current draw from the back pressure.   It's safe to assume that it's not a ghost nor is there an air flow control. within the fan housing!  Hopefully, someone can  explain what's happening and share it with the group.   Thanks, Ron W2CQM/3

To view this project  in development, with annotations,  visit my QRZ site and click on SHUTTERFLY  at the bottom of the page.
« Last Edit: December 07, 2017, 04:59:35 AM by W2CQM » Logged

Posts: 2638

« Reply #1 on: December 07, 2017, 05:04:47 AM »

The usual description is that the fan blades "stall", which means that they aren't moving the air, which means that it takes no energy to make them spin.

In a "stall", the air at the fan could be spinning around with the fan, or it could be flowing around the fan blades, but it's not flowing through the fan blades. (which causes more drag)

However it happens, less air movement = less drag on the fan blades = faster spinning motor. The vacuum at the input disappears, the pressure at the output drops.

You can also see this with outboard motors, when a boat is skimming along the water and hits a wave and the prop momentarily lifts out from the water the engine will speed up.  It is hard to imagine that when you block the input or output that the fan would speed up but but when you consider that the fan blade is in a quasi "stall"  condition and dragging less air the motor will spin faster due to less drag.

The problem is the concept is hard to imagine because at first thought it would seem just the opposite.  Put your hand over a shop vacuum to cut off the flow of air being sucked in, what happens?  It speeds up.

Posts: 4925

« Reply #2 on: December 07, 2017, 05:25:04 AM »

1uF would be unusually small for a run capacitor. 2uF, 3uF, and 5uF are the small sizes. 5uF is a very common value for condenser fans.

Posts: 44

« Reply #3 on: December 07, 2017, 08:09:13 AM »

Ron what colors are the three leads from the motor? Usually the white and black would be high connected to the source. The other wire would be a lower speed thus making it two speeds if provided. You would disconnect the black and connect the other wire for a slower speed. A fan performs according to its fan curve depending on static pressure. In general if you block the discharge the motor speed will increase because the fan is conveying no air and the blower wheel is spinning unloaded due to increased static pressure. Also you never want to let a squirrel cage blower run without a duct or some resistance on it as the blower will overload the motor because of reduced static pressure and handle more air than the rated max. It will follow the fan laws. Look on the internet for the fan laws if interested.
The starting cap has nothing to do with the operating speed , it just assists the motor to start.


Posts: 6364

« Reply #4 on: December 07, 2017, 08:55:21 AM »


I've never seen a 3-wire capacitor start blower.  Very interesting on what you eventually come up with.

If the nameplate where you got the speed information has a brand name, perhaps you could Google this for specific information?

I agree with RVN... the three wires are one: common - two: High speed and three: third wire is low speed.

I owned one of these single motor, two outlet blowers but it was single speed.  However, most of these types of blowers run pretty warm, which was initially disconcerting to me.  But after years of running the same type of blower (Only with a single port) I've accepted this higher than what I consider "normal" temp.

Normally, a capacitor start motor has two leads just for the capacitor and is cut out of the circuit by an internal switch when the motor comes up to speed.
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