On Thu, 15 Jul 2010 23:21:05 -0400, Tony wrote:

bud-- wrote:
Tony wrote:
bud-- wrote:
Tony wrote:
bud-- wrote:
Tony wrote:
Lee B wrote:

Current house came with a whole house fan (not an attic fan, but the
kind in a hallway ceiling that sucks air into the attic). I was
happy to
see that because I really liked the one in my old house. However
the one
in the old house had a variable speed dial on it, and the new
house's
fan has just one speed - high and loud with a simple off/on switch.

Is the variable speed a function of the fan itself or can that be
added
at the switch? I'd definitely be having an electrician do it, but
I'm
just trying to figure out if it's even doable, or if I'd end up
needing
an entire new fan. And if so, would replacing an existing fan and
switch
be difficult (ie cost me an arm and a leg)?


My fan has a 2 speed motor. The 2 speeds are from separate windings
- each speed has a different number of poles in the motor.

You can easily control a "universal" motor - the type with brushes.
Not likely that is what is in the fan.

Three phase motors can be controlled by changing the frequency. No
possibility there is a 3 phase motor in the fan. The control would
be relatively expensive. I have never seen info, but I suspect the
new variable speed furnace motors are of this basically this type.

You could probably use a variable frequency control for a standard
induction motor, but if the speed falls to where the start switch
in the motor turns on you will probably burn out the motor. And
probably relatively expensive.

If you use a "phase angle" controller, like a light dimmer, you
could control the speed - the motor speed falls farther from the
"rotating field" speed. Has the same problem with start switches. I
believe the torque falls rapidly as the speed drops, but the power
required for a fan, if I remember right, is about the 4th power of
the RPM.



What is the voltage and amperage ratings of the motor? Here are
some speed controllers to choose from.

http://www.amazon.com/s?ie=UTF8&rh=n...ntrol&p age=1


Briefly looking at the info for one of the speed controllers, it
says it is for shaded pole motors (which are used in clocks and my
bathroom fan) and permanent split capacitor motors, which is not
likely what is being used for the fan. Not obvious from what I saw
that the 10A speed control would work for a typical induction
motor, which is probably what powers the fan. These controls are
probably "phase angle" controllers - as above.

Outside of a 2 (or more speed) induction motor, I don't know how
you reliably control the speed of an induction motor that probably
powers the fan. How does the control work? How do you avoid
problems with the start switch?

Uh... hello? A shaded pole motor IS an induction motor. Motor
speed controllers will vary their speed by varying the frequency.
Try google.

http://www.google.com/search?num=50&...=&oq=&gs_rfai=


You will not find a shaded pole motor on a fan of any size - like a
"whole house" fan that makes too much noise.

The motor controller I commented on (from your previous post) is
almost certainly a "phase angle" control (like a light dimmer). It
does not change the frequency.


What is it like to guess instead of going by the facts?

Guessing?

Facts: a variable frequency drive converts the AC line to DC and then
inverts that to variable frequency AC. There is no way the enclosures
have enough space for the filter capacitors that would be required. Also
way to cheap. And all VFDs I have seen are for 3 phase motors.

They are "phase angle" controls.

Probably everything goes your way, if you don't read the specs.

I read the specs - "shaded pole" or "PSC motors". Neither of these
motors has a start switch. Find me a "whole house" fan that uses either.

Someone else said that shaded pole motors are about 1/4 HP max. In a
fast look at Grainger I only saw 1/5 HP max. Find a "whole house" fan
that only uses a 1/5 HP motor.


The controller I looked at was also stated to work with a permanent
split capacitor motor. If I remember right, they are a 2 winding
motor with a capacitor in series with one of the windings. There is
no start cap and no start switch. It is basically a 2-phase motor. (I
don't remember ever seeing one.)

Neither of the motors that the controller said it was good for has a
start switch. A "whole house" fan will almost certainly have an
induction motor with a start switch. Using a dimmer-type control, as
above, can easily burn out the motor. It is a misapplication
according to the limited information provided with the control.

There is a lot more info out there. I can't do all the work for you,
look up KB Electronics and read the specs. And if you look at the
picture, you will see that from the "off" position,the first "on"
position is "high". Yes it will start the motor.

If you set the speed too low will the motor start switch close? If it
does the motor is likely toast. The motors in the manufacturer's spec do
not have a start switch. There is a reason.

Where in the specs does it say the control is good for an induction
motor with a start switch that would be used on a "whole house" fan?

Seems to be a lot of people who disagree with you.

I was replying to those who said the speed controls I pointed to would
not work on an induction motor. The specs says it does. That's all. I
didn't argue that they would work for a whole house fan. My grinder is
1/2hp and it has an induction motor.

Moron, the thread is *about* whole house fans. Induction motor powered whole
house fans *CANNOT* be "dimmed".

Here is a 2HP induction motor. There are many more even larger.
http://www.biscuitplantspares.com/in...oducts _id=32

http://www.google.com/search?q=1%2F2...ient=firefox-a

Again, the motor speed controllers I posted a link to do work on
induction motors. That is a fact. That's all I'm saying. What did I
post that is not true?

What an idiot.