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