I am repurposing an old home ventilation fan to just preform extraction, but its too powerful and loud. I need to reduce the speed by about 25-30%, doesn't need to be variable. Its a PSC motor, do i need to increase the capacitance?

 

If it's a standard AC induction motor- change the drive belt pulley ratio.

The normal AC blower type motor is not friendly for simple or inexpensive speed change tricks.

 

The normal AC blower type motor is not friendly for simple or inexpensive speed change tricks.

Indeed not: http://www.designnews.com/document.asp?doc_id=221517

 

Its a direct drive axial fan, i think i can maybe add a choke, trying to find out now what value in Henrys of choke to use for a 100W 230V motor.

 

Its a direct drive axial fan, i think i can maybe add a choke, trying to find out now what value in Henrys of choke to use for a 100W 230V motor.

Ummm... No.

If it was that easy, you would see this trick used everywhere.
You are asking the motor to revolve at a different speed / frequency than it's designed to operate, AC motors like this cannot be controlled so easily.

The only reasonable way is to use a VFD, that's gonna work, but its way more expensive than a new fan...

 

You could reduce the voltage applied with dimmer, this will also reduce the current and starting torque.

 

If it's direct drive fan with a shaded pole or split capacitor type (most common on fans) reducing input voltage will reduce speed without causing motor problems.

 

You might be able to use a light dimmer to reduce the voltage and thus the fan speed.

If that generates too much of a buzz in the motor, you can use a series capacitor.
I've done that for a table fan.
You have to experimentally determine the capacitance value to get the reduction you want.
I think I used about 4μF for a 110V fan.
A 230V fan would likely use about half of that.
The cap needs to be an AC film type, such as a motor run capacitor, with a voltage rating greater than 350V for a 230VAC supply.

 

Be wary, if using a series cap, that the motor winding inductance and cap don't resonate at the mains frequency or a harmonic thereof, or very high voltages can be generated inside the motor and cause damage (don't ask me how I know )

 

Ummm... No.

If it was that easy, you would see this trick used everywhere.
You are asking the motor to revolve at a different speed / frequency than it's designed to operate, AC motors like this cannot be controlled so easily.

The only reasonable way is to use a VFD, that's gonna work, but its way more expensive than a new fan...

This leads me to believe that i could use a choke, and that it likely has been used in manufacturing plenty. Id say it has died out as a method of speed control as you need a large lump of copper to make one.

 

If it's direct drive fan with a shaded pole or split capacitor type (most common on fans) reducing input voltage will reduce speed without causing motor problems.

wont reducing the voltage increase the current? Ohms law and all? Resistance is constant, voltage drops, current increases?

 

wont reducing the voltage increase the current? Ohms law and all? Resistance is constant, voltage drops, current increases?

V = I * R
Voltage and current are proportional given constant resistance. Reducing the voltage reduces the current.

 

You are not going to control a furnace fan with a Triac controller, it was done in the past with a multi-winding motor, nowadays it is done with a ECM motor (Electronically Commutated) P.M. motor.
Max.

 

I love the choke, and I recall seeing this a long time ago, but being told it was an autotransformer, which I of course disagreed with. Of course along with the reduction in voltage and current and speed is logarithmic reduction in power, both incoming and at the shaft.

 

Its a direct drive axial fan, .

BTW It is not an axial fan, it's known as a Radial type.
Max.

 

BTW It is not an axial fan, it's known as a Radial type.
Max.

Also known as a squirrel cage or centrifugal fans.

 

When I tried to reduce the speed of a fan using a triac the fan slowed down as one would assume but the harmonic distortion introduced by the triac caused an annoying buzz. That is opposite of the desired result.

When using a Variac the fan's speed could easily be adjusted and the fan became quieter.

 

You are not going to control a furnace fan with a Triac controller, it was done in the past with a multi-winding motor,

Actually withe permanent split capacitor motors regardless of whether they are multi speed or not they tend to take triac phase angle control of their speed rather well. It's been done many times.