Variablle speed exhaust fan

[junkyardjeff]

Now that its cold and I still have some painting to do and was wondering if I cam make my exhaust fan variable speed so it wont **** all the heat out,does anyone know if I can hook a light dimmer in line to slow the motor down or is there anything else that will work better.

 

[Stuart in MN]

A dimmer switch will not work.

 

[Burl]

Use a rheostat (sp?).

 

[ddawg16]

Dimmer switch is nothing more than a chopper that gives a variable pulse to the motor. In most cases, it will cause oscilation or will just flat die.

Rheostat is a resistive wire (basically the same thing in your dash light dimmer) that reduces the voltage to the device.....you would need a real big one to handle the power of the fan.

Most multi-speed fans actually have multi-pole motors that you use for different speeds.

Use a damper on the input (not output). If you reduce the available air flow to the motor it will just pull less current. As an example, block the hose on your vacuum cleaner...hear the motor speed up? That is because the load on it is less. It's easy to prove....hook up an amp probe....you will see a big difference.

 

[junkyardjeff]

I just put a new motor on it a couple years ago and I wish I would of thought about a motor with multiple speeds,I forgot that we have a electric motor shop and see what they have to offer.

 

[Freejack]

Assuming you have a direct drive fan with a PSC motor, you need a triac-style fan speed controller, almost any HVAC distributor or motor shop should have something available.

Here's the selection from Johnstone Supply: http://www.johnstonesupply.com/corp/tabid/1783/default.aspx?product_group=1214

A bunch of similar products from Grainger: http://www.grainger.com/Grainger/fan-speed-control/hvacr-controls/hvacr/ecatalog/N-ce2?op=search

If you have a belt driven fan, its likely a split phase motor and you'll not be able to use a speed control, but you can change pulley sizes.

.

Most multi-speed fans actually have multi-pole motors that you use for different speeds.

Multispeed motors in direct drive fans are not actually multi-pole, but rather tap off different segments of the winding to create a weaker motor for the lower speeds.

Jake

 

[junkyardjeff]

I talked to the electric motor shop today and I dont think I wil be able to make it variable speed,its not a direct drive motor and has a starting switch so I cant hook up a rheostat and will have to change the speed by changing the pulleys

 

[Freejack]

Yep, a split phase motor has a start winding, so it wouldn't be too good to put it on a speed control.

In that case, as suggested earlier, if you do want to reduce the flow significantly, some sort of shutter would be an option.

Also, just as an FYI, if you need to replace the motor again, there are two speed split phase motors available.

Jake