[Amps] Blower noise

[Manfred Mornhinweg]

Steven, Jim, Drax, Mike, Alek, Russ, Chris, Roger,

> However, 12Vdc brushless motors usually have an inverter inside them, so 
> they're really AC motors (higher frequency) operated from a DC source; as 
> such slowing them down is best done by PWM  rather than just a series 
> resistance.

The data sheet of a Sunon blower with brushless DC motor specifically forbids
using PWM to control the speed, since it interferes with the motor's internal
electronics. Instead by controlling the voltage, most of these brushless DC
motors can be regulated over a considerable range. At too low a voltage, though,
they will stop suddenly. The electronics inside need a certain minimum voltage.

> Most of the 12Vdc blowers I've tried just stop running altogether at 7-8Vdc,

Yes, that's typical. But it should give enough control range for my needs.

> and using a series resistance to reduce the applied voltage at levels before
> they stop operating doesn't really slow them down.

It's better to actually regulate the voltage to regulate speed. Using 12V with a
series resistor reduces the whole torque/speed curve, and that results in poor
speed regulation, and startup problems.

When I run such DC fans at reduced speed, I use a reduced voltage, and if it's
very low, I add a circuit to give them one second or so at full voltage to start
up safely, then switch down to the low voltage.

> I use Bose QC15s... And I LOVE them....

I don't like headphones... Not even good ones!

> Put it in a closet or another room.

This baby wants its meters watched, so the screen current doesn't get high
enough to vaporize the screens. So I need it within sight, at the operating
position! Also for tuning it, it's handy to have it nearby. I have considered
some sort of soundproof enclosure with a cooler inside (water circuit) and a
transparent front door, but it seems like too much trouble. It should be better
to find a way to quieten that blower!

> I've also put just the blower in another location, and ran a flexible duct to
> the back of the amp.

Yes, that's indeed a possible solution. My garage is just under the shack, and
there is a 5 inch hole in the floor, for cables. I could place a blower in the
garage and run a duct to the amp. This will actually be my last resort solution,
if I can't silence the beast in any other way! What I don't like is installing
that duct, with a suitable fitting to the amp's sealed compartment under the
tubes. It would require cutting and hacking steel, and I would really prefer not
doing anything non-reversible to that amp. I tend to take a conservationist's
attitude to antique electronics! :-)

> Looking at what I think is your amplifier (National NCL-2000?)

Yes.

> the fan motor appears to be what I called a record player motor

Yes.

> I remember them as being relatively low RPM, maybe
> 1500-1600 RPM,

These motors turn slightly slower than the grid frequency. So, at 60Hz they run 
slightly below 3600 RPM, and at 50Hz, slightly below 3000 RPM. I measured mine 
at 2640 RPM if the air flow is unrestricted, going up to 2916 RPM when fully 
blocking the air flow.

I suppose it's possible to make such "record player motors" (shaded pole 
induction motors, in engineer's lingo) in 4-pole versions. Such a motor would 
indeed run at roughly 1600 RPM under load, at 60Hz. But I haven't seen any like 
that.

> so I am surprised at the noise level being produced.? My Drake
> L-4B runs a 1550 RPM motor and is very very quiet.? I do realize ceramic
> tubes like your 8122's need a bit more airflow than 3-500z's; hence, more
> noise.

The airflow isn't really much larger, I think. But ceramic tubes need 
pressurized air forced through their narrow fins. That's why they need a radial 
blower rather than an axial fan. And this is a more noisy animal!

> My noise issue was with an ITT AM-6155 amplifier.
> At that RPM, the fan blade noise equates to a
> high-pitched ringing sound.

Yes, that's what mine sounds like, too. A somewhat tinny ring.

> Add the air noise, and the result is one noisy
> fan!!! I replaced the s/s instrument bearings with standard sealed bearings.?

The NCL-2000's blower has friction bearings, with a nice oiling system. I don't 
think these bearings are contributing any significant noise.

> The next step was isolating
> the fan motor from the chassis using cutout pieces of an old mouse pad--major
> improvement there.

I would love to do that, but the stock blower fits tightly in the NCL-2000. I 
can't isolate the blower case. I might mount just the motor in some isolated 
supports, but that would require machining some parts. That would probably 
eliminate the line-frequency hum, but the most disturbing noise of this blower 
is the ring at about 900Hz, and that one doesn't come from the motor.

 > Playing with the 400hz power supply, the noise emanated
> by the motor? dropped significantly by *increasing* the fan rpm slightly.

I wonder whether mine would be quieter at 60Hz than at 50. It's quite possible.

> The last thing was isolating the amplifier from the table using a plastic
> real estate sign--like plastic cardboard.

I have it on a shelf above the table. The shelf flexes a lot under the weight of 
this amp, so there I have built-in decoupling of vibration! :-)

> Locating the amp so the remaining
> sound was directed away helped as well.

I use some baffles to block the direct noise between the amp's air inlet and my 
ears. But the baffles only reduce the noise by 2dB or so.

> A word of caution about
> using cheapee 12 volt fans in amplifiers......been there and done that.? I
> found that in the presence of RF, they can shut down.?

I don't think this will happen in my case, because the modern cheapie blower is 
small enough to fit completely under the chassis. I would close the hole in the 
chassis, through which the motor of the original blower pokes, with a piece of 
metal sheet. So the new blower will work in a relatively low level RF field. 
Sure, I have to test whether it really keeps running during TX!

> In the automotive world most belt driven engine (radiator) cooling fans these
> days have an asymmetric blade pattern for noise reduction. I have never seen
> this on other than an axial fan design. I wonder if it would work for a
> centrifugal fan? 

At least some modern radial blowers have alternating long and short blades. 
That's probably for the same effect. I haven't yet seen any with variable 
spacing between the blades, though.

Many car tires use the same trick to reduce noise. Asymetric patterns spread out 
the sound energy over a wider frequency spectrum, instead of concentrating it 
into a ringing whine. The total noise energy remains the same, but it's far less 
disturbing.

 > I believe the fan noise
 > from the squirrel cage blower, is air flow, turbulence, and a resonance
 > related to the number of blades the impeller speed, and possibly changes
 > to back pressure related to turbulence and air density which changes
 > with air temp.

Yes, that's my impression too. In particular, this blower produces the most 
objectionable noise when its airflow is restricted just to the point where the 
blades start stalling, and probably produce huge turbulence. Unfortunately this 
is precisely the level of restriction at which it actually works in the amp! At 
least at 50Hz. I wonder if at 60Hz it would be fully stalled, or fully 
un-stalled, but in any case it's probably quieter!

 > In the computer world they have come up with 140mm, low RPM fans with a
 > very low noise signature at less than 20 to 25 db.

But those are fans, not blowers. Yes, in general, a larger fan, running slower, 
is MUCH quieter, for a given cooling performance.

 >  They can run as much
 > as 70 cu ft per min IIRC and do it with surprising back pressure.

I can't believe that they can work against a significant back pressure. That 
pressure would act over their whole surface! Can you give some example, to look 
up its specs?

The two 8122 tubes in the NCL-2000, at my altitude, require something like 20 cu 
ft per min, at a pressure of about 0.6 inches of water. Add some loss, because 
the ceramic chimneys don't fit closely to the tubes, and it's probably 25 to 30 
cu ft per minute, at the same pressure. And that's clearly in blower's land, not 
in the range of an axial fan! Actually the optimal blower would be narrower, and 
with a larger diameter, than that fitted in the NCL-2000.

I think I will just shelf away the amp, until the new modern plastic DC blower 
arrives here, and then see if I can rig up that new blower in some way that's 
acoustically compatible with a modern ham shack! :-)

Manfred

========================
Visit my hobby homepage!
http://ludens.cl
========================