Selecting a quiet motor for an extraction fan

[Heretic]

This has a bit of a story, please bear with me.

I have been looking at getting a 'whole house fan', which is a large diameter fan that goes in your ceiling and extracts hot air from the living space, and expels it along with the hot air in the attic through vents. It is supposed to be quiet, since it can be left on overnight as a substitute for air conditioning. Heres an example of a commercial one:

In Australia one costs over $1000 even though they arent the most robust looking things. I thought I would do a DIY on the cheap, so I got hold of an 800mm extraction fan that looks a lot like this:

It looks a bit better made with stainless steel aerodynamically shaped blades, etc. and its even got the louvres as part of the unit. My rationale was that the diameter is large, I can run it slower for a given CFM, and maybe I can experiment with different input voltages to find the noise/CFM sweet spot.

So I fired it up the other day and the motor which looks like it might be a 0.37kW induction motor, and a serious one at that, is pretty loud, louder than I had hoped, and it has that characteristic sound of industrial equipment with that sort of 'whoop' as it spins up and then levels out at a uniform operating speed, sorry I dont know how else to describe it. I read that induction motors are the quietest, so not sure if it is or not. Heres a pic:

Not knowing a great deal about the different varieties of motors, I imagined (before getting the fan) that I might be able to treat it like say a universal motor, and ramp it up gently with a variac to find an operating speed and volume that I could live with, and then use a transformer to supply that voltage. But my attempts to spin it up slowly failed, as it seems that unless it gets close to the rated 240V it hums and jiggles back and forth and doesnt get enough momentum to start. And when I do get it running at full voltage it goes at full blast; it's actually quite loud and therefore unusable for my purpose.

I'm wondering if there is some other kind of motor that might be better for my situation, and I thought this would be the right place to ask. I found a video of a quiet motor here: [URL unfurl="true"]https://www.youtube.com/watch?v=VzPAhL1DMBo[/url]. Does anybody know what type this might be? Or some other type that I can vary with a triac speed controller across the whole speed range? Maybe a washing machine or air con compressor motor?

Any thoughts would be much appreciated!

 

[BrianPetersen]

"Variac" is just a transformer that changes the voltage (essentially a dimmer switch). It doesn't change the frequency. You are essentially trying to operate the motor at full frequency in an under-voltage condition. That's why the motor won't start unless it is near full voltage. At best, even if you get the motor running, attempting to run it at full frequency but under-voltage will burn up the motor as the motor desperately tries to pull more amps than the supply will give it.

A "variac" is suitable for a resistance load (e.g. heating elements and incandescent bulbs) it it is not suitable for operating a motor that "wants" to try to run at or near synchronous speed. That's why they're used as lighting dimmers.

There are some induction motors built with higher designed-in "slip" and higher starting torque that might somewhat function in this type of application but it's hardly an ideal scenario. A "universal" motor, which is really a wound-rotor-and-stator DC motor, also won't care what frequency is supplied and will have an operating speed dependent on the supply voltage.

What you really need is a "VFD", a variable frequency drive. It is quite a bit more expensive than a variac. That is what your video shows ...

 

[waross]

Look for a Variable Frequency Drive that will accept 240 Volts, single phase input. Then get a three phase induction motor.
The type of motor you presently have may have some magnetic noise when starting but is quite quiet when running at speed.
It WILL NOT run on a Variac. The speed of most induction motors is related to the supply frequency, not the voltage. An induction motor needs the optimum Volts per Hertz ratio. In you case, 240 Volts / 50 Hertz = a ratio of 4.9:1 That means that, theoretically, the motor will run at half speed if supplied with 120 Volts and 25 Hertz. Fine for a three phase motor, but there is an added complication with single phase motors. The starting winding has a couple of problems with reduced speed. The starting winding is disconnected by a centrifugal switch when the motor is close to rated speed. At half speed the starting winding may stay connected and burn out.
Have you run the motor uncoupled? Usually the fan itself makes much more noise than the motor.

A VFD and a three phase motor may be run at any speed from close to zero RPM up to 2 times or more rated speed. (Running many loads over-speed will overload the motor. Check the running current at your chosen speed. The motor may have cooling issues at slow speeds.)
There are VFDs on the market which will accept a single phase input and output three phase at varying Voltage and frequency.
Link
For a three phase motor:
Link
eBay is your friend,

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zlatkodo]

Look for fan with outer rotor motor + inexpensive triac regulator.
Winding Design and Repair

 

[itsmoked]

I sort of disagree with you BrianPetersen in that these days you can get a VFD for about $80 that would run that size 3ph motor on single phase and Variacs cost way more than that.

And Heretic; In my opinion you are going about this wrong. Those axial whole house fans suck in the audible arena. You will NEVER find one acceptably quiet! Plus half of them shake the whole house.

What I suggest is you scrounge the blower out of a furnace and use that instead. Furnace makers have spent a fortune developing the absolute quietest and most efficient air movers they can. Some come with motors that you can change the taps on to get about 7 different speeds. Newer ones have single phase VFDs that allow them to be started very quietly so you only notice the when the air velocity starts to make tiny noises out the registers.

I put a 97% furnace in my attic. I had an output plenum made so I could 'hose over' to each register like a giant spider perched in the attic. In that plenum I had two huge outlets added that mount 16" ARDs, (motor controlled butterfly valves).

When my thermostat calls for cooling (A/C) which, I don't actually have, the result is that the two ARDs open the plenum chamber directly into the attic space. The result is the air is sucked into the furnace return as normal but then it's all exhausted into the attic instead of returning to the living space thru all the registers. This cools the roasting attic and sucks air thru the entire house in all the blocked-open windows.

It's quiet, quieter than the normal heating because no air is blowing out the registers. It also is thermostatically controlled since it's the thermostat thinking it's running A/C. It can cycle automatically on temperature so maybe at 2AM it quietly tapers off then after a little more heat soaks out of the woodwork and furnishings it can cycle quietly back ON. It does that until about 5AM. I usually set the time to stop further 'cooling' about 6AM or as it starts to warm up it will continue to run and, of course, screw up everything by pulling in hot outside air starting mid morning.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Nice "Outside the Box" thinking Keith. I like it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[spciesla]

The belt driven units are quieter than the direct drive ones. However, neither are quiet. If you have one, you better enjoy "white noise" or get used to it. One thing you can do is mount the whole house fan structure (box) on foam or rubber to try to get some isolation from the ceiling joists.

Another option that I have heard about, but not witnessed are suspended axial flow fans in a flexible duct. So, instead of the fan right above the louvers, you would have a flexible duct running from the louvers and have the horizontal mounted fan suspended in the attic. These units are supposed to be quieter.

 

[Heretic]

Wow what a great collection of information and ideas, thanks very much to everybody that replied. I have been sitting here absorbing and sorting this new information for at least a couple of hours, watching videos on VFDs, researching furnace blowers etc. and I feel much more confident about solving this problem now and much better armed.


BrianPetersen and waross, YES, great explanations, the motor's behavior makes all kinds of sense now, and I see the value in a VFD in conjunction with a 3 phase motor. I checked out the links waross (thanks a bunch) and I think I will buy one of those exact VFDs or maybe the 0.75kw version they are also selling, which might give me more headroom. Also I found a new, low cost motor which might be a candidate too: [URL unfurl="true"]http://www.ebay.com.au/itm/0-37kw-0-5HP-1400rpm-shaft-14mm-Electric-motor-Three-phase-415v-/181695344372?hash=item2a4de2eef4:m:mA4VVuB727klp29TH_NAmCg[/url] Thanks for clearing up my misunderstandings regarding driving the induction motor.

zlatkodo thanks Im not sure what outer rotor motor means but I will do some research.

itsmoked - Great lateral solution! I have already bought the larger fan described in my first post so I am kind of committed to making it work. But I did some research on the furnace blowers and that seems like a great solution, so long as I could find a large enough unit. I will keep that in store if I cannot make the VFD solution work.

spciesla - Good idea too. I have already bought some medium density foam for that exact purpose, and I will probably disassemble the sheet metal frame and reassemble it with foam inserts to further dampen vibration of the frame. I think I will have to stand the fan up vertically because I don't think the bearings are designed for horizontal mounting in this fan's case, so I will have to make a housing to couple it to the horizontal vent. This will also reduce noise since the fan is not right above the vent.

Thanks to all again, I will keep investigating, I really appreciate the time and thought that went into everybody's answers.
All the best, Chris

 

[Tmoose]

I wonder what noise(s) you are hearing.
- Fan air handling noise coming from the duct inlet
- Fan blade pass noise coming from the duct inlet
- Noise caused by fan unbalance vibration being transmitted to the ceiling joists and then to the ceiling panels (loudspeaker)
- Noise caused by motor unbalance vibration being transmitted to the ceiling joists and then to the ceiling panels (loudspeaker)
- Noise caused by fan and motor sheave runout and belt thickness vibration being transmitted to the ceiling joists and then to the ceiling panels (loudspeaker)

If slowing the fan down and some degree of flexible/isolation mounting does not result in a satisfactory reduction, I'd be interested in understanding the frequencies of the problem noise/sound before making any more changes. A Radio Shack sound level meter used as a microphone,and free FFT software on a PC can process the sound and generated a sound "spectrum."

As a start, if vibration can be felt in the ceiling only when the fan is running, then transmitted vibration may be causing noise using the ceiling (or walls) as a loudspeaker, and the fan assembly or duct mounting need to be more flexible to "isolate" the ceiling from the fan.

 

[NebraskaFanGuy]

I do like Keith's idea above- interesting thought. Axial fans paired properly can be quiet but it can be difficult in a DIY situation to get it right. If you are set on the building it yourself, I would contact a fan manufacturer with your motor specs/rpm and let them manufacture a fan that will best suite your available power for the set diameter. After market fans can be had relatively inexpensive if you go directly to the manufacturer.