Whole house fan motor - variable speed

Electromatic

Senior Member
Location
Virginia
I've never gotten into the nitty-gritty with single-phase variable speed motors except to know that only certain kinds of them can have any speed control at all. One of our service technicians has a customer with a whole-house fan that they would like to have run at different speeds. Right now, the motor is an A.O. Smith 316P758 :


and is controlled just by an on/off wall switch. I'm thinking we cannot just swap out the switch with a speed controller.

What do I need to look for as far as a new motor and control (other than just googling "variable speed fan motor")? I know I'll have to match the frame and shaft sizes and get something in the same HP and RPM range. Are there advantages/disadvantages of looking for something with several discreet set speeds vs. something more analog with almost a rheostat type of control?

I know this is fairly simple, and I should be more well-versed in this kind of application. Any help would, as always, be appreciated.
 

Electromatic

Senior Member
Location
Virginia
Thanks.

It appears the existing motor is a split phase type. I am under the impression that that type of single phase AC motor cannot run at different speeds.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Do they really need 'infinite' variable speed or can they live with a stepped speed switch like Low-Med-High?
 

tom baker

First Chief Moderator
Staff member
Grainger is a great resource for small motors and speed controls. If you know the make, model, frame size, find a replacement motor and compatible speed control. Variable speed is possible but may not be worth the cost.
 

Electromatic

Senior Member
Location
Virginia
Thanks for the replies.
I don't think the customer needs infinitely variable speed. My impression is that the fan is just too much air, too loud running at the set motor speed and wants at least a slower, quieter operational option.
I have used Grainger before, and they are pretty good at cross-referencing motors. They can be pretty expensive, though.

I assume a 2-speed or other kind of multi-set-speed motor would need extra conductors from the controller to the motor. That's probably not a big deal.
Again, I'm not entirely well-versed on split-phase vs. capacitor start vs. permanent split capacitor, etc. I would guess a stepped-speed motor would be cheaper than something infinitely variable, correct?
 
I have used Grainger before, and they are pretty good at cross-referencing motors. They can be pretty expensive, though.
They are because they have so much stuff on the shelf ready to ship (same for McMaster, Fastenal, etc; less so IME for Global Industrial).

I assume a 2-speed or other kind of multi-set-speed motor would need extra conductors from the controller to the motor.
Most of the stepped-speed motors need one conductor per speed + the return.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Most of the stepped-speed motors need one conductor per speed + the return.
The wall mounted 'quiet' ones for my ceiling fans are all two wire devices.

Without more specifics on the motor we are just guessing.
 

Electromatic

Senior Member
Location
Virginia
I've never dealt with Zoro though they do come up quite a bit in search results. I think there was a thread on these forums about Grainger and Zoro being the same company but Grainger being the brick-and-mortar side of things.

The existing motor I'm dealing with/thinking of replacing is a 1/4HP, belt-drive connected to a whole-house fan.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
FWIW, when my high efficiency furnace was being put in, the tech was showing me how stuff worked in it. He explained that the blower motor was a DC motor and the reason was for easier speed control. Maybe there was a reason they didn't try the same thing with an AC motor?
 

Electromatic

Senior Member
Location
Virginia
Well, I found a 2-speed motor to match all other specs on Grainger/Zoro for about $150 and a couple of SPDT center-off options for switches.

There were a lot of internet search results involving variable speed whole house fans--almost all of them questioning as to how or if it could be done. It was surprising that it's not really offered as a product option for sale. I did find one complete whole house fan assembly with a PSC motor, but it only came with a two speed switch. I guess one could use a rheostat or other analog control with that type of motor, but I'm not going further down that hole for now.

Thanks again, all.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The problem with variable speed for residential applications is the complexity and/or cost. The motor needs to either be 3 phase and you add what's called a VFD to control the speed and convert 1 phase residential power to 3 phase for the motor, or if you want to stick to a single phase motor, it must be PSC (Permanent Split Capacitor) with a special single phase VFD or Shaded Pole with a dimmer type controller. VFDs don't like heat, so mounting one in the attic is not going to work, which means more complexity as to where to put it. Shaded Pole motors can be used with simple dimmer switch type controls, but lack power for "whole house" applications. So for the average homeowner, it's either too complex, too expensive or too wimpy to be worth doing with variable speed. Just using a multi-speed motor is the preferred solution.

That motor you showed by the way is what's called "Split Phase", which doesn't use a starting capacitor, but it DOES have a centrifugal switch, which, like a Capacitor Start motor, makes them unsuitable for variable speed because if you drop below the speed setting of the centrifugal switch (usually 90%), you drop back into starting mode, which is not designed to be used continuously.
 

Electromatic

Senior Member
Location
Virginia
So PSC motors will NOT work with a dimmer type control? I thought I read elsewhere that they could--that the slip and torque don't work out in many applications, but a fan application could (possibly anyway) use a PSC motor and a rheostat/dimmer type control.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
So PSC motors will NOT work with a dimmer type control? I thought I read elsewhere that they could--that the slip and torque don't work out in many applications, but a fan application could (possibly anyway) use a PSC motor and a rheostat/dimmer type control.
A PSC motor or a shaded pole motor are the only kind of single phase motors that can work with a speed control because they don't have a starting switch.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The motor that you are looking for will be called an ECM motor. The motor requires a matching controller ('three phase plus VFD') but you might find the controller integrated into the motor housing.

Here is an example, which is not suitable for your application because of speed and the belt drive, but which might point you in the right direction:


Good luck
Jon
 
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